CS5101A datasheet(31/39 Pages) CIRRUS

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CS5101A CS5102A

DS45F6

Figure 23. CS5101A Histogram Plot of 8192

Conversion Inputs

Figure 24. CS5102A Histogram Plot of 8192

Conversion Inputs

7.4

Noise

An A/D converter's noise can be described like that

of any other analog component. However, the con-

verter's output is in digital form so any filtering of its

noise must be performed in the digital domain. Dig-

itized samples of analog inputs are often consid-

ered individual, static snap-shots in time with no

uncertainty or noise. In reality, the result of each

conversion depends on the analog input level and

the instantaneous value of noise sources in the

ADC. If sequential samples from the ADC are

treated as a “waveform”, simple filtering can be im-

plemented in software to improve noise perfor-

mance with minimal processing overhead.

All analog circuitry in the CS5101A and CS5102A

is wideband in order to achieve fast conversions

and high throughput. Wideband noise in the

CS5101A and CS5102A integrates to 35

µV rms in

unipolar mode (70

µV rms in bipolar mode). This is

approximately 1/2 LSB rms with a 4.5V reference

in both modes. Figure 23 shows a histogram plot of

output code occurrences obtained from 8192 sam-

ples taken from a CS5101A in the bipolar mode.

Hexadecimal code 7FFE was arbitrarily selected

and the analog input was set close to code center.

With a noiseless converter, code 7FFE would al-

ways appear. The histogram plot of the device has

a “bell” shape with all codes other than 7FFE due

to internal noise. Figure 24 illustrates the noise his-

togram of the CS5102A.

In a sampled data system all information about the

analog input applied to the sample/hold appears in

the baseband from DC to one-half the sampling

rate. This includes high-frequency components

which alias into the baseband. Low-pass (anti-

alias) filters are therefore used to remove frequen-

cy 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 sam-

pling at higher than the desired word rate and av-

eraging

multiple

samples

for

each

word.

Oversampling spreads the device's noise over a

wider band (for lower noise density), and averag-

ing applies 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 maxi-

mum specified rate of 100 kSps (CS5101A) or

20 kSps (CS5102A) (for lowest noise density) and

digitally filtering to the desired signal bandwidth.

7.5

Aperture Jitter

Track-and-hold amplifiers commonly exhibit two

types of aperture jitter. The first, more appropriate-

ly termed “aperture window”, is an input-voltage-

dependent variation in the aperture delay. Its sig-

nal dependency causes distortion at high frequen-

7F F C

7F F D

7F F E

8000

8001

7F F F

7 FFB

20 48

40 96

61 44

81 92

Cou nt

No ise le s s

C S 510 1 A

C ode (H exade cim al)

C ou nts :

989

63 59

844

Con verte r

7FF E

7FF F 80 00(H )

80 02

80 03

80 01

7FFD

Co u nt

No is ele ss

C S 5102A

C o de

(H ex ad ec im a l)

C o unts:

17 27

4988

14 67

C o nverte r

81 92

61 44

40 96

20 48

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