Anti
-
Alias Filters
Using different filter selections (Wideband/Bessel IIR/Butterworth IIR/etc.) or different filter bandwidths can lead to phase mismatches between
channels.
Sample rate selection
Digital Filter (Anti-Alias)
A
F
y
(n)
n
-1
n
-1
Σ
x
(n)
z
-1
z
-1
Anti-Alias Sigma Delta ADC
1010
Analog input
A
F
1 out of N
Figure A.238:
Combined analog and digital anti
-
alias filter block diagram
Anti
-
aliasing is prevented by a steep, fixed frequency analog anti
-
alias filter integrated inside the Sigma Delta Analog to Digital Converter (ADC)
always sampling at a fixed sample rate. This setup avoids the need for other analog anti
-
alias filters.
Directly behind the ADC, the high precision digital filter is used as anti
-
alias protection before the digital downsampling to the desired user
sample rate is performed. The digital filter is programmed to a fraction of the user sample rate and automatically tracks any user sample rate
selection. Compared to analog anti
-
alias filters, the programmable digital filter offers additional benefits like higher order filter with steep roll
-
off, a larger selection of filter characteristics, noise
-
free digital output and no additional phase shifts between channels that use the same filter
settings.
Sigma Delta Wideband
When Sigma Delta wideband is selected there is always the anti
-
alias filter built
-
in the Sigma
Delta ADC (no digital filter) in the signal path. Therefore there is always anti
-
alias protection
when wideband is selected.
Bessel IIR
When Bessel IIR filter is selected, this is always a combination of the anti
-
alias filter built
-
in
the Sigma Delta ADC and a digital Bessel IIR filter to prevent aliasing at lower sample rates.
Bessel filters are typically used when looking at signals in the time domain. Best used for
measuring transient signals or sharp edge signals like square waves or step responses.
Butterworth IIR
When Butterworth IIR filter is selected, this is always a combination of the anti
-
alias filter
built
-
in the Sigma Delta ADC and a digital Butterworth IIR filter to prevent aliasing at lower
sample rates.
Best used when working in the frequency domain. When working in the time domain this
filter is best used for signals that are (close to) sine waves.
Elliptic IIR
When Elliptic IIR filter is selected, this is always a combination of the anti
-
alias filter built
-
in
the Sigma Delta ADC and a digital Elliptic IIR filter to prevent aliasing at lower sample rates.
Best used when working in the frequency domain. When working in the time domain this
filter is best used for signals that are (close to) sine waves.
Elliptic Band Pass IIR
When Elliptic Band Pass IIR filter is selected, this is always a combination of the built
-
in
anti
-
alias filter of the Sigma Delta ADC and a digital Elliptic Band Pass IIR filter.
Elliptic Band Pass filters are best used when working in the frequency domain. When
working in the time domain, this filter is best used for signals that are (close to) sine waves.
GEN3t
694
I3995-3.1 en HBM: public
