Rainbow Electronics MAX1402 User Manual
Page 30
when valid data is available, a minimum of three data-
word periods later.
The digital filter can be bypassed by setting the MDOUT
bit in the global setup register. When MDOUT = 1, the
raw output of the modulator is directly available at DOUT.
Filter Characteristics
The MAX1402 digital filter implements both a SINC
1
(sinx/x) and SINC
3
(sinx/x)
3
lowpass filter function. The
transfer function for the SINC
3
function is that of three
cascaded SINC
1
filters described in the z-domain by:
and in the frequency domain by:
where N, the decimation factor, is the ratio of the modu-
lator frequency f
M
to the output frequency f
N
.
Figure 10 shows the filter frequency response. The
SINC
3
characteristic cutoff frequency is 0.262 times the
first notch frequency. This results in a cutoff frequency
of 15.72Hz for a first filter notch frequency of 60Hz. The
response shown in Figure 10 is repeated at either side
of the digital filter’s sample frequency (f
M
) and at either
side of the related harmonics (2f
M
, 3f
M
, . . .).
The response of the SINC
3
filter is similar to that of a
SINC
1
(averaging filter) filter but with a sharper rolloff.
The output data rate for the digital filter corresponds
with the positioning of the first notch of the filter’s fre-
quency response. Therefore, for the plot of Figure 10
where the first notch of the filter is at 60Hz, the output
data rate is 60Hz. The notches of this (sinx/x)
3
filter are
repeated at multiples of the first notch frequency. The
SINC
3
filter provides an attenuation of better than
100dB at these notches.
Determine the cutoff frequency of the digital filter by the
value loaded into CLK, X2CLK, MF1, MF0, FS1, and FS0
in the global setup register. Programming a different
cutoff frequency with FS0 and FS1 does not alter the
profile of the filter response; it changes the frequency of
the notches. For example, Figure 11 shows a cutoff fre-
quency of 13.1Hz and a first notch frequency of 50Hz.
For step changes at the input, a settling time must be
allowed before valid data can be read. The settling time
depends upon the output data rate chosen for the filter.
The settling time of the SINC
3
filter to a full-scale step
input can be up to four times the output data period.
For a synchronized step input (using the FSYNC func-
tion or the internal scanning logic), the settling time is
three-times the output data period.
H(f)
1
N
sin N
f
f
sin
f
f
M
M
3
=
π
π
H(z)
1
N
1
z
1 – z
N
1
3
=
−
−
−
MAX1402
+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
30
______________________________________________________________________________________
-160
-140
-100
-120
-80
-60
-20
-40
0
0
40 60 80
20
100 120 140 160 180 200
FREQUENCY (Hz)
GAIN (dB)
f
CLKIN
= 2.4576MHz
MF1, 0 = 0
FS1, 0 = 0
f
N
= 50Hz
Figure 11. Frequency Response of the SINC
3
Filter
(Notch at 50Hz)
-160
-120
-140
-100
-80
-60
-20
-40
0
0
40 60 80
20
100 120 140 160 180 200
FREQUENCY (Hz)
GAIN (dB)
f
CLKIN
= 2.4576MHz
MF1, 0 = 0
FS1, 0 = 1
f
N
= 60Hz
Figure 10. Frequency Response of the SINC
3
Filter (Notch at
60Hz)