U.S. patent application number 11/388594 was filed with the patent office on 2007-09-27 for method and circuit for amplitude compensation in a digital-to-analog converter.
Invention is credited to Bruce E. Hofer.
Application Number | 20070222651 11/388594 |
Document ID | / |
Family ID | 38473270 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070222651 |
Kind Code |
A1 |
Hofer; Bruce E. |
September 27, 2007 |
METHOD AND CIRCUIT FOR AMPLITUDE COMPENSATION IN A
DIGITAL-TO-ANALOG CONVERTER
Abstract
An amplitude-compensated digital-to-analog converter circuit and
method of compensation. A unipolar digital-to-analog converter is
provided, having a reference connection and a pair of differential,
unipolar signal outputs. A sensing circuit senses the common mode
component of the differential outputs and produces a common mode
signal. A reference source produces a first reference signal, and a
differential control amplifier compares the common mode signal to
the first reference signal and applies a second reference signal
applied to the reference connection of the digital-to-analog
converter, thereby stabilizing the reference current for the
digital-to-analog converter. The circuit includes a single signal
output reference to ground and a multiple-pole low-pass filter.
Inventors: |
Hofer; Bruce E.; (Portland,
OR) |
Correspondence
Address: |
William A. Birdwell;BIRDWELL & JANKE, LLP
Suite 1400
1100 SW Sixth Ave.
Portland
OR
97204
US
|
Family ID: |
38473270 |
Appl. No.: |
11/388594 |
Filed: |
March 24, 2006 |
Current U.S.
Class: |
341/120 |
Current CPC
Class: |
H03M 1/66 20130101; H03M
1/0607 20130101 |
Class at
Publication: |
341/120 |
International
Class: |
H03M 1/10 20060101
H03M001/10 |
Claims
1. An amplitude-compensated digital-to-analog converter circuit,
comprising: a unipolar digital-to-analog converter, having a
reference connection and a pair of differential, unipolar signal
outputs that have a common mode component; a sensing circuit for
sensing the common mode component of the differential outputs and
producing a common mode signal; a reference source for producing a
first reference signal; and a differential control amplifier,
having a first control amplifier input, a second control amplifier
input and a control amplifier output, the first reference signal
being applied to the first control amplifier input, the common mode
signal being applied to the second control amplifier input, and the
control amplifier output producing a second reference signal
applied to the reference connection of the digital-to-analog
converter.
2. The circuit of claim 1, further comprising a feedback loop from
the control amplifier output to one of the first control amplifier
input or the second control amplifier input so as to provide an
inverted feedback signal.
3. The circuit of claim 2, wherein the feedback loop includes
capacitive reactance so that the control amplifier operates as an
integrator.
4. The circuit of claim 1, wherein the sensing circuit comprises a
summing circuit for summing respective signals produced by the
differential outputs of the digital-to-analog converter.
5. The circuit of claim 1, further comprising a first
transimpedance amplifier having an input and an output, and a
second transimpedance amplifier having an input and an output, one
of the differential outputs of the digital-to-analog converter
being applied to the input of the first transimpedance amplifier
and the other of the differential outputs of the digital-to-analog
amplifier being applied to the input of the second transimpedance
amplifier, the sensing circuit comprising a summing circuit for
summing the output of the first transimpedance amplifier and the
output of the second transimpedance amplifier.
6. The circuit of claim 5, wherein the first reference signal is a
voltage reference.
7. The circuit of claim 5, wherein the summing circuit comprises a
voltage divider connected between the output of the first
transimpedance amplifier and the output of the second
transimpedance amplifier, the common mode signal being produced by
a tap on the voltage divider.
8. The circuit of claim 7, wherein the tap is adjustable to cancel
out the ac components of the outputs of the first and second
transimpedance amplifiers.
9. The circuit of claim 1, wherein the reference source is a
temperature controlled device.
10. The circuit of claim 1, wherein the reference source includes a
device for adjusting the magnitude of the first reference
signal.
11. The circuit of claim 1, further comprising a
differential-to-single signal amplifier having a first
differential-to-single input, a second differential-to-single
signal input and a differential-to-single signal output, the output
of the first transimpedance amplifier being applied to the first
differential-to-single signal input, and the output of the second
transimpedance amplifier being applied to the second
differential-to-single signal input, the differential-to-single
signal amplifier producing a single output signal based on a
digital signal applied to the digital-to-analog converter.
12. The circuit of claim 10, further comprising a first pole
filtering feedback loop from the output of the first transimpedance
amplifier to the input thereof, a first pole filtering feedback
loop from the output of the second transimpedance amplifier to the
input thereof, and a second pole filtering feedback loop from the
output of the differential-to-single signal amplifier to an input
thereof, thereby producing a multi-pole low pass filter.
13. A method for compensating a digital-to-analog converter circuit
for changes in a reference value, comprising: providing the
digital-to-analog converter circuit with a unipolar
digital-to-analog converter, having a reference connection and a
pair of differential, unipolar signal outputs that have a common
mode component; sensing the common mode component of the
differential outputs and producing a common mode signal; producing
a first reference signal; comparing the common mode signal to the
first reference signal to produce a second reference signal; and
applying the second reference signal to the reference input of the
digital-to-analog converter.
14. The method of claim 13, further comprising integrating the
difference in amplitude between the first reference signal and the
common mode signal to produce the second reference signal.
15. The method of claim 13, wherein producing a first reference
signal includes providing reference source at least a portion of
whose temperature is controlled independently of the temperature of
the unipolar digital-to-analog converter.
16. The method of claim 13, wherein producing a reference signal
comprises producing a voltage reference signal.
17. The method of claim 13, wherein the comparing comprises
converting the differential outputs of the unipolar
digital-to-analog converter from currents to respective voltages,
and determining the difference of those respective voltages to
produce the common mode signal.
18. The method of claim 17, further comprising converting the
respective differential signal voltages to a single voltage.
19. The method of claim 13, further comprising low-pass filtering
the differential outputs of unipolar digital-to-analog converter.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to digital-to analog converter
circuits, and particularly to circuits that provide a more stable
reference signal for a digital-to-analog converter.
[0003] 2. Description of the Related Art
[0004] Accurate, stable and high resolution digital-to-analog
converters ("DAC") are needed for many applications. One
application for which they are needed is audio system testing. In
particular, they are needed for digital-to-analog converter
circuits that are central to an audio signal generator for
producing a test signal to be applied to the input of an audio
system under test. Since many audio system tests involve comparing
the output signal of the audio system to a signal applied to the
input of the audio system to determine such characteristics as the
linear transfer function, harmonic distortion and intermodulation
distortion, the accuracy with which such characteristics can be
determined depends on the stability of the input signal.
[0005] High resolution and performance integrated circuit ("IC")
DACs with differential current outputs often include an on-chip
voltage reference device that is used in conjunction with an
external resistor to produce a reference current and thereby set
the magnitude of the output currents in response to a given digital
input signal. However, for many applications, including but not
limited to precision audio testing, the internal voltage reference
device is not as stable as is needed. Changes in the operating
temperature of the DAC due, for example, to warm up, changes in
ambient temperature, and varying heat dissipation caused by varying
the sample rate will cause errors in the amplitude of the output
signal current.
[0006] One approach to this problem is to try to design more stable
voltage sources within the DAC itself, but system manufacturers are
dependent on IC manufacturers to do so. Accordingly, it would be
desirable to provide a digital-to-analog circuit with the stability
needed for a particular application that is not dependent on the
inherent stability of a component DAC IC.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention provides an amplitude-compensated
digital-to-analog converter circuit and method of compensation. It
includes a unipolar digital-to-analog converter having a reference
connection and a pair of differential, unipolar signal outputs. A
sensing circuit senses the common mode component of the
differential outputs and produces a common mode signal. A reference
source produces a first reference signal, and a differential
control amplifier compares the common mode signal to the first
reference signal and applies a second reference signal applied to
the reference connection of the digital-to-analog converter,
thereby stabilizing the reference voltage for the digital-to-analog
converter.
[0008] The circuit architecture also lends itself to the inclusion
of a single signal output referenced to ground and a multiple-pole
low-pass filter.
[0009] It is to be understood that this summary is provided as a
means for generally determining what follows in the drawings and
detailed description, and is not intended to limit the scope of the
invention. Objects, features and advantages of the invention will
be readily understood upon consideration of the following detailed
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic diagram of an amplitude compensated
digital-to-analog converter circuit according to a specific
embodiment of the present invention.
[0011] FIG. 2A shows an example of a typical signal produced at a
first, non-inverting output of a digital-to-analog converter in the
circuit of FIG. 1.
[0012] FIG. 2B shows the signal produced at a second, inverting
output of the digital-to-analog converter in the circuit of FIG. 1
that is complementary to the signal shown in FIG. 2A.
[0013] FIG. 2C shows the result of adding the signal of FIG. 2A and
the signal of FIG. 2B.
[0014] FIG. 3 is a schematic diagram of an example of reference
source for use in the circuit of FIG. 1.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0015] In the following description many details are set forth to
provide an understanding of the disclosed embodiments of the
invention. However, upon reviewing this disclosure, it will become
apparent to one skilled in the art that not all of the disclosed
details may be required to practice the claimed invention and that
alternative embodiments might be constructed without departing from
the principles of the invention.
[0016] Referring first to FIG. 1, a specific embodiment of a
digital-to-analog circuit 10 according to the present invention
comprises a DAC 12, a sensing circuit 14, an independent reference
source 16, and a DAC feedback control loop 18, having a
differential control amplifier 20. It also includes a
differential-to-single signal output amplifier 22 for providing an
output signal referenced to ground, though it is to be understood
that this output amplifier could be left out so as to provide a
differential output without departing from the principles of the
invention.
[0017] The DAC 12 is a high resolution and performance IC having a
digital input 24, a non-inverting differential signal output 26 and
a corresponding inverting differential output 28, the differential
outputs being unipolar so that, while their respective ac signal
components are inverted with respect to one another, they each have
a common mode dc component. In this specific embodiment, the DAC
outputs 26 and 28 are shown as current sources, but it is to be
understood that a DAC with voltage source differential outputs
could be employed without departing from the principles of the
invention. The DAC also includes an internal voltage reference
source 30 and a reference connection 32. Ordinarily, a resistor
would be connected between the reference connection 32 and ground
to set the reference current within the DAC.
[0018] FIG. 2A shows a plot with respect to time of a typical
signal produced at the non-inverting output 26 of the DAC in
response to a sinusoidal digital input. The ac component is
indicated by i.sub.+(t) and the dc component is indicated by
i.sub.dc. Similarly, FIG. 2B shows a plot of the signal produced at
the inverting output 28 of the DAC with respect to time. The ac
component is indicated by i.sub.-(t) and the dc component is again
indicated by i.sub.dc as the dc component is common to both
outputs, that is, it is a common mode component. As the outputs are
unipolar, the plots are always positive with respect to zero,
though it would make no material difference to the invention to
show them as always negative with respect to zero.
[0019] Returning to FIG. 1, the outputs 26 and 28 of the DAC are
applied to inputs of respective differential transimpedance
amplifiers 34 and 36. As will be understood by a person of skill in
the art, the transimpedance amplifiers convert input current from
the DAC to output voltage. These transimpedance amplifiers comprise
respective operational amplifiers 38 and 40, having respective
feedback loops 42 and 44 with appropriate impedances, as will also
be understood by a person of skill in the art. In this specific
embodiment the impedances have a resistive component represented by
respective resistors 46 and 48, and capacitive component
represented by respective parallel capacitors 50 and 52, the
purpose of which will be explained below.
[0020] The sensing circuit 14 in this specific embodiment is a
resistor network that works as a voltage divider and current
summing circuit. The network comprises two substantially identical
fixed resistors 54 and 56, and a variable resistor 58 having an
adjustable tap 60. The three resistors are connected in series,
with the variable resistor disposed between the two fixed resistors
and the opposite ends of the series network being connected
respectively to the outputs 62 and 64 of the transimpedance
amplifiers 34 and 36. As will be appreciated by a person of skill
in the art, the ac components of the differential signal appearing
at the respective outputs of the transimpedance amplifiers will
cancel one another when the tap is properly adjusted. However, the
tap is applied to the inverting input 66 of a differential
operational amplifier 68, so that the voltage applied to input 66
is equal to the common mode dc signal voltage produced at outputs
62 and 64 of transimpedance amplifiers 34 and 36, and currents from
the common mode dc signals add. The current addition is indicated
in FIG. 2C by i.sub.dcs.
[0021] The operational amplifier 68 is part of the differential
control amplifier 20, as will be explained in more detail below.
The non-inverting input 70 of the operational amplifier 20 receives
a signal from the reference source 16. An example of a voltage
reference source is shown in FIG. 3. It comprises a zener diode 72
in series with a resistor 74, the series pair being connected
between a power supply voltage V.sub.P and ground. As a person of
skill in the art will appreciate, a stable fixed reference voltage
will be produced by this circuit at connection 76.
[0022] In practice, the reference source is preferably a more
sophisticated commercial component that provides a precision
reference voltage that has a low temperature coefficient and may be
independently temperature controlled for stability. Such devices
are commonly available, as will be appreciated by a person of skill
in the art. One example of a suitable precision reference voltage
device with a low temperature coefficient is a MAX6126
ultra-high-precision, ultra-low-noise, series voltage reference
available from Maxim Integrated Products, of Sunnyvale, Calif.
However, any convenient voltage or current reference that provides
the desired stability could be used without departing from the
principles of the invention. The output of the voltage reference 78
is applied to one end of a variable resistor 80 whose other end is
connected to ground. The tap 82 of the variable resistor is
connected to input 70 of the operational amplifier 68, as mentioned
above. Consequently, the variable resistor 80 can be used to adjust
the reference voltage applied to input 70 of the operational
amplifier 68.
[0023] Returning again to FIG. 1, the control amplifier comprises
the operational amplifier 68 and a feedback loop 82, and
effectively uses the resistors 54, 56 and 58 of the sensing circuit
14, and the variable resistor 80, as input resistors from the
sensing circuit and reference source, respectively. The feed back
loop preferably has a capacitive reactance, represented by
capacitor 84, so that the control amplifier acts as an integrator.
This serves to drive the difference between the two input voltages,
that is, the reference voltage and the dc common mode voltage, to
zero and thereby simplifies the design of the feedback control loop
18. However, a non-integrating control amplifier could be used
without departing from the principles of the invention.
[0024] In this specific embodiment, the output of the control
amplifier 20 is applied to an inverting amplifier 86, comprising an
operational amplifier 88, an input resistor 90, a feedback resistor
92, and an output 94, as will be understood by a person of skill in
the art. However, this is described for the sake of completeness,
and it is to be understood that a different circuit design could be
used that would not need the inverting amplifier without departing
from the principles of the invention.
[0025] The output 94 of the inverting amplifier 86 is applied
through a feedback resistor 95 to the reference connection 32 of
the DAC and a reference resistor 96 is connected from that input to
ground, as mentioned above. The current through the feedback
resistor 95 from the DAC feedback control loop 18 adjusts the DAC
reference current so that the common mode voltage applied to input
66 of control amplifier 20 matches the reference voltage from the
reference source 16, as adjusted with the variable resistor 80,
thereby providing the desired reference current stability for the
digital-to-analog converter circuit 10.
[0026] In addition to the foregoing, which provides the amplitude
compensation of the invention, the digital-to-analog converter
circuit may include the differential-to-single signal output
amplifier 22. As will be understood by a person of skill in the
art, the two differential outputs 62 and 64 from the transimpedance
amplifiers 34 and 36, respectively, are applied through respective
input resistors 98 and 100 to the non-inverting input 102 and the
inverting input 104 of operation amplifier 106. Each of the inputs
has a feedback loop comprising resistors 108 and 110, and
capacitors 112 and 114, respectively, so that a single signal
referenced to ground is produced at the output 116 of the
digital-to-analog converter circuit 10.
[0027] It will be appreciated that the capacitors 50 and 52 of the
respective transimpedance amplifiers 38 and 40, and the capacitors
112 and 114 of the differential-to-single signal output amplifier
22, introduce a multi-pole low-pass filter. While the filter is not
necessary for the operation of the digital-to-analog converter
circuit, and these capacitors could be eliminated without affecting
the reference stability or the provision of a single-signal output,
they provide an added advantage that is made convenient by the
overall circuit architecture. It will be appreciated that, while a
two-pole filter is shown, additional amplifier stages could be
added to produce a filter having additional poles.
[0028] Preferably, the DAC 12 is physically isolated from the
reference source 16 so that heat from the DAC does not affect the
reference source. Also, while the reference source may rely on a
low temperature coefficient to maintain stability, it may also, or
alternatively, include a temperature control device such as a
cooling thermocouple. As an additional alternative, the reference
source may be provided with an external temperature control device,
such as a cooling thermocouple and control circuit.
[0029] It will be appreciated by a person of skill in the art that
the foregoing not only describes a specific digital-to-analog
converter circuit, but also a method for compensating a
digital-to-analog converter circuit for changes in a reference
value, particularly variations in the internal reference voltage of
the a digital-to-analog converter IC due to changes in operating
temperature.
[0030] The terms and expressions that have been employed in the
foregoing specification are used therein as terms of description
and not of limitation, and there is no intention, in the uses of
such terms and expressions, to exclude equivalents of the features
shown and described or portions thereof, it being recognized that
the scope of the invention is defined and limited only by the
claims which follow.
* * * * *