Texas Instruments MSP430x4xx - Manual (Page 409)

SD16 Operation
21-6 SD16
21.2.6 Digital Filter
The digital filter processes the 1-bit data stream from the modulator using a
SINC
3 comb filter. The transfer function is described in the z-Domain by:
H(z) +
ǒ
1
OSR
1 * z *OSR
1 * z *1
Ǔ 3
and in the frequency domain by:
HǒfǓ +
ȧ
ȱ
Ȳ
sinc
ǒ
OSRp f
fM
Ǔ
sinc
ǒ
p f
fM
Ǔ ȧ
ȳ
ȴ 3
+
ȧ
ȡ
Ȣ
1
OSR
sin
ǒ
OSR p f
fM
Ă
Ǔ
sin
ǒ
p f
fM
Ă
Ǔ ȧ
ȣ
Ȥ 3
where the oversampling rate, OSR, is the ratio of the modulator frequency f
M
to the sample frequency f
S
. Figure 21−2 shows the filter’s frequency response
for an OSR of 32. The first filter notch is at f
S
= f
M
/OSR. The notch’s frequency
can be adjusted by changing the modulator’s frequency, f
M
, using
SD16SSELx and SD16DIVx and the oversampling rate using SD16OSRx.
The digital filter for each enabled ADC channel completes the decimation of
the digital bit-stream and outputs new conversion results to the corresponding
SD16MEMx register at the sample frequency f
S
.
Figure 21−2. Comb Filter’s Frequency Response with OSR = 32
−140
−120
−100
−80
−60
−40
−20
0
Frequency
GAIN [dB]
f
S
f
M