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AB-089

更新时间： 2024-11-18 23:26:55

品牌	Logo	应用领域
其他 - ETC		/
页数	文件大小	规格书
2页	24K
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AB-089 - A CLARIFICATION OF USE HIGH-SPEED S/H TO IMPROVE SAMPLING ADC PERFORMANCE

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AB-089 数据手册

AP P LICATION BULLETIN

Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706

Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132

A CLARIFICATION OF USE HIGH-SPEED S/H TO

IMPROVE SAMPLING ADC PERFORMANCE

by ??, (602) 746-7???

You know it’s going to be one of those days when you see

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.

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.

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.

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 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

–70

–75

–80

MAGNITUDE SPECTRUM

–10

–20

–85

ADS7800

–30

–40

–90

–95

–50

–60

–70

SHC804/ADS7800

–100

–80

–90

0.9k

32.6k

64.2k

95.9k

127.5k

–100

–110

–120

Frequency (Hz)

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 –85dB up to the Nyquist fre-

quency.

Frequency (kHz)

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 –76dB.

AB-089

Printed in U.S.A. January, 1994

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