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APPLICATION BULLETIN
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A CLARIFICATION OF USE HIGH-SPEED S/H TO
IMPROVE SAMPLING ADC PERFORMANCE
by ??, (602) 746-7???
You know it鈥檚 going to be one of those days when you see
the printed version of an article you wrote and realize that
you have made an error in the presentation of your ideas.
This is what happened to me as I read
Use High Speed S/H
to Improve Sampling ADC Performance (Design Update,
Vol. 1, No. 1). This article will rectify the situation and
further illuminate some important performance aspects of
the ADS7800.
After having spent three weeks staring at the display of an
Audio Precision distortion analyzer and trying to track down
the jitter sources in a digital audio reconstruction board, I did
the experiments with the ADS7800 and SHC804. In the
former project, I could not distinguish between harmonic
distortion and other spurious frequency components, due to
the nature of the measurement. In the latter, all we did
measure was harmonic distortion (aside from SNR). And
this is where I made my mistake; so let me set the record
straight and tell you:
In all but the most contrived cases,
jitter does NOT cause HARMONIC distortion!
Jitter causes spurious frequency components, and the level
of the spurs is dependent upon the rms value of the jitter, and
the frequency of the signal being digitized, as explained in
the article in Vol. 1, No. 1. This will cause a degradation in
SNR.
Therefore, the graph from
Design Update,
Vol. 1, No. 1
showing the improvement in THD with the SHC804 preced-
ing the ADS7800 really does not illustrate the results of
reducing jitter. The correct results can be seen in Figure 1.
The level of spurious frequencies is improved significantly
by using the lower-jitter sampling mechanism of the SHC804.
So why does the THD improve by adding the SHC804 if
harmonic distortion is not caused by jitter? The cause of the
harmonic distortion in this case is a function of the sample/
hold in the ADS7800. The on-resistance of the sampling
switch changes with input signal amplitude, causing a chang-
ing phase lag in the sampling mechanism. This results in
even-order harmonic distortion. A close look at Figure 2 will
reveal the largest spur is the second harmonic of the input
signal. (A complete explanation of this phenomenon, as well
as other information on using the ADS7800 for dynamic
applications, will be the subject of a forthcoming Applica-
tion Note).
NON-HARMONIC PEAK SPUR vs FREQUENCY
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Non-Harmonic Peak Spur (dBFSR)
鈥?5
MAGNITUDE SPECTRUM
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鈥?5
鈥?0
鈥?5
SHC804/ADS7800
鈥?00
0.9k
32.6k
64.2k
Frequency (Hz)
95.9k
127.5k
0
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鈥?0
Magnitude (dBFSR)
ADS7800
鈥?0
鈥?0
鈥?00
鈥?10
鈥?20
0
32
64
Frequency (kHz)
96
128
FIGURE 1. The Level of Non-Harmonic Spurious Signal
Components is Improved by Using the Lower-
Jitter Sampling Mechanism of the SHC804. Note
that even without the SHC804, non-harmonic
spurs are below 鈥?5dB up to the Nyquist fre-
quency.
FIGURE 2. Spectral Performance of ADS7800 Digitizing a
Full-Scale 100kHz Sine Wave at a 256kHz Sam-
pling Rate. The largest frequency component
(aside from the fundamental) is the second har-
monic at 鈥?6dB.
漏
1994 Burr-Brown Corporation
AB-089
Printed in U.S.A. January, 1994
Application Bulletin Number 89
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