8Analog Interface
8 – 7
The high pass filter is a 4th-order elliptic filter with a passband cutoff
at 150 Hz. Stopband attenuation is 25 dB. This filter has the following
specifications:
Filter type: 4th-order high pass elliptic IIR
Sample frequency: 8.0 kHz
Passband cutoff: 150.0 Hz
Passband ripple: ±0.2 dB
Stopband cutoff: 100.0 Hz
Stopband ripple: –25.00 dB
(Note that these specifications apply only to this filter, and not to the
entire DAC. 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 8.3 shows the frequency response of the high pass filter.
8.3.1.2 Interpolation Filter
The DAC’s anti-imaging interpolation filter contains two stages. The
first stage is is an IIR low pass filter that interpolates the data rate from
8 kHz to 40 kHz and removes images produced by the interpolation
process. The output of this stage is then interpolated to 1.0 MHz and
fed to the second stage, a sinc
4
digital filter that attenuates images
produced by the 40 kHz to 1.0 MHz interpolation process.
The IIR low pass filter is a 10th-order elliptic filter with a passband
edge at 3.70 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 DAC. 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.)
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