CS5321 datasheet(22/36 Pages) CIRRUS

22 / 36 page

CS5321/22

DS454F2

2.8 Performance

Figure 22, 23 and 24 illustrate the spectral perfor-

mance of the CS5321/22 and chipset when operat-

ing from a 1.024 MHz master clock. Ten 1024

point FFTs were averaged to produce the plots.

Figure 22 illustrates the chip set with a 100 Hz,

-20 dB input signal. The sample rate was set at 1

kHz. Dynamic range is 122 dB.

The dynamic range calculated by the test soft-ware

is reduced somewhat in Figures 23 and 24 because

of jitter in the signal test oscillator. Jitter in the

100 Hz signal source is interpreted by the signal

processing software to be increased noise.

The choice of master clock frequency will affect

performance. The CS5321 will exhibit the best Sig-

nal to Distortion performance with slower modula-

tor sampling clock rates as slower sample rates

allow more time for amplifier settling.

For lowest offset drift, the CS5321 should be oper-

ated with MCLK = 1.024 MHz and HBR = 1. Slow-

er modulator sampling clock rates will exhibit

more offset drift. Changing MCLK to 512 kHz

(HBR = 1) or changing HBR to zero (MCLK =

1.024 MHz) will cause the drift rate to double. Off-

set drift is not linear over temperature so it is diffi-

cult to specify an exact drift rate. Offset drift

characteristics vary from part to part and will vary

as the power supply voltages vary. Therefore, if the

CS5321 is to be used in precision dc measurement

applications where offset drift is to be minimized,

the power supplies should be well regulated. The

CS5321 will exhibit about 6 ppm/°C of offset drift

with MCLK = 1 and HBR = 1. Gain drift of the

CS5321 itself is about 5 ppm/°C and is not affected

by either modulator sample rate or by power supply

variation.

Figure 22. 1024 Point FFT Plot with -20 dB Input, 100 Hz

Input, ten averages

D yna m ic R ange = 1 22.0 dB

HB R = 1

OF S T = 0

LP W R = 0

500

-20

-40

-60

-80

-100

-120

-140

-160

-180

Figure 23. 1024 Point FFT Plot with Full Scale Input,

100 Hz Input, HBR = 1, ten averages

S /D = 11 6 .0 dB

S /N = 11 8 .4 dB

S /N+ D = 11 4 .2 d B

HB R = 1

OF S T = 0

LP W R = 0

500

-20

-40

-60

-80

-100

-120

-140

-160

-180

see text

Figure 24. 1024 Point FFT Plot with Full Scale Input,

100 Hz Input, HBR = 0, ten averages

500

-20

-40

-60

-80

-100

-120

-140

-160

-180

S /D = 12 2 .7 d B

S /N = 11 7 .1 d B

S /N + D = 11 6 .4 d B

HB R = 0

OF S T = 0

LP W R = 0

see text

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