### Q

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

### A

Yes, an anti-alias filter is required. However, because a sigma delta converteroversamples 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

limited.