Datasheet

MAX11040K/MAX11060
24-/16-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADCs
13
Maxim Integrated
Digital Filter
The devices contain an on-chip digital lowpass filter
that processes the data stream from each modulator
and generates the high-resolution output data. The low-
pass filter frequency response is determined by the
programmable output data rate. At the nominal 16ksps
output data rate, the -3dB bandwidth of the filter is
3.4kHz. The passband flatness is better than ±0.1dB
from 0 to 1.74kHz. The notches are located at 5.75kHz
and 7.195kHz. These frequencies scale linearly with the
output data rate. See Figure 2 and Table 1 for the fre-
quency response at different data rates.
Since the transfer function of a digital filter is repeatable
and predictable, it is possible to correct for frequency-
dependent attenuation in downstream software. See
the
Compensating for the Rolloff of the Digital Filter in a
Typical FFT Analysis section. The transfer function is
defined by the following equation:
where:
Gain is the filter gain.
f
AIN
is the analog input frequency.
f
SAMPLE
is the programmed output data rate, nominally
16kHz.
f
XINCLOCK
is the clock frequency at XIN, nominally
24.576MHz.
FIR_Gain (f
AIN
) is the normalized gain of the FIR filter
with the following filter coefficients, as a function of the
analog input frequency f
AIN
. These coefficients are
applied at the output data rate:
+ 0.022
- 0.074
- 0.036
+ 0.312
+ 0.552
+ 0.312
- 0.036
- 0.074
+ 0.022
Gain f
f
f
f
f
AIN
SAMPLE
AIN
SAMPLE
()
sin
=
××
⎛
⎝
⎜
⎞
⎠
⎟
π
XXINCLOCK
AIN
XINCLOCK
f
f
××
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎜
⎜
⎜
⎜
⎞
sin π
⎠⎠
⎟
⎟
⎟
⎟
⎟
×
()
3
_()FIR Gain f
AIN
Table 1. Bandwidth vs. Output Data Rate
OUTPUT DATA
RATE (ksps)
-3dB BANDWIDTH
(kHz)
-0.1dB BANDWIDTH
(kHz)
0.5 0.11 0.05
1 0.21 0.11
2 0.42 0.22
4 0.85 0.43
8 1.69 0.87
10 2.11 1.09
12 2.54 1.31
16 3.38 1.74
32 6.78 3.48
64 13.5 6.96
MAX11040K fig02
f
AIN
/f
SAMPLE
GAIN (dB)
0.240.200.160.120.080.04
-5
-4
-3
-2
-1
0
1
-6
00.28
Figure 2. Digital Filter Response