11..3.2.

Digital Filter

In Symbolic Diagram of the Delta-Sigma Analog-to-Digital Converter, 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 FS511 digital filter is:

Equation 2

 









2

2

s i n

s i n

1















S

S

f

f

f

f

N

N

f

H





Where N is TAP of the digital filter.

Suppose the sampling rate of the ADC is 83.3KHz, the TAP of the digital filter is 8192. We can find the

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

Equation 3

Hz

10

8192

Hz

83333

1







N

f

f

S

Z

The Frequency Response Diagram of FS511 Digital Filter

0

20

40

60

80

100

-200

-150

-100

-50

0

Hz

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

From above figure., 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 16384, the first zero-frequency is at 5Hz.

The output rate is selected by TPS [2:0].

TPS [2:0]

TAP (N)

ADC Output Rate and First Zero Frequency（Hz）

111

16384

5

110

8192

10

101

4096

20

100

2048

40

011

1024

80

010

512

160

001

256

320

000

128

640