FS970X datasheet(27/46 Pages) FORTUNE

27 / 46 page

FS970X

Rev. 4.4

27/46

There are two this kind of programmable digital filters in

FS970X, namely, COMB1 and COMB2. Their output is

SUM1 and SUM2 respectively. The number of COMB1is

higher, often used to measure high resolution. And the

number of COMB2 is lower; its high-speed, low-resolution

output can be used on peak-hold sampling and analog bar

graph.

The numbers of COMB1 and COMB2 are both

programmable. They can be set by TPS1 and TPS2

respectively, as shown in Chart 16. Taking the sampling

frequency of 166.7 kHz as an example, the location of the

first zero-point is calculated as Chart 17.

The actual resolution of each status is defined by the actual

measurement.

COMB1（TPS1）

COMB2（TPS2）

TPSX<1:0> Number（N） 1st 0-point frequency

（

Hz）

Number（N） 1st 0-point frequency

（

Hz）

16384

10.17

256

651.17

8192

20.34

128

1302.34

4096

40.68

2604.68

2048

81.40

5209.38

Chart

16. Digital Filter number setup and 1

st zero-point location（F

S=166.7 kHz）.

Because of the delay of the digital filter, the bandwidth of the signal pulse needs to be greater than four-times

output period of the input signal of ADC. For example, when set TPS2 to 00, the output period of COMB2 will

be:

Formula 10......................................................

192

sec

5209



Therefore, the pulse bandwidth of the input signal must be greater than

769

192





so that the

value can be effectively converted by ADC. When the sampling frequency (FS) is 166.7 kHz, the output reading

of SUM2 from COMB2 can detect to the smallest pulse bandwidth and TPS2, as shown in Chart 17.

TPS2<1:0>

Output frequency

5.2 kHz

2.6 kHz

1.3 kHz

0.65 kHz

Detectable smallest pulse bandwidth

0.769 ms

1.53 ms

3.06 ms

6.12 ms

Chart 17. The relationship between detectable smallest pulse bandwidth and TPS2.

-160

-140

-120

-100

-80

-60

-40

-20

 Graph 12、Digital filter frequency response of 970x

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