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Re: Aliasing in sigma-delta ADC output?

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Hi Alexxx,

 

With a sigma delta ADC, the point of aliasing will be at the output data rate/2.

 

When FS = 480 (sinc4 filter, chop off), the first notch is at 10 Hz. If you continuously convert on a single channel (zero latency off), conversions are available at 10 Hz. Any tones at 10 Hz are aliased back to dc. However, the notch at 10 Hz means that there is no aliasing back to dc.

 

If the above configuration is used and zero latency is turned on, the output data rate is reduced to 2.5 Hz when the sinc4 filter is used with chop off. The filter response remains unchanged so there is no attenuation at 1.25 Hz.

 

What data rate do you require in your application? I am aware that loadcells have tones due to mechanical vibration in the 20-40 Hz band. If you use the AD7195 with an output data rate of 10 Hz, for example, the filter provides over 40 dB of rejection above 10 Hz. This should be sufficient to attenuate the tones due to mechanical vibration.

 

We use sinc filters with the low speed sigma delta ADCs as the sinc filter has low latency. In our wideband sigma delta ADCs, we have used FIR type filters which give a response similar to brickwall filters, i.e. over 90 dB attenuation above output data rate/2. This type of filter does have a longer latency though. The AD7764 is such a part.

 

Regards,

 

Johnny


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