Digital Filter

In Fig. 10, the 1-bit output of the comparator should be fed to the digital low pass filter to do

decimation to find the high resolution multiple-bit digital output. The transfer function of the FS501

digital filter is:

Equation 5

 









H f

N

N f f

f f

S

S















1

2

2

sin

sin





Where N is TAP of the digital filter.

Suppose the sampling rate of the ADC is 166KHz, the TAP of the digital filter is 16600. We can find

the frequency response diagram of the digital filter as shown in Fig. 12. The first zero is at:

Equation 6

f

f

N

Z

S

1

166000 Hz

16600

10 Hz







Fig. 12. The Frequency Response Diagram of FS501 Digital Filter

The zero points fall at multiples of 10Hz. The digital filter will filter out all the signals near the zero points. From

Fig. 12, we can find that the noises at 50Hz and 60Hz are suppressed very well. If the sampling rate is 83KHz

and the TAP of the filter is 16600, the first zero-frequency is at 5Hz.

There are two programmable TAP digital filters in FS970x, and they are COMB1 and COMB2; the outputs are

SUM1 and SUM2 respectively. The TAP of COMB1 is higher than that of COMB2; therefore COMB1 is used for

high resolution measuring, and COMB2 is used for low resolution but high speed measuring.

The TAP’s

of COMB1 and COMB2 are programmable, and can be set by TPS1 and TPS2 respectively; they

are shown in Table 16. The first zero-frequency is computed by taking 83.3 kHz as sampling frequency and

applies Equation 5. At TPS1=TPS2=11, the resolutions of high speed and low speed are 18-bit and 12-bit

respectively. The resolutions for other conditions are according to the actual measuring.

0

20

40

60

80

100

-200

-150

-100

-50

0

Hz