MAX11040_10 Datasheet, PDF (11/32 Pages) Maxim Integrated Products – 24-Bit, 4-Channel, Simultaneous-Sampling,Cascadable,Sigma-Delta ADC

24-Bit, 4-Channel, Simultaneous-Sampling,
Cascadable, Sigma-Delta ADC
Digital Filter
The MAX11040 contains an on-chip digital lowpass fil-
ter 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.
1
0
-1
-2
-3
-4
-5
-6
0
0.4 0.8 0.12 0.16 0.20 0.24 0.28
fAIN/fSAMPLE
Figure 2. Digital Filter Response
Table 1. Bandwidth vs. Output Data Rate
OUTPUT DATA
RATE (ksps)
0.5
1
2
4
8
10
12
16
32
64
-3dB BANDWIDTH
(kHz)
0.11
0.21
0.42
0.85
1.69
2.11
2.54
3.38
6.78
13.5
-0.1dB BANDWIDTH
(kHz)
0.05
0.11
0.22
0.43
0.87
1.09
1.31
1.74
3.48
6.96
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:
Gain(f_AIN)
=
⎛
⎜
⎜
⎜
⎜
⎝
f_SAMPLE
f_XINCLOCK
×
×
⎛
sin_⎝⎜
π
⎛
sin_⎝⎜
π
×
×
f_AIN
f_SAMPLE
⎞
⎠⎟
f_AIN
f_XINCLOCK
⎞
⎠⎟
⎞³
⎟
⎟
⎟
⎟
⎠
where:
Gain is the filter gain.
f_AIN is the analog input frequency.
f_SAMPLE is the programmed output data rate, nomi-
nally 16kHz.
f_XINCLOCK is the clock frequency at XIN, nominally
24.576MHz.
FIR_Gain (f_AIN) is the normalized gain of the FIR fil-
ter with the following filter coefficients, as a function
of the analog input frequency f_AIN. These coeffi-
cients are applied at the output data rate:
+ 0.021484375
- 0.072265625
- 0.035156250
+ 0.304687500
+ 0.539062500
+ 0.304687500
- 0.035156250
- 0.072265625
+ 0.021484375
Latency
The digital filter determines the latency of the
MAX11040. Latency is defined as the time between the
effective point in time that a sample is taken and when
the resulting digital data is available for reading.
The latency of the converter is specified by the follow-
ing equation:
Latency = (6 x t_DOUT) + (PHI x 1.3µs) + 30µs
where t_DOUT is the data output period (inverse of the
programmed sample rate), determined by f_XINCLOCK
and the selected output data rate, and PHI is the pro-
grammed sampling instant delay for the channel in
question (0 ≤ PHI ≤ 255). The latency is approximately
405µs at 16ksps.
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