Anti
-
Alias Filters
Using different filter selections (Wideband/Bessel/Bessel IIR) or different filter bandwidths can result in phase mismatches between channels.
FLASH ADC
Sample Rate Selection
Analog Input
Digital Filter (Anti-Alias)
1 out of N
A
F
Analog Anti-Alias Filter
1010
A
F
y
(n)
n
-1
n
-1
Σ
x
(n)
z
-1
z
-1
Figure A.16:
Combined analog and digital anti
-
alias filter block diagram
Anti
-
aliasing is prevented by a steep, fixed frequency analog anti
-
alias filter in front of the Analog to Digital Converter (ADC). The ADC always
samples at a fixed sample rate. The fixed sample rate of the ADC avoids the need for different analog anti
-
alias filter frequencies.
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 supports a range of fixed bandwidth anti
-
alias filters. 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.
Wideband
When wideband is selected, there is neither an analog anti
-
alias filter nor any digital filter
in the signal path. Therefore, there is no anti
-
alias protection when wideband is selected.
Wideband should not be used if working in a frequency domain with recorded data.
Using wideband, enhanced resolution is not supported at lower sample rates.
Bessel (Fc @
-
3 dB)
This analog Bessel filter can be used to reduce the higher bandwidth signals, but is also
used to prevent aliasing at the 100 and 50 MS/s sample rates. For lower sample rates, the
digital Bessel IIR filter must be used to prevent aliasing. Bessel filters are typically used
when looking at signals in the time domain. They are best used for measuring transient
signals or sharp edge signals like square waves or step responses.
Using the Bessel filter, enhanced resolution is not supported at lower sample rates.
Bessel IIR (Fc @
-
3 dB)
When Bessel IIR filter is selected, this is always a combination of an analog Bessel anti
-
alias filter and a digital Bessel IIR filter to prevent aliasing at lower sample rates. This can
only be used for sample rates up to 50 MS/s. Bessel filters are typically used when looking
at signals in the time domain. They are best used for measuring transient signals or sharp
edge signals like square waves or step responses.
Enhanced resolution is supported by using over sampling combined with a digital filter at
the following sample rates: 15 bit resolution at 25 MS/s and lower, 16 bit resolution at
10 MS/s and lower.
GEN3i
I3763-3.1 en HBM: public
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