8 Analog Interface
8 – 4
8.2.2.1 Decimation Filter
The ADC’s anti-aliasing decimation filter contains two stages. The first
stage is a sinc
4
digital filter that increases resolution to 16 bits and
reduces the sample rate to 40 kHz. The second stage is an IIR low pass
filter.
The IIR low pass filter is a 10th-order elliptic filter with a passband
edge at 3.7 kHz and a stopband attenuation of 65 dB at 4 kHz. This
filter has the following specifications:
Filter type: 10th-order low pass elliptic IIR
Sample frequency: 40.0 kHz
Passband cutoff*: 3.70 kHz
Passband ripple: ±0.2 dB
Stopband cutoff: 4.0 kHz
Stopband ripple: –65.00 dB
* The passband cutoff frequency is defined to be the last point in the
passband that meets the passband ripple specification.
(Note that these specifications apply only to this filter, and not to the
entire ADC. The specifications can be used to perform further analysis
of the exact characteristics of the filter, for example using a digital filter
design software package.)
Figure 8.2 shows the frequency response of the IIR low pass filter.
0
–100
5000
–80
2000
–40
–60
–20
4400380032002600
FREQUENCY – Hz
LOG MAGNITUDE – dB
Figure 8.2 IIR Low Pass Filter Frequency Response
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