QAD7124-8 Should an anti-alias filter be used with the ADC?
AYes, an anti-alias filter is required. However, because a sigma delta converter
oversamples the analog input, the design of the anti-alias filter is greatly
simplified compared to an ADC that samples at the Nyquist rate (Maximum signal
Bandwidth * 2).
The AD7124-8 uses a digital filter. As the filter is digital, the frequency
response is reflected around the sampling frequency. This means that the filter
will provide 0dB of attenuation at frequencies which are integer multiples of
the sampling frequency. Therefore, an anti-alias filter in the analog domain is
required to adequately attenuate these frequencies; usually a single pole
(possibly a 2 pole) RC filter is all that is required. A 1KΩ resistor in series
with each analog input, a 0.1ìF capacitor from AINP to AINM, and a 0.01 ìF
capacitor from each analog input pin to AVSS is recommended when the analog
input channel is buffered.
When the converter is operated in unbuffered mode (gain = 1), the inputs look
directly into the sampling capacitor of the modulator. The modulator is
continually charging and discharging the sampling capacitor. If the time
constant of the anti-aliasing filter is too large, the modulator may be unable
to fully charge the sampling capacitor and gain errors will result. To prevent
the R-C combination from introducing errors, the R and C values used must be