MAX1400_02 Datasheet, PDF (27/34 Pages) Maxim Integrated Products – +5V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC

+5V, 18-Bit, Low-Power, Multichannel,
Oversampling (Sigma-Delta) ADC
Digital Filter
The on-chip digital filter processes the 1-bit data
stream from the modulator using a SINC³ or SINC³ fil-
ter. The SINC filters are conceptually simple, efficient,
and extremely flexible, especially where variable reso-
lution and data rates are required. Also, the filter notch
positions are easily controlled, since they are directly
related to the output data rate (1 / data word period).
The SINC¹ function results in a faster settling response
while retaining the same frequency response notches
as the default SINC³ filter. This allows the filter to settle
faster at the expense of resolution and quantization
noise. The SINC¹ filter settles in one data word period.
With 60Hz notches (60Hz data rate), the settling time
would be 1 / 60Hz or 16.7ms whereas the SINC³ filter
would settle in 3 / 60Hz or 50ms. Toggle between these
filter responses using the FAST bit in the global setup
register. Use SINC¹ mode for faster settling and switch
to SINC³ mode when full accuracy is required. Switch
from the SINC¹ to SINC³ mode by resetting the FAST
bit low. The DRDY signal will go false and will be
reasserted 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 MAX1400 digital filter implements both a SINC¹
(sinx/x) and SINC³ (sinx/x)³ lowpass filter function. The
transfer function for the SINC³ function is that of three
cascaded SINC¹ filters described in the z-domain by:
H(z)
=


1
^N
1− z^−N
1− z^−1
³


and in the frequency domain by:


H(f)
=
¹


N


sin_Np
f
f_M


³



sin_ p
f
f_M





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
SINC³ 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 SINC³ filter is similar to that of a
SINC¹ (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
SINC³ 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 differ-
ent cutoff frequency with FS0 and FS1 does not alter
the profile of the filter response; it changes the frequen-
cy of the notches. For example, Figure 11 shows a cut-
off frequency 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 SINC³ filter to a full-scale step
0
f_CLKIN = 2.4576MHz
-20
MF1, 0 = 0
FS1, 0 = 1
-40
f_N = 60Hz
-60
-80
-100
-120
-140
-160
0 20 40 60 80 100 120 140 160 180 200
FREQUENCY (Hz)
Figure 10. Frequency Response of the SINC³ Filter (Notch at
60Hz)
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