CS5101A datasheet(28/40 Pages) CIRRUS

28 / 40 page

shows a histogram plot of output code occur-

rences obtained from 8192 samples taken from a

CS5101A in the bipolar mode. Hexadecimal code

7FFE was arbitrarily selected and the analog in-

put was set close to code center. With a noiseless

converter, code 7FFE would always appear. The

histogram plot of the device has a "bell" shape

with all codes other than 7FFE due to internal

noise. Figure 22 illustrates the noise histogram of

the CS5102A.

In a sampled data system all information about

the analog input applied to the sample/hold ap-

pears in the baseband from dc to one-half the

sampling rate. This includes high-frequency com-

ponents which alias into the baseband. Low-pass

(anti-alias) filters are therefore used to remove

frequency components in the input signal which

are above one-half the sample rate. However, all

wideband noise introduced by the CS5101A and

CS5102A still aliases into the baseband. This

"white" noise is evenly spread from dc to one-

half the sampling rate and integrates to 35

µV rms

in unipolar mode.

Noise in the digital domain can be reduced by

sampling at higher than the desired word rate and

averaging multiple samples for each word. Over-

sampling spreads the device’s noise over a wider

band (for lower noise density), and averaging ap-

plies a low-pass response which filters noise

above the desired signal bandwidth. In general,

the device’s noise performance can be maximized

in any application by always sampling at the

maximum specified rate of 100 kHz (CS5101A)

or 20 kHz (CS5102A) (for lowest noise density)

and digitally filtering to the desired signal band-

width.

Aperture Jitter

Track-and-hold amplifiers commonly exhibit two

types of aperture jitter. The first, more appropri-

ately termed "aperture window", is an input

voltage dependent variation in the aperture delay.

Its signal-dependency causes distortion at high

frequencies. The proprietary architecture of the

CS5101A and CS5102A avoids applying the in-

put voltage across a sampling switch, thus

avoiding any "aperture window" effects. The sec-

ond type of aperture jitter, due to component

noise, assumes a random nature. With only

100 ps peak-to-peak aperture jitter, the CS5101A

and CS5102A can process full-scale signals up to

7FFC

7FFD

7FFE

8000

8001

7FFF

7FFB

2048

4096

6144

8192

Count

Noiseless

CS5101A

Code (Hexadecimal)

Counts:

989

6359

844

Converter

Figure 21. 5101A Histogram Plot of 8192 Conversion

Inputs

7FFE

7FFF 8000(H)

8002

8003

8001

7FFD

Count

Noiseless

CS5102A

Code (Hexadecimal)

Counts:

1727

4988

1467

Converter

8192

6144

4096

2048

Figure 22. 5102A Histogram Plot of 8192 Conversion

Inputs

CS5101A CS5102A

DS45F2

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