SD16_A Operation
22-6 SD16_A
22.2.7 Digital Filter
The digital filter processes the 1-bit data stream from the modulator using a
SINC3 comb filter. The transfer function is described in the z-Domain by:
H(z)+ǒ1
OSR 1*z*OSR
1*z*1Ǔ3
and in the frequency domain by:
HǒfǓ+ȧ
ȱ
Ȳ
sincǒOSRpf
fMǓ
sincǒpf
fMǓȧ
ȳ
ȴ
3
+ȧ
ȡ
Ȣ
1
OSR
sinǒOSR p f
fMĂǓ
sinǒp f
fMĂǓȧ
ȣ
Ȥ
3
where the oversampling rate, OSR, is the ratio of the modulator frequency fM
to the sample frequency fS. Figure 222 shows the filters frequency response
for an OSR of 32. The first filter notch is at fS = fM/OSR. The notchs frequency
can be adjusted by changing the modulators frequency, fM, using
SD16SSELx and SD16DIVx and the oversampling rate using the SD16OSRx
and SD16XOSR bits .
The digital filter for each enabled ADC channel completes the decimation of
the digital bit-stream and outputs new conversion results to the SD16MEM0
register at the sample frequency fS.
Figure 222. Comb Filters Frequency Response with OSR = 32
140
120
100
80
60
40
20
0
Frequency
GAIN [dB]
fSfM