U.S. patent application number 11/313549 was filed with the patent office on 2007-06-21 for capacitive level shifting for analog signal processing.
This patent application is currently assigned to Impinj, Inc.. Invention is credited to Jay A. Kuhn.
Application Number | 20070139244 11/313549 |
Document ID | / |
Family ID | 38157112 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070139244 |
Kind Code |
A1 |
Kuhn; Jay A. |
June 21, 2007 |
CAPACITIVE LEVEL SHIFTING FOR ANALOG SIGNAL PROCESSING
Abstract
An analog processing block is arranged to receive an input
signal through an AC coupling circuit. A digitally programmable
voltage reference (DPVR) circuit is arranged to provide selection
of a voltage reference that is DC coupled through a high impedance
circuit to the AC coupling circuit. The input to the analog
processing block includes the AC coupled input signal and the DC
level from the selected voltage reference such that the DC level is
effectively shifted for the analog processing block. The analog
processing block may include any number of analog functions
including: buffering, level shifting, scaling, integrating, and
analog-to-digital conversion for digital signal processing, to name
a few. A digital control logic circuit and a trim map can be
arranged to control adjustments to the DPVR such that the effects
of any non-ideal conditions on the analog processing block are
minimized. The trim map may include non-volatile memory
devices.
Inventors: |
Kuhn; Jay A.; (Seattle,
WA) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
Impinj, Inc.
Seattle
WA
|
Family ID: |
38157112 |
Appl. No.: |
11/313549 |
Filed: |
December 20, 2005 |
Current U.S.
Class: |
341/155 |
Current CPC
Class: |
H03M 1/1061 20130101;
H03M 1/361 20130101 |
Class at
Publication: |
341/155 |
International
Class: |
H03M 1/12 20060101
H03M001/12 |
Claims
1. An apparatus that includes facility for level shifting an input
signal, comprising: an AC coupling circuit that is coupled between
a first node and a second node, wherein the first node is arranged
to receive the input signal, and wherein the AC coupling circuit is
arranged to AC couple the input signal to the second node; an
analog processing block that includes an input terminal that is
coupled to the second node, wherein the input terminal of the
analog processing block comprises a gate of a field effect
transistor; a digitally programmed voltage reference circuit that
is arranged to provide a reference signal to a third node in
response to a digital control signal, wherein the digitally
programmed voltage reference circuit comprises an analog reference
voltage generator circuit that is coupled to a voltage divider
circuit that includes a set of tap-points, wherein each of the set
of tap-points in the voltage divider circuit are coupled to a
respective input of a multiplexer circuit, wherein one of the set
of tap points is selectively coupled to the third node in response
to the digital control signal; and a high impedance DC coupling
circuit that is coupled between the second node and the third node
such that a DC level associated with the second node is adjusted by
the digitally programmed voltage reference circuit such that the
input terminal of the analog processing block has a precisely
adjusted DC level.
2. The apparatus of claim 1, wherein the analog processing block
includes at least one of: an amplifier circuit that includes an
amplifier input that is coupled to the second node, and a
comparator circuit that includes a comparator input that is coupled
to the second node.
3. The apparatus of claim 1, wherein the AC coupling circuit
comprises one of a capacitor circuit, a single capacitor, a
plurality of capacitors, a transistor circuit that is arranged to
operate as a capacitor, a poly-silicon capacitor, and a metal plate
capacitor.
4. The apparatus of claim 1, wherein the high impedance DC coupling
circuit comprises at least one of: a first resistor circuit, a
transistor circuit that is biased to operate as a second resistor
circuit, a doped poly-silicon resistor circuit, an un-doped
poly-silicon resistor circuit, a diffusion resistor circuit, an
implant resistor circuit, and a thin film resistor circuit.
5. The apparatus of claim 1, wherein the high impedance DC coupling
circuit has a sufficiently high impedance to shield the digitally
programmed voltage reference circuit from signal effects of the
second node.
6. The apparatus of claim 1, wherein the high impedance DC coupling
circuit has a resistance characteristic that is greater than one
mega-ohm.
7. The apparatus of claim 1, further comprising: a trim map circuit
that is arranged in cooperation with the digitally programmed
voltage reference circuit such that previously determined offset
levels associated with the input terminal of the analog processing
block are adjusted by the digitally programmed voltage reference
circuit in response to an output from the trim map circuit.
8. The apparatus of claim 1, wherein the voltage divider circuit
comprises: a resistor array that is comprised of a selected
material type, and wherein the analog reference voltage generator
is a bandgap circuit that employs resistors comprised of the
selected material type.
9. The apparatus of claim 1, further comprising a current trim
circuit that is arranged to provide an offset current to at least
one of the set of tap-points in response to at least one of the
digital control signal and another digital control signal that is
different from the digital control signal.
10. The apparatus of claim 9, further comprising a trim map circuit
that is arranged in cooperation with the current trim circuit such
that the offset current that is provided to the at least one of the
set of tap-points is identified with a previously determined
setting in the trim map circuit.
11. The apparatus of claim 1, further comprising: a trim map
circuit that is arranged in cooperation with a digital control
logic circuit, wherein the trim map circuit comprises at least one
of: a memory circuit, a non-volatile memory (NVM) circuit, a fuse
circuit, an anti-fuse circuit, a ROM circuit, an EPROM circuit, an
EEPROM circuit, a programmable logic device (PLD) circuit, a latch
circuit, a register circuit, and a random access memory (RAM)
circuit.
12. The apparatus of claim 1, the high impedance DC coupling
circuit comprising an un-doped poly-silicon resistor circuit that
is arranged such that no diffusion contacts the second node of the
apparatus.
13. The apparatus of claim 1, wherein the digitally programmed
voltage reference circuit comprises: a resistor array that is
configured to selectively generate the reference signal, wherein
the resistor array comprises at least one of: a series resistor
circuit, a parallel resistor circuit, a combined series/parallel
circuit, a resistor ladder circuit, and an R-2R resistor ladder
circuit.
14. An apparatus that includes facility for level shifting an input
signal for signal processing, comprising: a first AC coupling
circuit that is arranged to couple the input signal to a first
input node such that the first input node has a first input signal
that is responsive to the input signal; a second AC coupling
circuit that is arranged to AC couple the input signal to a second
input node such that the second input node has a second input
signal that is responsive to the input signal; a first processing
block that includes a first input terminal that is coupled to the
first input node, wherein the first processing block includes at
least one of: a first amplifier circuit and a first comparator
circuit, and wherein the first processing block is arranged to
provide a first output signal in response to the first input
signal; a second processing block that includes a second input
terminal that is coupled to the second input node, wherein the
second processing block includes at least one of: a second
amplifier circuit and a second comparator circuit, and wherein the
second processing block is arranged to provide a second output
signal in response to the second input signal, wherein the first
processing block and the second processing block have matched
operating functions; a first digitally programmed voltage reference
circuit that is arranged to provide a first reference signal to a
first reference node in response to a first digital control signal;
a second digitally programmed voltage reference circuit that is
arranged to provide a second reference signal to a second reference
node in response to a second digital control signal; a first high
impedance DC coupling circuit that is coupled between the first
reference node and the first input node, wherein the first high
impedance DC coupling circuit is arranged to couple the first
reference signal to the first input node such that the first input
signal at the input terminal of the first processing block has a
first offset level that is adjusted by the first reference signal;
and a second high impedance DC coupling circuit that is coupled
between the second reference node and the second input node,
wherein the second high impedance DC coupling circuit is arranged
to couple the second reference signal to the second input node such
that the second input signal at the second input terminal of the
second processing block has a second offset level that is adjusted
by the second reference signal, wherein the second offset level is
different from the first offset level.
15. The apparatus of claim 14, wherein the first high impedance DC
coupling circuit is arranged to shield the first digitally
programmed voltage reference circuit from the first input node, and
wherein the second high impedance DC coupling circuit is arranged
to shield the second digitally programmed voltage reference circuit
from the second input node.
16. The apparatus of claim 14, wherein the first high impedance DC
coupling circuit and the second high impedance DC coupling circuit
each have a nominal resistance of at least one hundred (100)
Mega-Ohms.
17. The apparatus of claim 17, wherein the first digitally
programmed voltage reference circuit and the second digitally
programmed voltage reference circuit are combined in a common
digitally programmed voltage reference circuit.
18. The apparatus of claim 17, wherein the common digitally
programmed voltage reference circuit includes a voltage divider
circuit that includes a plurality of tap-points, a first
multiplexer circuit that is arranged to select one of a first set
of the plurality of tap points as the first reference signal, and a
second multiplexer circuit that is arranged to select one of a
second set of the plurality of tap points as the second reference
signal.
19. The apparatus of claim 18, wherein a selected one of the tap
points from the first set of tap points is in common with another
selected one of the tap points from the second set of tap
points.
20. The apparatus of claim 14, further comprising a trim map
circuit that is arranged to select the first reference signal and
the second reference signal in response to the first digital
control signal and the second digital control signal, respectively
such that previously determined offset voltages associated with the
first input node and the second input node are selected via the
trim map circuit.
21. A method for level shifting an input signal for signal
processing, the method comprising: ac coupling the input signal to
a plurality of input nodes, wherein each of the input nodes is
associated with a respective processing circuit that includes
either an amplifier or a comparator, wherein the operating
functions of the processing circuits are matched to one another;
selecting a trim setting associated with each input node for each
respective processing circuit; initializing at least one digitally
programmed voltage reference to select a plurality of reference
signals in response to the selected trim setting; dc coupling the
selected reference signals to each of the input nodes via a
plurality of respective high impedance circuits such that the each
signal each respective input node is level shifted in response to
the respective reference signal; and operating each processing
circuit with the input signal, wherein the operation of the
processing circuit is enhanced by DC level shifting the input
signal at the input node via the plurality of reference
signals.
22. The method of claim 21, further comprising: retrieving the trim
setting from a trim map circuit in response to a digital control
signal such that previously determined offset levels may be
selected in response to the digital control signal, wherein the
trim map circuit comprises at least one of: a memory circuit, a
non-volatile memory (NVM) circuit, a fuse circuit, an anti-fuse
circuit, a ROM circuit, an EPROM circuit, an EEPROM circuit, a
programmable logic device (PLD) circuit, a latch circuit, a
register circuit, and a random access memory (RAM) circuit.
23. The method of claim 21, further comprising: retrieving the trim
setting from a signal that is provided from an external source.
24. A method for determining offset levels for level shifting an
input signal for signal processing, the method comprising: cycling
through selection of an input node for signal processing, wherein
the input node corresponds to one of a plurality of input nodes,
and for each selected input node: ac coupling the input signal to
the selected input node, wherein the input node is associated with
a high impedance input of a processing circuit from a plurality of
matched processing circuits that each include at least one of an
amplifier and a comparator; selecting a trim setting associated
with the selected input node for the processing circuit;
initializing a digitally programmed voltage reference to select a
reference signal in response to the selected trim setting, wherein
the selected reference signal is associated with a DC level for
offsetting the input signal at the high impedance input of the
processing circuit; dc coupling the selected reference signal to
the input node via a high impedance circuit such that the input
node is level shifted in response to the reference signal;
operating the processing circuit with the input signal, wherein the
operation of the processing circuit is responsive to the
combination of the DC level and the AC coupled input signal at the
high impedance input; evaluating an output associated with the
processing circuit for the selected trim setting; and cycling
though additional trim settings until the evaluated output
associated with the processing circuit attains a desired
result.
25. The method of claim 24, further comprising: storing trim
settings associated with the processing circuit in a trim map
circuit that comprises at least one of: a memory circuit, a
non-volatile memory (NVM) circuit, a fuse circuit, an anti-fuse
circuit, a ROM circuit, an EPROM circuit, an EEPROM circuit, a
programmable logic device (PLD) circuit, a latch circuit, a
register circuit, and a random access memory (RAM) circuit.
26. The method of claim 24, further comprising: storing trim
settings associated with the processing circuit in an external trim
data store.
27. The apparatus of claim 14, further comprising a decoder circuit
that is coupled to the outputs of the first processing block and
the second processing block, and further arranged to provide a
digital output signal for a flash conversion of the input signal.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to signal
processing. More particularly, the present invention is related to
a method and apparatus for adjusting a DC level associated with an
AC coupled input signal for signal processing.
BACKGROUND
[0002] Analog signal processing circuits are often desired in
electronic systems where transmission and reception of signals can
have very specific system requirements. Often times, signals are
encoded, encrypted, modulated, phase-shaped, or otherwise
manipulated for transmission in the system. The various signal
manipulations can be performed to improve signal reception as well
as to prevent unauthorized reception. The resulting signal may be
transmitted to a receiving circuit, where the receiving circuit
must understand how to extract the signal from the
transmission.
[0003] The various described signal manipulations in the electronic
system can be performed by analog function blocks or digital
function blocks. Example analog function blocks include buffers,
amplifiers, filters, level shifters, demodulators and integrators,
to name a few. Example digital signal processing blocks include
such functions as: decrypting, decoding, demodulating, filtering,
scaling, to name a few. Depending on the desired signal processing
functions, the selected analog function blocks can vary
significantly such that custom-built circuits are necessary. In
contrast, digital signal processing functions can be provided by a
general-purpose processor, such as a CPU or a DSP core, where the
processor is arranged to provide the various signal processing
functions using algorithmic processing that can be changed with
software. It is often preferable to translate the analog signals
into the digital domain such that digital signal processing can be
utilized.
[0004] An analog-to-digital converter (ADC) is a circuit that
performs the translation between analog and digital signals. An
example of a conventional ADC circuit (900) is illustrated in FIG.
9. ADC circuit 900 includes a resistor reference ladder and a
comparator array. The resistor reference ladder is arranged to
divide an analog reference voltage (V.sub.REF) into a number of
different reference voltages (V.sub.REF1-V.sub.REFN). Each of the
comparators (CP.sub.1-CP.sub.N) in the comparator array is arranged
to compare the analog input signal (V.sub.IN) to a respective one
of the reference voltages (V.sub.REF1-V.sub.REFN). Each of the
comparators (CP.sub.1-CP.sub.N) has a respective output
(OUT.sub.1-OUT.sub.N) which indicates whether or not the analog
input signal (V.sub.IN) is greater than its respective reference
voltage. The various comparator outputs (OUT.sub.1-OUT.sub.N)
provide a thermometer decode that indicates a value associated with
the magnitude of the analog input signal (V.sub.IN).
[0005] Another example of a conventional ADC circuit (1000) is
illustrated in FIG. 10. ADC circuit 1000 includes a level shifting
resistor ladder, a comparator array, a buffer, and a current
source. The buffer is arranged to provide a buffered version of the
analog input signal (V.sub.IN) as signal V.sub.IN1. The current
source is arranged to provide a current (I.sub.LS) coupled to the
output of the buffer through the level shifting resistor ladder
such that tap points in the level shifting resistor ladder each
correspond to a DC level shifted version of the buffered input
signal (V.sub.IN1). In other words, V.sub.IN2=V.sub.IN1+LS.sub.1,
V.sub.IN3=V.sub.IN2+LS.sub.2, and
V.sub.INN=V.sub.IN(N-1)+LS.sub.N-1. Each of the comparators
(CP.sub.1-CP.sub.N) is arranged to compare a respective one of the
analog input signals (V.sub.IN1-V.sub.INN) to the reference voltage
(V.sub.REF) to provide a respective one of the outputs
(OUT.sub.1-OUT.sub.N). The various comparator outputs
(OUT.sub.1-OUT.sub.N) again provide a thermometer decode that
indicates a value associated with the magnitude of the analog input
signal (V.sub.IN).
[0006] Yet another example of a conventional ADC circuit (1100) is
illustrated in FIG. 11. ADC circuit 1100 includes a resistor
reference ladder, an array of sampling capacitors
(CS.sub.1-CS.sub.N), a first array of switches (S.sub.11-S.sub.N1),
a second array of switches (S.sub.12-S.sub.N2), an array of a
comparator array (CP.sub.1-CP.sub.N), and an array of inverting
amplifiers (I.sub.1-I.sub.N) The resistor reference ladder is
substantially identical to that described previously with respect
to FIG. 9. The switches and capacitors are arranged to sample the
difference between a respective one of the reference voltages
(V.sub.REF1 through V.sub.REFN) and the analog input signal
(V.sub.IN), while the inverting amplifier circuit is arranged to
amplify the difference for input to a respective one of the
comparators (CP.sub.1-CP.sub.N). For example, capacitor CS.sub.1 is
arranged to sample the difference between V.sub.IN and V.sub.REF1,
while capacitor CS.sub.2 is arranged to sample the difference
between V.sub.IN and V.sub.REF2, etc. Each of the comparators is
arranged to compare the sampled difference voltage to a signal
ground to provide a respective output (OUT.sub.1-OUT.sub.N). The
various comparator outputs (OUT.sub.1-OUT.sub.N) again provide a
thermometer decode that indicates a value associated with the
magnitude of the analog input signal (V.sub.IN).
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Non-limiting and non-exhaustive embodiments are described
with reference to the following drawings:
[0008] FIG. 1 is a schematic diagram of an analog processing
circuit that employs capacitive level shifting in accordance with
the present disclosure.
[0009] FIG. 2 is a schematic diagram of another analog processing
circuit that employs capacitive level shifting in accordance with
the present disclosure.
[0010] FIG. 3 is a schematic diagram of a digitally programmable
voltage reference for an analog processing circuit that employs
capacitive level shifting in accordance with the present
disclosure.
[0011] FIG. 4 is a schematic diagram of another digitally
programmable voltage reference for an analog processing circuit
that employs capacitive level shifting in accordance with the
present disclosure.
[0012] FIG. 5 is a schematic diagram of still another digitally
programmable voltage reference for an analog processing circuit
that employs capacitive level shifting in accordance with the
present disclosure.
[0013] FIG. 6 is a schematic diagram of yet another analog
processing circuit that employs capacitive level shifting in
accordance with the present disclosure.
[0014] FIG. 7 is a schematic diagram of yet still another analog
processing circuit that employs capacitive level shifting in
accordance with the present disclosure.
[0015] FIG. 8 is a process flow diagram for an analog processing
circuit that employs capacitive level shifting in accordance with
the present disclosure.
[0016] FIG. 9 is a schematic diagram of a conventional
analog-to-digital converter circuit.
[0017] FIG. 10 is a schematic diagram of another conventional
analog-to-digital converter circuit.
[0018] FIG. 11 is a schematic diagram of still another conventional
analog-to-digital converter circuit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0019] Various embodiments of the present invention will be
described in detail with reference to the drawings, where like
reference numerals represent like parts and assemblies throughout
the several views. Reference to various embodiments does not limit
the scope of the invention, which is limited only by the scope of
the claims attached hereto. Additionally, any examples set forth in
this specification are not intended to be limiting and merely set
forth some of the many possible embodiments for the claimed
invention.
[0020] Throughout the specification and claims, the following terms
take at least the meanings explicitly associated herein, unless the
context clearly dictates otherwise. The meanings identified below
are not intended to limit the terms, but merely provide
illustrative examples for the terms. The meaning of "a," "an," and
"the" includes plural reference, the meaning of "in" includes "in"
and "on." The term "connected" means a direct electrical connection
between the items connected, without any intermediate devices. The
term "coupled" means either a direct electrical connection between
the items connected or an indirect connection through one or more
passive or active intermediary devices. The term "circuit" means
either a single component or a multiplicity of components, either
active and/or passive, that are coupled together to provide a
desired function. The term "signal" means at least one current,
voltage, charge, temperature, data, or other signal.
[0021] Briefly stated, an analog processing block is arranged to
receive an input signal through an AC coupling circuit. A digitally
programmable voltage reference (DPVR) circuit is arranged to
provide selection of a voltage reference that is DC coupled through
a high impedance circuit to the AC coupling circuit. The input to
the analog processing block includes the AC coupled input signal
and the DC level from the selected voltage reference such that the
DC level is effectively shifted for the analog processing block.
The analog processing block may include any number of analog
functions including: buffering, level shifting, scaling,
integrating, and analog-to-digital conversion for digital signal
processing, to name a few. A digital control logic circuit and a
trim map can be arranged to control adjustments to the DPVR such
that the effects of any non-ideal conditions on the analog
processing block are minimized. The trim map may include
non-volatile memory devices.
[0022] The present disclosure applies generally to analog
processing circuits, and has particular application in systems that
have non-ideal effects from a variety of sources such as
semiconductor processing based variations. Such processing
variations may be manifested in voltage and/or current based
offsets in circuits such as amplifier blocks and comparator blocks
to name a few. It is understood and appreciated that offsets are
merely one example of a non-ideal effect and others are also
contemplated. The present disclosure contemplates all varieties of
non-ideal effects by evaluating their net result at the output of
the various signal-processing circuits.
[0023] After production of the circuits for the present disclosure,
a test signal can be applied to the circuit such that the output of
the signal processing circuits can be evaluated for each of the
available DC level adjustments from the DPVR. By adjusting the DC
level at the input side of such signal processing circuits and
evaluating the effects on the outputs of those circuits, a
preferred DC level can be identified. The various settings for the
DPVR circuit can be stored in a trim map for later retrieval during
operation in the field. The trim map can be implemented on-chip or
off-chip, and may include non-volatile memory (NVM) structures,
fusing circuits, anti-fusing circuits, read-only memory (ROM)
circuits, programmable read-only memory (PROM) circuits, erasable
programmable read-only memory (EPROM) circuits, electrically
erasable programmable read-only memory (EEPROM) circuits, or any
other appropriate means as will be understood by the present
disclosure.
[0024] The present disclosure has evaluated many conventional
analog-to-digital converter (ADC) circuits, and other signal
processing circuits, to identify problems that have resulted in the
present disclosure. A limited number of such examples are
illustrated by the ADC circuits of FIGS. 9-11.
[0025] FIG. 9 illustrates a flash-type converter as previously
described. The input signal (V.sub.IN) is compared against a number
(N) of reference voltage (V.sub.REF1-V.sub.REFN) that is generated
using the resistor reference ladder. The comparators
(CP.sub.1-CP.sub.N) convert the input signal (V.sub.IN) to a
digital quantity that results from the collection of outputs
(OUT.sub.1-OUT.sub.N) from the comparators (CP.sub.1-CP.sub.N).
Ideally, the reference voltages are exact at the input of the
comparators, the comparators are very fast, and there is no input
referred offset on the comparator inputs.
[0026] A practical implementation of circuit 900 will always have
some finite error in each reference voltages due to mismatches in
various devices (e.g., mismatches between resistors in the resistor
reference ladder). Also, each comparator will have an input
referred offset voltage that may vary from comparator to
comparator, and also may vary based on the common-mode voltage for
each comparator. Conventional efforts to reduce the input referred
offset often use large area transistors on the input differential
pair, which requires larger chip areas and results in increased
parasitic capacitance on the inputs to the comparator. Negative
effects of the increased parasitic capacitance include: slower
performance due to input slewing, and noise from the comparators
(e.g., the high gain of the comparator results in significant
voltage swings that look like switching noise that can "kickback")
that can couple back to the resistor reference ladder. The values
of the resistors in the resistor reference ladder can be reduced to
minimize the effect of noise from the comparators at the cost of
increased minimum operating current in the resistors. The
comparators of circuit 900 must be able to accommodate a variety of
common-mode voltages to work over the full range of VREF.sub.1
through VREF.sub.N. Thus the comparators must either be matched to
one another using a robust design that can accommodate the full
common-mode range, or different comparator circuits must be used
for each of the different DC operating voltage ranges.
[0027] FIG. 10 illustrates another flash-type converter as
previously described. The input signal (V.sub.IN) is converted into
a number (N) of input voltage (V.sub.IN1-V.sub.INN) that is
generated using the level shifting resistor ladder. The comparators
(CP.sub.1-CP.sub.N) compare the level shifted inputs to a reference
voltage (V.sub.REF) to convert the input signal (V.sub.IN) to a
digital quantity that results from the collection of outputs
(OUT.sub.1-OUT.sub.N) from the comparators (CP.sub.1-CP.sub.N).
Ideally, the level shifted input voltages (VIN1-VINN) are precisely
level shifted at the input of the comparators, the comparators are
very fast, and there is no input referred offset on the comparator
inputs.
[0028] Similar to the discussion above for FIG. 9, a practical
implementation of circuit 1000 will always have some finite error
in the level shifting due to mismatches in various devices. For
example, mismatches between resistors in the resistor ladder result
in an error in the level shifting between each resistor. Any
variations in the magnitude of the current from the current source
will also result in an error in the level shifting. Any offset
errors from the buffer will also result in an error in the level
shifting. Similar to that described previously, the values of the
resistors may need to be reduced to improve the speed or bandwidth
of the circuit and minimize kickback. One disadvantage is the
addition of the buffer that drives the bottom of the resistor
ladder, where the buffer consumes additional power, and must be
robustly designed to accommodate the input range for V.sub.IN.
[0029] FIG. 11 illustrates another flash-type converter as
previously described. The reference voltages (V.sub.REF1 through
V.sub.REFN) from the resistor ladder are sampled by the capacitors
(CS.sub.11 through CS.sub.1N) during one time interval (e.g.,
switches S.sub.11 through S.sub.1N in one position), while the
input voltage (V.sub.IN) is sampled at another time interval (e.g.,
switches S.sub.11 through S.sub.1N in another position). Thus, the
capacitors (CS.sub.11 through CS.sub.1N) effectively sample the
difference between the input signal (V.sub.IN) and the reference
voltages (V.sub.REF1 through V.sub.REFN). Switches
S.sub.12-S.sub.N2 are arranged to zero the inverting amplifiers
(I.sub.1 through I.sub.N) when the reference voltages are sampled
such that the circuit is auto-zeroing.
[0030] Similar to the discussion above for FIGS. 9 and 10, a
practical implementation of circuit 1100 will always have some
finite error due to mismatches in various devices. For example,
mismatches between resistors in the resistor ladder result in an
error in the reference voltages. Although offset errors from the
inverting amplifiers are not an issue since this offset is
effectively zeroed, mismatches in the gain of the various
amplifiers may be an issue. The kickback issues previously
described are a very big issue for circuit 1100, since switches are
necessary to perform the sampling and auto-zeroing functions.
Moreover, the switches add a further parasitic load on the resistor
ladder circuit, requiring additional setting time for stable
sampling operation. The overall effect is that the noise coupled
back to the reference ladder is significant and lower valued
resistors need to be used for high speed operation.
[0031] The above described limitations, problems, as well as
others, have been evaluated and considered as part of the present
disclosure. While the circuits that are described herein are shown
as single-ended implementations, it is understood that
fully-differential implementations are also considered within the
scope of this disclosure. For fully-differential implementations,
buffers, amplifiers, comparators and/or any other pertinent circuit
blocks will include both differential inputs and differential (or
complementary) outputs. Moreover, it is also understood that each
signal node in a fully-differential circuit includes a
complementary signal node that is 180 degrees out of phase with
respect to the other signal node. The described figures illustrate
single-ended circuits merely as a convenience, while expecting both
single-ended and fully-differential circuit implementations to be
within the scope of this disclosure.
[0032] FIG. 1 is a schematic diagram of an analog processing
circuit (100) that employs capacitive level shifting in accordance
with the present disclosure. The analog processing circuit (100)
includes: an optional buffer circuit, an ac coupling circuit, an
analog processing block, an analog signal processing circuit, a
digital signal processing circuit, a digitally programmed voltage
reference (DPVR), a trim map circuit, a mode control circuit, and a
high impedance DC coupling circuit (Z).
[0033] There are a number of ways of implementing the analog
processing circuit (100). In some embodiments of circuit 100, the
analog processing block includes a gain block and a comparator.
[0034] The buffer circuit is arranged to receive an input signal
(INPUT1) and provide a buffered input signal (INPUT2) to an input
side of the AC coupling circuit. The AC coupling circuit provides
an AC coupled input signal (INAC) in response to signal INPUT2. The
gain block is arranged to receive the ac coupled input signal
(INAC) and provide an amplified signal (AOUT) that is gain scaled
relative to the ac coupled input signal (INAC). The comparator
circuit is arranged to compare the amplified signal (AOUT) to a
reference level (VX) and provide a comparator output signal to
terminal (CPOUT) as signal DIN. The analog signal processing
circuit is arranged to provide signal processing functions to any
number of a variety of input signals, including but not limited to
signals: INAC, AOUT, and DIN. The digital signal processing circuit
is arranged to provide signal processing functions in response to
any number of digital input signals, including but not limited to
signal DIN.
[0035] The DPVR circuit is arranged to provide a DC voltage level
(VREF) to the output side of the AC coupling circuit, where the DC
voltage level (VREF) is adjusted in response to a programming
signal (TRIM SETTING) when enabled by an enable signal (EN). The
trim setting signal is provided by the trim map circuit in response
to a control signal (T_CTL) and an indexing signal that corresponds
to data associated with trimming (TRIM_DATA) for the system. The
control signal (T_CTL), the indexing signal (TRIM_DATA), and the
enable signal (EN) are provided by the digital control logic, which
is response to a program control signal (PGM_CTL) and a data signal
(ex_DATA). The program control signal (PGM_CTL) may be provided by
a mode control circuit that only allows programming during a
predetermined mode. Contemplated modes will be described later. The
above described system is merely one example system that is
contemplated, and many others are also considered within the scope
of the present disclosure. The presently disclosed invention
contemplates that the DC level associated with the output side of
the AC coupling circuit is adjusted by the DPVR circuit through the
high impedance DC coupling circuit such that the AC coupled signal
can be further processed by some other analog processing mechanisms
such as, but not limited to: the analog signal processing circuit,
the comparator, the digital signal processor, an others. The DPVR
circuit is arranged to adjust the DC level for the AC coupled
signal such that the overall effect on the analog processing
mechanisms are adjusted. For example, the gain amplifier and the
comparator circuit may each have offset voltages that effect the
processing of the analog input signal and can potentially yield an
error in the decision of the comparator relative to signal VX. The
DPVR is used to adjust the DC signal level such that these and
other offsets, and/or other non-ideal effects, are compensated such
that the comparator can make an appropriate decision. Other
examples are also contemplated; where the offsets and other
non-ideal errors are adjusted out of the system using the DC offset
adjust from the DPVR through the high impedance DC coupling
circuit.
[0036] The buffer circuit is illustrated as an operational
amplifier circuit (AMP1) that is arranged in a unity gain
configuration as an inverting amplifier. In another example, a
non-inverting buffer can be used. In still another example, the
buffer circuit is eliminated. Many other input signal conditioners
are also contemplated, including but not limited to any combination
of, buffer circuits, voltage dividers, gain scalers, and level
shifters, to name a few.
[0037] The AC coupling circuit is illustrated as a capacitor
circuit (C), that may be implemented as any reasonable means
including but not limited to: a single capacitor, a plurality of
capacitors, one or more transistors that are configured to operate
as a capacitor circuit, poly-silicon based capacitors, metal plate
based capacitors, and others.
[0038] In some embodiments, the high impedance DC coupling circuit
(Z) and the capacitance of the AC coupling circuit form a high pass
filter. It is advantageous to choose values for the impedance of
the high impedance DC coupling circuit (Z) and the capacitance of
the AC coupling circuit so that the high pass filter allows the
lowest frequency of interest to pass through the capacitor. This
lowest frequency typically has values between 1 kHZ and 1 MHz,
depending on the application.
[0039] In some embodiments, the impedance of the high impedance DC
coupling circuit (Z) can be chosen to be as high as possible such
that the capacitance of the AC coupling circuit can be as low as
possible, and area utilized by the capacitor in the AC coupling
circuit can be conserved. Advantageously, it is possible if the
high impedance DC coupling circuit (Z) is implemented from
materials that result in high resistance, such as an undoped
polysilicon resistor.
[0040] The gain amplifier circuit is illustrated as a non-inverting
gain amplifier that includes an operational amplifier circuit
(AMP2) and two resistor circuits (RF and RA). Resistor circuits RF
and RA are series coupled between the output of AMP2 and a circuit
ground (e.g., GND), with a common-node coupled to the inverting
input terminal of AMP2. The non-inverting input terminal of AMP2 is
arranged to receive an input signal (e.g., INAC). Although the
example gain amplifier circuit is arranged as a non-inverting
amplifier, inverting gain amplifiers are also contemplated. In some
examples, the gain amplifier may be unnecessary and thus
eliminated.
[0041] FIG. 2 is a schematic diagram of another analog processing
circuit (200) that employs capacitive level shifting in accordance
with the present disclosure. Analog processing circuit 200 includes
capacitor circuits (C1-CN), differential circuits (X21-X2N), high
impedance DC coupling circuits (Z1-ZN), a digitally programmed
voltage reference circuit (X1), an optional analog signal
processor, an optional decoder logic circuit, and an optional
digital signal processor circuit.
[0042] Each of the capacitor circuits (C1-CN) is arranged to AC
couple an input signal (INPUT) from node N1 to a respective
internal node (NC1-NCN) as one of signals INAC1-INACN. The
digitally programmed voltage reference circuit (X1) is arranged to
provide a number (N) of reference voltage signals (VREF1-VREFN) at
a corresponding one of nodes NZ1-NZN. Each of the high impedance DC
coupling circuits (Z1-ZN) is arranged to DC couple a respective one
of the reference voltage signals (VREF1-VREFN) from a respective
one of nodes NZ1-NZN to a corresponding one of nodes NC1-NCN. Each
of the differential circuits (X21-X2N) is arranged to compare a
respective one of the input signals (INAC1-INACN) to a fixed
reference potential (e.g., GND) to provide a corresponding one of
the output signals (OUT1-OUTN). An analog signal processor can be
arranged to evaluate a number of signals, including but not limited
to INAC1-INACN and OUT1-OUTN. A decoder logic circuit can be
arranged to evaluate the output signals (OUT1-OUTN) to provide a
decoded logic signal (DIN) to the digital signal processor. The
digital signal processor can be arranged to communicate with the
analog signal processor as may be desired in an implementation.
[0043] In one example the differential circuits are comparators,
while in another example the differential circuits are amplifiers.
For example, circuit 200 can be arranged for use as a flash
comparator circuit, where differential circuits X21-X2N are
comparator circuits, the decoder logic is a thermometer decoder,
and each of the comparator outputs (OUT1-OUTN) corresponds to the
results of the comparison of the AC coupled input signals
(INAC1-INACN) to the fixed reference potential (e.g., GND). For
this example, the AC coupled input signals (INAC1-INACN) each have
a DC level that corresponds to a respective one of the reference
voltages (VREF1-VREFN). The DC levels are digitally adjusted (or
digitally trimmed) so that the digital signal processor identifies
the proper decode for a given input signal (INPUT). Similar
arrangements can be made for analog signal processing for the case
when differential circuits X21-X2N are amplifier circuits.
[0044] The digitally programmed voltage reference circuit is
responsive to a programming signal (TRIM SETTING) when enabled by
an enable signal (EN), similar to that previously described. In one
example, an array of DPVR circuits (X11-X1N) are used to generate
the reference voltages (VREF1-VREFN), where each of the DPVR
circuits is responsive to a respective one of enable control
signals (PG1-PGN) and a corresponding one of the program setting
signals (TRIM1-TRIMN).
[0045] FIG. 3 is a schematic diagram of a digitally programmable
voltage reference (300) for an analog processing circuit that
employs capacitive level shifting in accordance with the present
disclosure. The digitally programmable voltage reference (DPVR)
includes a resistor array, a multiplexer (MUX), and an analog
reference circuit.
[0046] DPVR 300 is arranged to respond to a programming signal
(TRIM_SETTING), from a digital control logic circuit. The digital
control logic is arranged to provide the programming signal
(TRIM_SETTING) in response to a programming control signal
(PGM_CTL) and the evaluation of a data setting that can either be
externally provided as ex_DATA, or previously stored as TRIM_DATA.
In one example implementation, the digital control logic can store
ex_DATA as TRIM_DATA in a TRIM DATASTORE circuit such as a
non-volatile memory (NVM) circuit, a fuse circuit, an anti-fuse
circuit, an EPROM circuit, an EEPROM circuit, a programmable logic
device (PLD) circuit, a latch circuit, a register circuit, a random
access memory (RAM) circuit, as well as others.
[0047] The analog reference circuit can be arranged to provide a
reference voltage such as signal AREF or a reference current such
as signal IREF. The resistor array includes a number of series
coupled resistors (RD1-RDN), which are arranged in a voltage
divider configuration to provide a plurality of reference voltages
(e.g., VREF1-VREFN) at tap points in the voltage divider. The
multiplexer (MUX) includes an array of switching circuits; where
each switching circuit is arranged to selectively couple one of the
reference voltages (VREF1-VREFN) to an output of the multiplexer as
reference voltage VREF. Reference voltage VREF can then be coupled
through a high-impedance DC coupling circuit (Z) as previously
described. The multiplexer includes a decoder circuit (MUX DECODE)
that is arranged to provide control signals to each of the
switching circuits for selective activation in response to the
programming signal (TRIM SETTING). The analog reference circuit can
be a bandgap voltage reference, where the resistors in the bandgap
circuit (e.g., RA1 and RA2) are manufactured from the same material
as resistors RD1-RDN such that temperature related effects on the
reference voltage (VREF) are minimized.
[0048] FIG. 4 is a schematic diagram of another digitally
programmable voltage reference (400) for an analog processing
circuit that employs capacitive level shifting in accordance with
the present disclosure. Digitally programmable voltage reference
400 includes a voltage reference circuit (VREF), an array of
resistors (R1-RN), a multiplexer (MUX), and a current trim
circuit.
[0049] Resistors R1-RN are arranged as a voltage divider circuit
similar to that described previously. A voltage reference (AREF),
or optionally a current reference (IREF), is coupled to the
resistor array such that each tap point (P0-PN) of the resistor
array (R1-RN) corresponds to a different voltage value that is
coupled to an input of the multiplexer (MUX). The output of the
multiplexer (MUX) corresponds to a selected reference voltage
(VREF) that is coupled through a high impedance DC coupling circuit
(Z) to another point such as the ac coupling circuit previously
discussed. The multiplexer (MUX) is responsive to a selection
control signal (DVSELECT) from the digital control logic.
[0050] The current trim circuit is arranged to provide one of
current I0-IN to tap points P0-PN such that the voltage reference
circuit can be digitally trimmed by the current trim circuit. The
current trim circuit is responsive to the programming signals
(TRIM_SETTING) from the trim map circuit. The digital control logic
circuit is arranged to control the selection from the trim map
circuit and the current trim circuit via signal DICTL and DTRIM,
respectively. The digital control logic and the trim map circuit
are operated in a similar fashion as that discussed previously.
[0051] FIG. 5 is a schematic diagram of still another digitally
programmable voltage reference (500) for an analog processing
circuit that employs capacitive level shifting in accordance with
the present disclosure. DPVR 500 includes an array of resistors,
switches S1-SX, a digital control logic circuit, a trim map
circuit, and a differential amplifier circuit.
[0052] The array of resistors are arranged as an R-2R resistor
ladder that has an input voltage corresponding to AREF, yielding
tap point voltages of AREF/2, AREF/4, . . . , AREF/2.sup.N. Each of
the switches (S1-SX) is arranged to selectively couple a respective
leg of the R-2R ladder to either a circuit ground (e.g., GND) or to
an inverting input of the differential amplifier circuit in
response to a respective one of the control signals (CTL1-CTLX). A
unit resistor (R) is couopled in the feedback path between the
output of the differential amplifier and the inverting input of the
differential amplifier circuit such the resulting reference voltage
(VREF) is provided at the output of the amplifier as a binary
programmed value such as:
VREF=AREF*(b.sub.1/2.sup.N+b.sub.2/2.sup.(N-1)+ . . .
b.sub.x-1/2+b.sub.x), where b.sub.1-b.sub.x correspond to binary
values associated with control signals CTL1-CTLX. The digital
control logic is arranged to provide the control signals similar to
that previously described, such that the DPVR provides a VREF that
is determined by the trim map circuit's stored values.
[0053] FIG. 6 is a schematic diagram of yet another analog
processing circuit (600) that employs capacitive level shifting in
accordance with the present disclosure. Analog processing circuit
600 is substantially similar to analog processing circuit 200 from
FIG. 2. The operation of the remaining circuit components are
substantially the same in operation, with like components being
similarly labeled.
[0054] The DPVR circuit from FIG. 2 is replaced with a resistor
array and an array of multiplexer circuits (MUX1-MUXN). The first
multiplexer circuit (MUX1) is arranged to provide a first reference
voltage (VREF1) to a first high-impedance coupling circuit (R1).
The second multiplexer circuit (MUX2) is arranged to provide a
second reference voltage (VREF2) to a second high-impedance
coupling circuit (R2). The last multiplexer circuit in the array
(MUXN) is arranged to provide a last reference voltage (VREFN) to
the last high-impedance coupling circuit (RN). Each of the
multiplexers is arranged to couple one of the tap points from the
resistors array to the respective high impedance coupling circuit.
In some instances, the multiplexers can have overlapping voltage
ranges as depicted in FIG. 6.
[0055] FIG. 7 is a schematic diagram of yet still another analog
processing circuit (700) that employs capacitive level shifting in
accordance with the present disclosure. Analog processing circuit
700 includes a high-impedance DC coupling circuit (Z), an ac
coupling circuit (C), an analog processing block, and a DPVR
circuit, and is operated in a similar manner as that described
previously with respect to FIG. 3. The DPVR circuit includes an
analog reference circuit, a resistor array (R1-RN) and a
multiplexer (MUX). The DPVR circuit is programmable via a digital
control logic circuit and a trim map circuit.
[0056] The trim map circuit may include a number of NVM circuits.
Each of the NVM circuit in the trim map are arranged to control one
bit (e.g., b.sub.0-b.sub.N+1) for the multiplexer control signal
(MUX_CTL). The digital control logic can be arranged to program the
NVM circuits using control pulses (e.g., PULSE1, PULSE2, etc.),
where the pulses can either inject charge, or remove charge from
the NVM circuit to effect programming. Once programmed, the NVM
circuits can be accessed to retrieve the program setting to select
the appropriate multiplexer settings.
[0057] It will be appreciated that a number of advantages result
from the disclosure, over the prior art. For one, the above
described errors due to device mismatches are reduced by the
correction voltage provided by the trim map and the DPVR. Not only
are the errors reduced, but large area devices are not needed to
correct them, either. So, area is also saved.
[0058] In addition, kick back is substantially eliminated because
the high Z coupling circuit isolates the DPVR from signal at the
coupled node.
[0059] Moreover, the individual analog processing blocks can all
work at the same DC voltage level. This avoids design effort to
accommodate large common mode range, or designing multiple blocks
to handle different supply ranges. In addition, no switches are
needed, which would add to kick back and add parasitic load.
[0060] Methods of the invention are now described according to
embodiments. The methods can be implemented in a number of ways,
such as by the above-described circuits.
[0061] FIG. 8 is a process flow diagram (800) for an analog
processing circuit that employs capacitive level shifting in
accordance with the present disclosure. The level analog processing
circuit may typically be arranged to operate with a number of
operating modes. Example operating modes includes programming mode
(PGM), calibration mode (CAL). However, additional operating modes
may also be used such as store program mode, as well as others.
[0062] The circuit initially identifies the selected operating mode
at block 810. Processing flows from block 810 to decision block
820. Processing continues to block 821 when the programming mode
(PGM) is identified. At block 821 the trim settings are retrieved
such as from a trim map circuit as previously described.
Alternatively, the trim setting can be retrieved from an external
source. Continuing to block 822, the retrieved trim setting is used
to initialize the selection of an appropriate voltage reference
(VREF) such as from the DPVR circuit previously described. The
voltage reference (VREF) is then coupled through a high impedance
DC coupling circuit to the analog processing block (e.g., see FIG.
7). Continuing to block 823, the analog processing block can now be
operated properly.
[0063] Processing continues from decision block 820 to decision
block 830 when the programming mode (PGM) is not identified.
Decision block 830 determines whether the calibration mode (CAL)
has been selected. Processing continues to block 831 when the
calibration mode is selected. At block 831, the reference voltage
(VREF) from the DPVR circuit is selected. At block 832, a test
signal is coupled to the input node as an AC signal (e.g., INAC).
Continuing to block 833, the analog processing block processes the
signal INAC. Flowing to block 834, the output of the analog
processing block is evaluated. Decision block 835 determines if the
desired result of the evaluation has been achieved. When additional
processing is desired, processing continues from decision block 835
to block 836, where the trim settings are adjusted and processing
continues back at block 831.
[0064] When additional processing is not desired, processing
continues from decision block 835 to block 840. At block 840, the
trim settings that have been identified as satisfactory are stored
for later use (e.g., stored in a trim map, placed in a data file
for later use, Non Volatile Memory (NVM), Random Access Memory
(RAM), etc.). Processing flows from block 840 to block 890, which
is a termination block.
[0065] When the calibration mode is not detected at decision block
830, processing continues to block 850 where other modes can be
processed as may be desired. Examples of other operating mode
includes a program store mode, where externally provided trim
settings can be stored in a trim map circuit, or perhaps a load
setting mode, where externally provided trim settings are loaded
into a temporary memory (e.g., a latch circuit, a register, etc.)
for adjusting the operational settings as may be desired.
* * * * *