Application Note 132
February 2011
Fidelity Testing for A→D Converters
Proving Purity
Jim Williams and Guy Hoover
Introduction
The ability to faithfully digitize a sine wave is a sensitive
test of high resolution A→D converter fidelity. This test
requires a sine wave generator with residual distortion
products approaching one part-per-million. Additionally,
a computer-based A→D output monitor is necessary to
read and display converter output spectral components.
Performing this testing at reasonable cost and complexity
requires construction of its elements and performance
verification prior to use.
Overview
Figure 1 diagrams the system. A low distortion oscillator
drives the A→D via an amplifier. The A→D output interface
formats converter output and communicates with the
computer which executes the spectral analysis software
and displays the resulting data.
Oscillator Circuitry
The oscillator is the system’s most difficult circuit design
aspect. To meaningfully test 18-bit A→Ds, the oscillator
must have transcendentally low levels of impurity, and
these characteristics must be verified by independent
means. Figure 2 is basically an “all inverting” 2kHz Wien
bridge design (A1-A2) adapted from work by Winfield Hill
of Harvard University. The original designs J-FET gain
control is replaced with a LED driven CdS photocell isolator, eliminating J-FET conductivity modulation introduced
errors and the trim required to minimize them. Band limited
A3 receives A2 output and DC offset bias, providing output
via a 2.6 KHz filter which drives the A→D input amplifier.
Automatic gain control (AGC) for the A1-A2 oscillator is
taken from the circuit output (“AGC sense”) by AC-coupled
A4 which feeds rectifier A5-A6. A6’s DC output represents
the AC amplitude of the circuit output sine wave. This
value is balanced against the LT®1029 reference by current
summing resistors which terminate into AGC amplifier A7.
A7, driving Q1, closes a gain control loop by setting LED
current, and hence CdS cell resistance, to stabilize oscillator output amplitude. Deriving gain control feedback from
the circuit’s output maintains output amplitude despite the
attenuating band-limiting response of A3 and the output
filter. It also places demands on A7 loop closure dynamics.
Specifically, A3’s band limiting combines with the output
filter, A6’s lag and ripple reduction components in Q1’s
base to generate significant phase delay. A 1μF dominant
pole at A7 along with an RC zero accommodates the
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and PScope
is a trademark of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
SOFTWARE
2kHz
OSCILLATOR
AqD
INPUT
AMPLIFIER
AqD
DATA
ACQUISITION/
INTERFACE
COMPUTER
AND
DISPLAY
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100MHz
CLOCK
Figure 1. Block Diagram of A→D Spectral Purity Test System. Assuming a Distortionless
Oscillator, Computer Displays Fourier Components Due to Amplifier and A→D Infidelities
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Application Note 132
OSCILLATOR
7.87k 0.01μF
0.1%
0.0068
0.01μF
5pF
7.87k
0.1%
40.2k
0.1%
OUTPUT AMP AND FILTER
12k*
–
19.1k
909Ω* 0.1%
249Ω
A1
LT1468
+
–
9.09k*
+
–
100Ω
+
A2
LT1468
+
36k
5%
100μF
0.5V
0V
+
A7
LT1006
100k
28k
4.3k
5%
LT1029
5V
10μF
+
+
4V BIASED
ON 0.5V
+
470Ω
5%
–
Q1
2N3904
100Ω
5%
≈2kHz
DC OFFSET
15μF
15V
15V
5V
4.5V
47μF
1μF
470Ω
5%
≈2kHz
TO AqD
AMPLIFIER
INPUT
0.47
12k
12k
TRIM 100Ω ADJ
FOR ≈5V
ACROSS HERE
130Ω
A3
LT1468
–15V
AGC
1μF
10k
NO CERAMIC CAPACITORS ANYWHERE. USE MYLAR OR SILVER MICA
10k
5k
–
1N4148
A6
LT1122
10k
A5
LT1122
+
= SILONEX NSL32SR3
10k
1N4148
–
A4
LT1122
0.1
AGC
SENSE
+
+
0.01μF = POLYSTYRENE PERFERRED.
POLYPROPELENE, PPS ACCEPTABLE.
OUTER FOIL TO AMPLIFIER OUTPUT
–
BYPASS EACH AMPLIFIER WITH 10μF OS-CON
1M
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ALL
= 1% METAL FILM UNLESS NOTED
RECTIFIER
* = IRC TYPE CAR-5, 1%
0.1%
= IRC TYPE CAR-5
RESELECT 909Ω IN SERIES WITH 100Ω TRIM IF NECESSARY
Figure 2. Wien Bridge Oscillator Uses Inverting Amplifiers in Signal Path, Achieves 3ppm Distortion. LED Photocell Replaces Usual
J-FET as Gain Control, Eliminating Conductivity Modulation Induced Distortion. A3 Associated Filtering Attenuation is Compensated
by Sensing AGC Feedback at Circuit Output. DC Offset Biases Output Into A→D Input Amplifier Range
delay, achieving stable loop compensation. This approach
replaces closely tuned high order output filters with simple
RC roll-offs, minimizing distortion while maintaining output
amplitude1.
Eliminating oscillator related components from the LED
bias is essential to maintaining low distortion. Any such
residue will amplitude modulate the oscillator, introducing
impure components. The band-limited AGC signal forward
path is well filtered and the heavy RC time constant in Q1’s
base provides a final, steep roll-off. Figure 3, Q1’s emitter
current, shows about 1nA of oscillator related ripple out
of a 10mA total, less than 0.1ppm.
Note 1. This is loosely akin to forcing food through a meat grinder to
produce purée.
AN132-2
A = 2V/DIV
ON 0.5V DC
B = 10nA/DIV
ON 10mA DC
30kHz BW
HORIZ = 500μs/DIV
AN132 F03
Figure 3. Oscillator (Trace A) Related Residue (Trace B), Just
Discernible in Q1 Emitter Noise, ≈1nA, About 0.1ppm of LED
Current. Characteristic, Deriving From Heavy AGC Signal Path
Filtering, Prevents Modulation Products From Influencing
Photocell Response
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Application Note 132
The oscillator achieves its performance using only a single
trim. This adjustment, which centers AGC capture range,
is set in accordance with the schematic note.
Verifying Oscillator Distortion
Verifying oscillator distortion necessitates sophisticated
measurement techniques. Attempts to measure distortion
with a conventional distortion analyzer, even a high grade
type, encounter limitations. Figure 4 shows oscillator
output (Trace A) and its indicated distortion residuals at
the analyzer output (Trace B). Oscillator related activity is
faintly outlined in the analyzer noise and uncertainty floor.
The HP-339A employed specifies a minimum measurable
distortion of 18ppm; this photograph was taken with the
instrument indicating 9ppm. This is beyond specification,
and highly suspect, because of the pronounced uncertainties introduced when measuring distortion at or near
equipment limits2. Specialized analyzers with exquisitely
low uncertainty floors are needed to meaningfully measure
oscillator distortion. The Audio Precision 2722, specified
at a 2.5ppm Total Harmonic Distortion + Noise (THD + N)
limit (1.5ppm typical), supplied Figure 5’s data. This figure
A = 2V/DIV
B = 9ppm
INDICATED
DISTORTION
HORIZ = 200μs/DIV
AN132 F04
Figure 4. HP-339A Distortion Analyzer Operating Beyond Its
Resolution Limit Provides Misleading Distortion Indication
(Trace B). Analyzer Output Contains Uncertain Combination of
Oscillator and Instrument Signatures and Cannot Be Relied
Upon. Trace A Is Oscillator Output
indicates a Total Harmonic Distortion (THD) of –110dB,
or about 3ppm. Figure 6, taken with the same instrument,
shows THD + N of 105dB, or about 5.8ppm. In Figure 7’s
final test, the analyzer determines the oscillator’s spectral
components with the third harmonic dominating at –112dB,
or about 2.4ppm. These measurements provide confidence
in applying the oscillator to A→D fidelity characterization.
A→D Testing
A→D testing routes oscillator output to the A→D via its
input amplifier. The test measures distortion products
produced by the input amplifier/A→D combination. A→D
output is examined by the computer, which quantitatively
indicates spectral error components in Figure 8’s display.3
The display includes time domain information showing the
biased sine wave centered into the converter’s operating
range, a Fourier transform indicating spectral error components and detailed tabulated readings. The LTC®2379
18-Bit A→D/LT6350 amplifier combination under test
produces 2nd harmonic distortion of –111dB, about
2.8ppm, with higher frequency harmonics well below
this level. This indicates the A→D and its input amplifier
are operating properly and within specifications. Possible
harmonic cancellation between the oscillator and amplifier/
A→D mandates testing several amplifier/A→D samples
to enhance measurement confidence4.
Note 2. Distortion measurements at or near equipment limits are full of
unpleasant surprises. See LTC Application Note 43, “Bridge Circuits”,
Appendix D, “Understanding Distortion Measurements” by Bruce Hofer of
Audio Precision.
Note 3. Input amplifier/A→D converter, computer data acquisition and
clock boards, necessary for testing, are available from LTC. Software code
may be downloaded at www.linear.com. See Appendix A “Tools for A→D
Fidelity Testing” for details.
Note 4. Review text section, “Verifying Oscillator Distortion” and
footnote 2 for relevant commentary.
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Application Note 132
AN132 F05
Figure 5. Audio Precision 2722 Analyzer Measures Oscillator THD at –110dB, About 3ppm
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Application Note 132
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Figure 6. AP-2722 Analyzer Measures Oscillator THD + N at ≈ –105dB, About 5.8ppm
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Application Note 132
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Figure 7. AP-2722 Spectral Output Indicates 3rd Harmonic Peak at –112.5dB, ≈2.4ppm
Figure 8. Figure 1’s Test System Partial Display Includes Time Domain Information, Fourier
Spectral Plot and Detailed Tabular Readings for LTC2379 18-Bit A→D Driven by LT6350 Amplifier
AN132 F08
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Application Note 132
APPENDIX A
Tools for A→D Fidelity Testing
Circuit boards for implementing the text’s A→D testing
are available. Table 1 lists the board functions and their
part numbers. The computer software, PScope™, is also
available from Linear Technology and may be downloaded
at www.linear.com.
Table 1
BOARD FUNCTION
PART NUMBER
LT6350/LTC1279 Amp/A→D
DC-1783A-E
Interface
DC718
100MHz Clock*
DC1216A-A
Oscillator
To Be Released
*Any stable, low phase noise 3.3V clock capable of driving 50Ω may be
used.
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
AN132-7
Application Note 132
“Summon a vision and declare it pure.”
–Theodore Roethke
“Four for Sir John Davies”
1953
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Linear Technology Corporation
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