ETC AB-089

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by ??, (602) 746-7???
You know it’s 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.
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
Therefore, the graph from Design Update, Vol. 1, No. 1
showing the improvement in THD with the SHC804 preceding 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.
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!
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 changing 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 Application Note).
Frequency (Hz)
FIGURE 1. The Level of Non-Harmonic Spurious Signal
Components is Improved by Using the LowerJitter Sampling Mechanism of the SHC804. Note
that even without the SHC804, non-harmonic
spurs are below –85dB up to the Nyquist frequency.
1994 Burr-Brown Corporation
Frequency (kHz)
FIGURE 2. Spectral Performance of ADS7800 Digitizing a
Full-Scale 100kHz Sine Wave at a 256kHz Sampling Rate. The largest frequency component
(aside from the fundamental) is the second harmonic at –76dB.
Printed in U.S.A. January, 1994
Application Bulletin Number 89
Magnitude (dBFSR)
Non-Harmonic Peak Spur (dBFSR)
This phenomenon is present in all monolithic sampling
ADCs—not just the ADS7800. In fact, we spent considerable time and effort here at Burr-Brown creating a design
that minimized this distortion. A monolithic sampling 12-bit
converter yielding spectral results such as those in Figure 2
is remarkable! (To add insult to injury, the FFTs originally
published were performed on an early version of the
ADS7800, which gave a much larger second harmonic. I
was quickly informed that we had improved the ADS7800
since then, and had never shipped ADS7800s like the one I
had used for my experiments.)
With the SHC804 in front of the ADS7800, the ADS7800’s
input sample/hold sees essentially DC, reducing harmonic
distortion. (Request PDS-1018 for ADS7800 and PDS-512
for SHC804.)
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.