FS9701B datasheet(26/46 Pages) FORTUNE

26 / 46 page

FS970X

Rev. 4.4

26/46

With suitable gain option control, all kinds of measurement can be applied to the best dynamic range of ADC.

Chart 15 shows the setup of three typical functions and ADG<5:0> in the application of DMM.

1st function 2nd function 3rd function

ADG<5:0>

01_0011

11_0111

11_1000

Reference voltage gain（GREFi）



1.0



1.25



1.25

Input voltage gain（GSIGi）



1.0



1.25



0.1

Chart 15

970x Typical ADC function setup.

The measurement transfer functions of each function is:

Formula 7

ref

REFi

SIGi





The reference voltage of each function and the gain approximation of the input voltage are shown in Chart 15.

The actual precise gain value and offset voltage should be obtained from calibration.

Digital Filter

As shown in Graph 9, the 1-bit output from the comparator must go through a digital low-pass filter and perform

calculation similar to arithmetic average to become a high-resolution multi-bit resolution. The transfer function

of digital filter used by 970x is:

Formula 8

 









H f

N f f

f f















sin



Whereas, N is the number of the filter (TAP).

Assuming the sampling frequency of the ADC is 166kHz and the number of the filter is 16600, the frequency

response Graph of the filter is shown as Graph 9. The first zero-point would be found at:

Formula 9

166000 Hz

16600

10 Hz



Thereafter, all the integer multiple points of the first zero-point occur zero-point. The signal around the

zero-point will be completely filtered out by the filter. So the frequency responses as shown in Graph 12, all

have good suppressing effect to the noise of 50 Hz and 60 Hz. By the same token, assuming the sampling

frequency is 83kHz, and the number of the filter is still 16600, then, the first zero-point position can be

calculated as 5 Hz.

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