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AD7124-8: Should an anti-alias filter be used with the ADC?

Q.  Should an anti-alias filter be used with the ADC?

A.  Yes, 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 0.1μF capacitor from AINP to AINM, and a 0.01 μF 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 limited.