U.S. patent application number 15/087711 was filed with the patent office on 2017-10-05 for digitally-controlled transimpedance amplifier (tia) circuit and methods.
The applicant listed for this patent is Avago Technologies General IP (Singapore) Pte. Ltd.. Invention is credited to Georgios Asmanis, Faouzi Chaahoub.
Application Number | 20170288618 15/087711 |
Document ID | / |
Family ID | 59929241 |
Filed Date | 2017-10-05 |
United States Patent
Application |
20170288618 |
Kind Code |
A1 |
Asmanis; Georgios ; et
al. |
October 5, 2017 |
DIGITALLY-CONTROLLED TRANSIMPEDANCE AMPLIFIER (TIA) CIRCUIT AND
METHODS
Abstract
A digitally-controlled transimpedance amplifier (TIA) circuit is
provided in which a plurality of feedback loops are digitally
controlled, including, but not limited to, the DC offset
cancellation loop, the variable gain control loop, and the TIA
feedback impedance adjustment loop. The digitally-controlled TIA
circuit includes digital loop-control circuitry that consumes less
area on the TIA IC chip than the analog circuitry traditionally
used to perform the feedback loop control in the analog domain. In
addition, because digital logic continues to shrink as IC processes
continue to evolve, the size of the IC chip packages will further
decrease over time, leading to a smaller footprint in systems in
which they are employed. The digital loop control circuitry is also
capable of independently varying the gains of multiple gain stages
of the variable gain control circuit to provide better control over
the gain stages and better overall performance of the TIA
circuit.
Inventors: |
Asmanis; Georgios; (Lake
Forrest, CA) ; Chaahoub; Faouzi; (San Jose,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Avago Technologies General IP (Singapore) Pte. Ltd. |
Singapore |
|
SG |
|
|
Family ID: |
59929241 |
Appl. No.: |
15/087711 |
Filed: |
March 31, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03G 3/3084 20130101;
H03F 2200/405 20130101; H03F 3/08 20130101; H03G 3/001 20130101;
H03F 2200/375 20130101; H04B 10/07955 20130101; H04B 10/6933
20130101; H03F 1/34 20130101; H04B 10/6931 20130101 |
International
Class: |
H03F 3/08 20060101
H03F003/08; H03G 3/00 20060101 H03G003/00; H03F 1/34 20060101
H03F001/34 |
Claims
1. A digitally-controlled transimpedance amplifier (TIA) circuit
comprising: a first TIA having a first input terminal and a first
output terminal; a first variable impedance having first and second
terminals electrically coupled to the first input and first output
terminals, respectively; at least a first variable gain amplifier
(VGA) circuit having a second input terminal and a second output
terminal, the second input terminal being electrically coupled to
the first output terminal; an output driver circuit having a third
input terminal and a third output terminal, the third input
terminal being electrically coupled to the second output terminal;
and digital loop-control circuitry comprising analog-to-digital
conversion (ADC) circuitry, digital controller circuitry and
digital-to-analog conversion (DAC) circuitry, the ADC circuitry
being electrically coupled to the first, second and third output
terminals, the ADC circuitry and converting first, second and third
output signals outputted from the TIA, the VGA circuit and the
output driver circuit into at least first, second and third digital
signals, the digital controller circuitry processing the first,
second and third digital signals and producing P digital control
signals, where P is a positive integer that is equal to N+2, where
N is a positive integer, the DAC circuitry being electrically
coupled to the digital controller circuitry, the DAC converting the
P digital control signals into P analog control signals,
respectively, and wherein P-2 of the analog control signals are fed
back to the VGA circuit and used to vary a gain of at least one
gain stage of the VGA circuit, and wherein one of the P analog
control signals is fed back to direct current (DC) offset
cancellation circuitry for use as a DC offset cancellation value by
the DC offset cancellation circuitry.
2. The digitally-controlled TIA circuit of claim 1, wherein the ADC
circuitry also receives a received signal strength indicator (RSSI)
signal output from an RSSI circuit that measures a signal strength
of an electrical analog signal produced by a photodetector that is
electrically coupled to the first input terminal of the first TIA,
and wherein the ADC converts the RSSI signal into a digital RSSI
signal, and wherein the digital controller processes the RSSI
signal to produce one of the P-2 analog control signals that is fed
back to the TIA and used to vary an impedance of the first variable
impedance.
3. The digitally-controlled TIA circuit of claim 2, wherein one or
more of the P-2 analog control signals is fed back to the VGA
circuit to cause the gain of one or more gain stages of the VGA to
be adjusted to perform offset cancellation to prevent the third
output signal from saturating.
4. The digitally-controlled TIA circuit of claim 1, wherein the
digital controller performs a loop-control algorithm that processes
the first, second and third digital signals to algorithmically
convert the first, second and third digital signals into the P
digital control signals.
5. The digitally-controlled TIA circuit of claim 2, wherein the
digital loop-control circuitry includes at least a first lookup
table (LUT), and wherein the digital controller produces the P
digital control signals by using the first, second and third
digital signals to retrieve the P digital control signals from said
at least a first LUT.
6. The digitally-controlled TIA circuit of claim 5, further
comprising: P sample-and-hold (S/H) circuits electrically coupled
to the DAC circuitry, wherein the analog control, signals are
maintained by the respective S/H circuits for respective
predetermined time periods and in a predetermined sequence such
that the analog control signals are fed back to the VGA circuit, to
the first variable impedance and to the DC offset cancellation
circuitry for respective predetermined periods of time in a
predetermined sequence.
7. The digitally-controlled TIA circuit of claim 6, wherein the
digital controller comprises a state machine having at least a VGA
sub-state machine, a DC offset cancellation sub-state state, an
RSSI sub-state machine, and wherein the sub-state machines operate
in a round-robin sequence such that the respective analog control
signals are sampled and held by the respective S/H circuits in a
round-robin sequence for respective predetermined periods of
time.
8. A digitally-controlled transimpedance amplifier (TIA) circuit
comprising: a first TIA having a first input terminal and a first
output terminal; a first variable impedance having first and second
terminals electrically coupled to the first input and first output
terminals, respectively; at least a first variable gain amplifier
(VGA) circuit having a second input terminal and a second output
terminal, the second input terminal being electrically coupled to
the first output terminal, the VGA circuit having N variable gain
stages where N is a positive integer; an output driver circuit
having a third input terminal and a third output terminal, the
third input terminal being electrically coupled to the second
output terminal; and digital loop-control circuitry comprising
analog-to-digital conversion (ADC) circuitry, digital controller
circuitry and digital-to-analog conversion (DAC) circuitry, the ADC
circuitry being electrically coupled to the first, second and third
output terminals and to an output terminal of a received signal
strength indicator (RSSI) circuit that measures a signal strength
of an electrical analog signal produced by a photodetector that is
electrically coupled to the first input terminal of the first TIA
and outputs an RSSI output signal from an RSSI output terminal of
the RSSI circuit, the ADC circuitry converting first, second, third
output signals and the RSSI output signals out from said first,
second, third output terminals and from the RSSI output terminal
into at least first, second, third and fourth digital signals, the
digital controller circuitry processing the first, second, third
and fourth digital signals and producing P digital control signals,
where P is equal to at least N+2, the DAC converting the P digital
control signals into P analog control signals, respectively, and
wherein P-2 of the analog control signals are fed back to the VGA
circuit and used to vary gains of one or more of the N variable
gain stages, respectively, and wherein one of the P analog control
signals is fed back to direct current (DC) offset cancellation
circuitry for use as a DC offset cancellation value by the DC
offset cancellation circuitry, and wherein one of the P-2 analog
control signals is fed back to the TIA and used to vary an
impedance of the first variable impedance.
9. The digitally-controlled TIA circuit of claim 8, wherein P is
equal to at least N+3, and wherein one or more of the analog
control signals is fed back to the VGA circuit to cause the gain of
one or more variable gain stages of the VGA to be adjusted to
perform offset cancellation to prevent the third output signal from
saturating.
10. The digitally-controlled TIA circuit of claim 9, wherein the
digital controller performs a loop-control algorithm that processes
the first, second, third and fourth digital signals to
algorithmically convert the first, second, third and fourth digital
signals into the P digital control signals.
11. The digitally-controlled TIA circuit of claim 8, wherein the
digital loop-control circuitry includes at least a first lookup
table (LUT), and wherein the digital controller produces the P
digital control signals by using the first, second, third and
fourth digital signals to retrieve the P digital control signals
from said at least a first LUT.
12. The digitally-controlled TIA circuit of claim 11, further
comprising: P sample-and-hold (S/H) circuits electrically coupled
to the DAC circuitry, wherein the analog control signals are
maintained by the respective S/H circuits for respective
predetermined time periods and in a predetermined sequence such
that the analog control signals are fed back to the VGA circuit, to
the first variable impedance and to the DC offset cancellation
circuitry for respective predetermined periods of time in a
predetermined sequence.
13. The digitally-controlled TIA circuit of claim 12, wherein the
digital controller comprises a state machine having at least a VGA
sub-state machine, a DC offset cancellation sub-state state, an
RSSI sub-state machine, and wherein the sub-state machines operate
in a round-robin sequence such that the respective analog control
signals are sampled and held by the respective S/H circuits in a
round-robin sequence for respective predetermined periods of
time.
14. A method of performing digital feedback loop control in a
transimpedance amplifier (TIA) circuit, the method comprising: in
digital loop-control circuitry, receiving first, second, third and
fourth output signals from a first TIA, a first variable gain
amplifier (VGA) circuit, an output driver circuit, and a received
signal strength indicator (RSSI) circuit, respectively, the first
TIA having a first input terminal and a first output terminal,
wherein a variable impedance is electrically coupled to the TIA in
a feedback loop interposed between the first input terminal and the
first output terminal, the VGA circuit having a second input
terminal and a second output terminal, the second input terminal
being electrically coupled to the first output terminal, the VGA
circuit having N variable gain stages where N is a positive
integer, the output driver circuit having a third input terminal
and a third output terminal, the third input terminal being
electrically coupled to the second output terminal; with
analog-to-digital conversion (ADC) circuitry of the digital
loop-control circuitry electrically coupled to the first, second
and third output terminals and to an output terminal of a received
signal strength indicator (RSSI) circuit that measures a signal
strength of an electrical analog signal produced by a
photodetector, receiving first, second, third and fourth signals
output from the TIA, the VGA circuit, the output driver circuit and
the RSSI circuit, respectively, and converting the first, second,
third and fourth output signals into at least first, second, third
and fourth digital signals; in a digital controller of the digital
loop-control circuitry, processing the first, second, third and
fourth digital signals and producing P digital control signals,
where P is equal to at least N+2; with a digital-to-analog
converter (DAC) of the digital loop-control circuitry, converting
the P digital control signals into P analog control signals,
respectively; and with the digital loop control circuitry, feeding
P-2 of the analog control signals back to the VGA circuit for use
in varying a gain of one or more of the N variable gain stages,
respectively, feeding one of the P analog control signals back to
direct current (DC) offset cancellation circuitry for use as a DC
offset cancellation value by the DC offset cancellation circuitry,
and feeding one of the P-2 analog control signals back to the TIA
for use in varying the variable impedance.
15. The method of claim 14, wherein P is equal to at least N+3, and
wherein the method further comprises feeding one or more of the
analog control signals back to the VGA circuit to cause the gain of
one or more of the variable gain stages of the VGA to be adjusted
to perform offset cancellation to prevent the third output signal
from saturating.
16. The method of claim 14, wherein the step of processing the
first, second, third and fourth digital signals and producing P
digital control signals is performed by a digital controller that
executes a loop-control algorithm that processes the first, second,
third and fourth digital signals to algorithmically convert the
first, second, third and fourth digital signals into the P digital
control signals.
17. The method of claim 14, wherein the step of processing the
first, second, third and fourth digital signals and producing P
digital control signals includes using the first, second, third and
fourth digital signals to retrieve the P digital control signals
from at least a first lookup table (LUT) of the digital
loop-control circuitry.
18. The method of claim 17, further comprising: with P
sample-and-hold (S/H) circuits electrically coupled to the DAC
circuitry, maintaining the analog control signals for respective
predetermined time periods and in a predetermined sequence such
that the analog control signals are fed back to the VGA circuit, to
the variable impedance and to the DC offset cancellation circuitry
for respective predetermined periods of time in a predetermined
sequence.
19. The method of claim 18, wherein the digital controller
comprises a state machine having at least a VGA sub-state machine,
a DC offset cancellation sub-state machine, an RSSI sub-state
machine, and wherein the sub-state machines operate in a
round-robin sequence such that the respective analog control
signals are sampled and held by the respective S/H circuits in a
round-robin sequence for respective predetermined periods of
time.
20. The method of claim 17, wherein P is equal to at least N+3, and
wherein the method further comprises feeding one or more of the
analog control signals back to the VGA circuit to cause the gain of
one or more variable gain stages of the VGA to be adjusted to
perform offset cancellation to prevent the third output signal from
saturating.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The invention relates to transimpedance amplifier (TIA)
circuits. More particularly, the invention relates to a
digitally-controlled TIA circuit.
BACKGROUND OF THE INVENTION
[0002] A typical optical receiver (Rx) includes at least one
photodiode that detects an optical signal and converts it into an
electrical current signal and at least one transimpedance amplifier
(TIA) that converts the electrical current signal into an
electrical voltage signal. The photodetector, which is typically a
P-intrinsic-N (PIN) photodiode, produces an electrical current
signal in response to light detected by the photodetector. The TIA
converts this electrical current signal into an output voltage
signal having some gain, commonly referred to as transimpedance
gain. This output voltage signal is further processed by other
circuitry of the optical Rx (e.g., a limiting amplifier (LA), clock
and data recover (CDR), etc.).
[0003] The TIA circuit typically includes several control loops for
improving performance, such as a direct current (DC) offset
cancellation loop, an automatic variable gain amplifier (VGA) stage
loop, and a TIA feedback impedance adjustment loop. Each of these
loops utilizes analog components such as operational amplifiers (Op
Amps), capacitors and resistors to implement the analog
functionality needed. Such components are complex and require a
large area on an integrated circuit (IC) chip in order to implement
them with accuracy. In high-speed TIA circuits, such analog control
circuitry can occupy the majority of the IC chip area, which
increases the cost of the IC chip solution and leads to parasitic
capacitances that degrade RF performance and/or increase power
consumption. In addition, using analog methods to tune the feedback
resistance of the TIA feedback impedance adjustment loop is not
precise and suffers from limitations due to the precision and order
of the adjustment function, the number of elements that can be
controlled, etc.
[0004] A need exists for a TIA circuit having control loops that
have greater precision and improved performance, that can be
implemented in a smaller area on the IC chip, and that can be
manufactured with improved IC chip yield and at reduced IC chip
costs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 illustrates a block diagram of a TIA circuit in
accordance with an illustrative embodiment.
[0006] FIG. 2 illustrates a block diagram of a digitally-controlled
TIA circuit 200 in accordance with another illustrative
embodiment.
[0007] FIG. 3 illustrates a flow diagram of the method performed by
the TIA circuit in accordance with an illustrative embodiment.
[0008] FIG. 4 illustrates a flow diagram of the method performed by
the TIA circuit in accordance with another illustrative
embodiment.
[0009] FIG. 5 illustrates a block diagram of the digital controller
shown in FIG. 2 implemented as a state machine.
DETAILED DESCRIPTION
[0010] In accordance with illustrative embodiments, a
digitally-controlled TIA circuit is provided in which a plurality
of feedback loops are digitally controlled, including, but not
limited to, the DC offset cancellation loop, the VGA loop, and the
TIA feedback impedance adjustment loop.
[0011] A few illustrative embodiments of the offset measurement and
cancellation circuitry and method will now be described with
reference to the FIGS. 1-5, in which like reference numerals
represent like elements, features or components.
[0012] As used in the specification and appended claims, the terms
"a," "an," and "the" include both singular and plural referents,
unless the context clearly dictates otherwise. Thus, for example,
"a device" includes one device and plural devices.
[0013] It should be noted that when an element is referred to
herein as being "connected to" or "coupled to" or "electrically
coupled to" another element, it can be directly connected or
coupled, or intervening elements may be present.
[0014] FIG. 1 illustrates a block diagram of a digitally-controlled
TIA circuit 100 in accordance with an illustrative embodiment. The
TIA circuit 100 includes a TIA 102, a variable feedback impedance
103 connected between an output terminal of the TIA 102 and the
inverting input terminal of the TIA 102, a VGA circuit 104 having
an input terminal that is connected to the output terminal of the
TIA 102, an output driver 105 having an input terminal that is
connected to an output terminal of the VGA circuit 104, DC offset
cancellation circuitry 106 connected to the inverting input
terminal of the TIA 102, and digital loop-control circuitry
120.
[0015] A PIN diode 107, which is external to the TIA circuit 100,
has an anode that is connected to the inverting terminal of the TIA
102 and a cathode that is connected to a Received Signal Strength
Indicator (RSSI) circuit 108 and to a PIN supply voltage,
V.sub.PIN. The RSSI circuit 108 is external to the TIA circuit 100.
The RSSI circuit 108 outputs an indicator signal, RSSI, which is
indicative of the optical power level of the incident light
striking the PIN diode 107. The RSSI signal is used to produce a
voltage signal, V.sub.RF, which, in turn, is used to vary a
variable impedance of the TIA 102, which is typically an
operational amplifier (Op Amp). For ease of illustration, the
variable impedance 103 of the TIA 102 is represented by the
feedback resistor, but typically includes one or more variable
resistors in parallel with one or more variable capacitors (not
shown). The circuitry that is used to convert the RSSI signal into
the voltage signal V.sub.RF is not shown because such circuitry is
well known and is outside the scope of the invention.
[0016] The invention is not limited with respect to the
configuration of the RSSI circuit 108. Known RSSI circuits exist
for determining the optical power level of the incident light based
on a measurement of the electrical current produced by a PIN diode.
A typical RSSI circuit includes an Op Amp and a filter circuit. The
filter circuit is typically a first-order filter circuit comprising
a resistor and a capacitor connected in series. The filter circuit
acts as a low-pass filter that removes high frequency noise from
the supply voltage, V.sub.PIN. By sensing the voltage across the
resistor, the input current signal output by the PIN diode 107 to
the RSSI circuit 108 is sensed. The input current signal is
proportional to the optical power level of the light striking the
PIN diode 107. Hence, the RSSI circuit 108 detects the input
optical power and outputs the RSSI signal. The variable impedance
103 of the TIA 102 is then adjusted based on the RSSI signal to
increase the gain of the TIA 102 when the input optical power level
is lower and to decrease the gain of the TIA 102 when the input
optical power level is higher.
[0017] The anode of the PIN diode 107 is also connected to the DC
offset cancellation circuitry 106, which is represented by a single
n-type Metal Oxide Semiconductor Field Effect Transistor (NMOS).
Although a single NMOS is shown to represent the DC offset
cancellation circuitry 106, the DC offset cancellation circuitry
may include additional or other circuitry not shown in FIG. 1. DC
offset cancellation circuits used in TIA circuits to cancel out the
DC component of the signal output from the PIN diode are well
known. The invention is not limited with respect to the circuit
configuration of the DC offset cancellation circuitry. As will be
understood by persons of skill in the art, in view of the
description provided herein, a variety of circuit configurations
may be used to perform DC offset cancellation.
[0018] In accordance with this illustrative embodiment, an
analog-to-digital converter (ADC) 126 of the digital loop-control
circuitry 120 samples the outputs of the RSSI circuit 108, the TIA
102, the VGA circuit 104, and the output driver circuit 105 and
converts the analog signals into respective digital values. The
digital controller 121 then performs a loop-control algorithm that
processes these digital values to produce respective digital
loop-control values. The respective digital loop-control values are
then converted by the digital-to-analog converter (DAC) 127 of the
digital loop-control circuitry 120 into respective analog
loop-control values, DCC, V.sub.RF and VGA. The analog loop-control
values, DCC, V.sub.RF and VGA, are output as feedback signals to
the DC offset cancellation circuitry 106, to the TIA 102, and to
the VGA circuit 104, respectively, to cause those circuits to be
adjusted accordingly. Specifically, the analog loop-control value,
DCC, drives the gate of the NMOS 106 to cancel the DC offset at the
inverting terminal of the TIA 102, the analog loop-control value,
V.sub.RF, varies the variable impedance 103 of the TIA 102 to vary
the gain of the TIA 102 and the analog loop-control signal, VGA,
varies the gain of the VGA circuit 104 to perform offset
cancellation.
[0019] The digital controller 121 may be any type of digital
computational device such as, for example, a microprocessor, a
microcontroller, a digital signal processor, an application
specific integrated circuit (ASIC), a field programmable gate array
(FPGA). In such devices, the loop-control algorithm is typically
implemented in software or in a combination of software and
firmware. The software or firmware is stored in a non-transitory
computer-readable memory device (not shown) that is internal to or
external to the digital controller 121.
[0020] An alternative to the digital controller 121 performing a
loop-control algorithm that processes the digital values output
from the ADC 126 to produce respective digital loop-control values,
the loop-control algorithm performed by the digital controller 121
may use one or more lookup tables (LUTs) that store the digital
loop-control values that are retrieved from the LUTs by the digital
controller 121 and output to the DAC 127. An illustrative example
of such an algorithm will now be described with reference to FIG.
2.
[0021] FIG. 2 illustrates a block diagram of a digitally-controlled
TIA circuit 200 in accordance with another illustrative embodiment.
The TIA circuit 200 includes the TIA 102, the variable impedance
103, a VGA circuit 204 having an input terminal that is connected
to the output terminal of the TIA 102, an output driver 105 having
an input terminal that is connected to an output terminal of the
VGA circuit 204, the DC offset cancellation circuitry 106 connected
to the inverting input terminal of the TIA 102, and digital
loop-control circuitry 220.
[0022] In accordance with this embodiment, the digital loop-control
circuitry 220 uses one or more lookup tables (LUTs) 225 to store
values used by the digital controller 221 to produce the
loop-control values that are fed back to the VGA circuit loop, the
DC offset cancellation loop, and the feedback impedance adjustment
loop. The ADC 226 converts the analog values output from the RSSI
circuit 108, the TIA 102, the VGA circuit 204, and the output
driver circuit 105 and converts the analog values into respective
digital values. The digital controller 221 uses the digital values
to retrieve respective digital loop-control values from the LUTs
225. The digital loop-control circuitry 220 includes P
sample-and-hold (S/H) circuits 222.sub.1-222.sub.P that hold the
respective loop-control values output from the digital controller
221 for respective periods of time, where P is a positive integer
that is equal to the number of loops that are being controlled by
the digital controller 221. In accordance with this illustrative
embodiment, P=N+2 loops are being controlled, where N is equal to
the number of variable gain stages that are in the VGA circuit 204.
The S/H circuits 222.sub.1-222.sub.P are periodically refreshed so
that errors are not introduced due to current leakage.
[0023] One of the benefits of this illustrative embodiment is that
each gain stage 204.sub.1-204.sub.N of the VGA circuit 204 is a
variable gain stage that is individually adjusted based on a
respective loop-control value, VGA.sub.1-VGA.sub.N, output from the
S/H circuits 222.sub.3-222.sub.P, respectively. Having this level
of granularity in adjusting the VGA circuit 204 provides more
precise control over the linearity and overload performance of the
TIA circuit 200. In contrast, with typical VGA circuits, a single
value is fed back to the VGA circuit and used to equally adjust the
gain of all of the gain stages of the VGA circuit.
[0024] In accordance with this illustrative embodiment, the digital
controller 221 is implemented as a state machine. Implementing the
digital controller 221 as a state machine enables it to be
implemented with a relatively small amount of digital logic in a
relatively small area compared to, for example, a microprocessor or
microcontroller. Consequently, the TIA circuit 200 consumes a very
small area on the TIA IC chip (not shown) on which the TIA circuit
200 is integrated. An illustrative embodiment of the state machine
is described below with reference to FIG. 3.
[0025] The state machine of the digital controller 221 uses values
stored in the LUTs 225 as loop-control values, which are then
converted into analog loop-control values by the DAC 227 and output
to the S/H circuits 222.sub.1-222.sub.P. The LUTs 225 preferably
store multi-dimensional values so that loop-control function or
virtually any order can be implemented by the digital loop-control
circuitry 220. This allows greater precision to be achieved in
controlling the respective feedback loops of the TIA 102, the VGA
circuit 204 and the DC offset cancellation circuit 106.
[0026] In accordance with this illustrative embodiment, the ADC 226
of the digital loop-control circuitry 220 samples the outputs of
the RSSI circuit 108, the TIA 102, the VGA circuit 204 and the
output driver circuit 105 and converts these analog signals into
respective digital values. The state machine of the digital
controller 221 then uses these digital values to lookup respective
digital loop-control values in the LUTs 225. The respective digital
loop-control values retrieved from the LUTs 225 are then converted
by the DAC 227 into respective analog loop-control values, which
are then output to the respective S/H circuits 222.sub.1-222.sub.P.
The analog loop-control values held in the S/H circuits
222.sub.1-222.sub.P are provided as feedback signals to the DC
offset cancellation circuitry 106, to the variable impedance 103,
and to the variable gain stages 204.sub.A-204.sub.N, respectively,
causing those circuits to be adjusted accordingly.
[0027] There are several benefits to the digital loop-control
circuitry 220 shown in FIG. 2. One benefit is that it consumes less
area on the TIA IC chip compared to TIA IC chips that perform the
feedback loop control in the analog domain. In addition, because
digital logic continues to shrink as IC processes continue to
evolve, the size of the IC chip packages will further decrease over
time, leading to a smaller footprint in the systems in which they
are employed. Another benefit is realized through use of the LUTs
225. The LUTs 225 can be implemented in, for example, one-time
programmable (OTP) register files on each TIA IC chip. Such
register files can be formed in IC chips with very high yield,
leading to a higher overall chip yield, which reduces costs.
Another advantage of the LUTs 225 is that, as stated above, the
values stored in them can be multi-dimensional. The use of these
multi-dimensional values improves algorithmic efficiency and
overall performance.
[0028] FIG. 3 illustrates a flow diagram of the method performed by
the TIA circuit in accordance with an illustrative embodiment. In
accordance with this illustrative embodiment, an ADC samples the
outputs of an RSSI circuit, a TIA, a VGA circuit, and an output
driver circuit, as indicated by block 301. The ADC converts the
analog signals into respective digital values, as indicated by
block 302. The digital controller of the digital loop control
circuitry then performs a loop-control algorithm that processes the
digital values to produce respective digital loop-control values,
as indicated by block 303. The respective digital loop-control
values are then converted by a DAC into respective analog
loop-control values, as indicated by block 304. The analog
loop-control values are then applied as feedback signals to DC
offset cancellation circuitry, to the TIA, and to the VGA stages,
respectively, to cause those circuits to be adjusted accordingly,
as indicated by block 305.
[0029] FIG. 4 illustrates a flow diagram of the method performed by
the TIA circuit in accordance with another illustrative embodiment.
In accordance with this illustrative embodiment, an ADC samples the
outputs of an RSSI circuit, a TIA, a VGA circuit, and an output
driver circuit, as indicated by block 401. The ADC converts the
analog signals into respective digital values, as indicated by
block 402. The digital controller of the digital loop control
circuitry then performs a loop-control algorithm that uses the
digital values to retrieve respective digital loop-control values
from one or more LUTs, as indicated by block 403. The respective
digital loop-control values are then converted by a DAC into
respective analog loop-control values, as indicated by block 404.
The analog loop-control values are then applied as feedback signals
to DC offset cancellation circuitry, to the TIA, and to the VGA
stages, respectively, to cause those circuits to be adjusted
accordingly, as indicated by block 405.
[0030] The portions of the methods represented by blocks 304 and
404 are implemented in a combination of hardware (e.g., a
microprocessor, microcontroller, ASIC, DSP, FPGA, combinational
logic, a state machine, etc.) and software and/or firmware. The
software and/or firmware is stored on a non-transitory
computer-readable medium that is either internal to or external to
the digital controllers 121, 221. In FIGS. 1 and 2, memory elements
130 and 230, respectively, represent computer-readable mediums that
are external to the digital controllers 121 and 221, respectively,
and that may be used to store computer instructions comprising the
software/firmware.
[0031] FIG. 5 illustrates a block diagram of the digital controller
221 shown in FIG. 2 implemented as a state machine 500. In
accordance with this illustrative embodiment, the state machine 500
comprises five sub-state machines, namely, an initialization
sub-state machine 501, an RSSI sub-state machine 502, a DC offset
cancellation sub-state machine 503, a VGA sub-state machine 504,
and an offset cancellation sub-state machine 505. In accordance
with this illustrative embodiment, the sub-state machines 501-505
perform their corresponding functions in a round-robin sequence.
The initialization sub-state machine 501 is first in the
round-robin sequence. During an initialization period, the samples
acquired by the ADC 226 are converted into digital values, which
are then forwarded by the sub-state machine 501 to the respective
sub-state machines 502-505. The initialization sub-state machine
501 then hands off control to the RSSI sub-state machine 502.
[0032] The RSSI sub-state machine 502 uses the digital loop-control
value forwarded to it by the initialization sub-state machine 501
to retrieve a respective digital loop-control value from the LUTs
225. The retrieved digital loop-control value is then forwarded to
the DAC 227, which converts the digital loop-control value into an
analog loop-control value, V.sub.RF, and outputs it to the
respective S/H circuit 222.sub.2. The S/H circuit 222.sub.2
maintains the analog loop-control value at that value for a
predetermined time period so that variable impedance 103 is held at
a predetermined impedance value for the predetermined time period.
Once the predetermined time period has ended, the RSSI sub-state
machine 502 hands off control to the DC offset cancellation
sub-state machine 503.
[0033] The DC offset cancellation sub-state machine 503 uses the
digital loop-control value forwarded to it by the initialization
sub-state machine 501 during the initialization period to retrieve
a respective digital loop-control value from the LUTs 225. The
retrieved digital loop-control value is then forwarded to the DAC
227, which converts the digital loop-control value into an analog
loop-control value, DCC, and outputs it to the respective S/H
circuit 222.sub.1. The S/H circuit 222.sub.1 maintains the analog
loop-control value at that value for a predetermined time period so
that DC offset cancellation circuitry 106 provides a DC offset
cancellation value to the inverting terminal of the TIA 102 for the
predetermined time period to remove the DC component from the
signal provided by the PIN diode 107 to the TIA 102. Once the
predetermined time period has ended, the DC offset cancellation
sub-state machine 503 hands off control to the VGA sub-state
machine 504.
[0034] The VGA sub-state machine 504 uses the digital loop-control
value forwarded to it by the initialization sub-state machine 501
to retrieve N digital loop-control values from the LUTs 225. The
retrieved digital loop-control value are then forwarded to the DAC
227, which converts the digital loop-control values into N analog
loop-control values, VGA.sub.1-VGA.sub.N, respectively, and outputs
them to the respective S/H circuits 222.sub.3-222.sub.P. The S/H
circuits 222.sub.3-222.sub.P maintain the analog loop-control
values for a predetermined time period so that the gains of the
respective gain stages 204.sub.1-204.sub.N are held constant for
the predetermined time period. Once the predetermined time period
has ended, the VGA sub-state machine 504 hands off control to the
offset cancellation sub-state machine 505.
[0035] In contrast to the DC offset cancellation sub-state machine
503, the purpose of the offset cancellation sub-state machine 505
is to remove an offset at the input of the TIA 102 that can cause
the output of the output driver circuit 105 to become saturated.
This is prevented by adjusting the gain of one or more of the VGA
gain stages 204.sub.1-204.sub.N based on the signal output from the
output driver circuit 105.
[0036] When the VGA sub-state machine 504 hands off control to the
offset cancellation sub-state machine 505, the offset sub-state
machine 505 uses the digital loop-control value forwarded to it by
the initialization sub-state machine 501 to retrieve one or more
digital loop-control values from the LUTs 225. The retrieved
digital loop-control value(s) is then forwarded to the DAC 227,
which converts the digital loop-control value(s) into one or more
respective analog loop-control value(s), VGA.sub.1-VGA.sub.N, and
outputs the respective value(s) to the respective S/H circuits
222.sub.3-222.sub.P, thereby adjusting the gain of one or more of
the gain stages 204.sub.1-204.sub.N. The S/H circuits
222.sub.3-222.sub.P maintain the analog loop-control values for a
predetermined time period so that the gain(s) of the respective
gain stage(s) 204.sub.1-204.sub.N is held constant for the
predetermined time period. Once the predetermined time period has
ended, the offset cancellation sub-state machine 504 returns
control to the initialization sub-state machine 505, which then
performs re-initialization of the state machine 500.
[0037] It should be noted that although the illustrative
embodiments have been described with reference to a few
illustrative embodiments for the purpose of demonstrating the
principles and concepts of the invention. Persons of skill in the
art will understand how the principles and concepts of the
invention can be applied to other embodiments not explicitly
described herein. It should also be noted that the circuits and
method described above with reference to FIGS. 1-5 are merely
examples of suitable circuit configurations and methods that
demonstrate the principles and concepts of the invention. As will
be understood by those skilled in the art in view of the
description being provided herein, many modifications may be made
to the embodiments described herein while still achieving the goals
of the invention, and all such modifications are within the scope
of the invention.
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