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[moved] can we avoid antialiasing filter?

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curious_mind

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Hi

I am pretty new to signal processing. The following are my queries

I have two analog inputs a and b , input ranging between 0-3V DC. All I have to do is to take ratio of this signal in a microcontroller. To filter out noise , I am planning to use 2nd order IIR filter with 10Hz cut off. Now should I use an anti-aliasing filter at all. Since my measurement is ratiometric, can I safely assume that aliased components will be nullified due to division of a/b (common mode)?
 

KlausST

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Hi,

It's the nature of alias frequencies, that they can be the whole range from DC to fs/2.

Thus you may attenuate all frequencies at the digital side above 10Hz with your filter, but if the alias frequency (frequencies) is in the range of 0Hz..10Hz you have no chance to suppress them on the digital side.
Depending on sample frequency and expected noise suppression ... a simple RC may be sufficient.

For more detailed answers we need the the specification of your measurement system.

Klaus
 

FvM

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Since my measurement is ratiometric, can I safely assume that aliased components will be nullified due to division of a/b (common mode)?
Not at all.

Firstly a/b ≠ (a+c)/(b+c)

Secondly, the microcontroller ADC inputs are sampled sequentially, each input sees a different aliasing signal phase and thus a different downsampled signal.

If you have relevant input signal components >fs/2, you need a low pass filter. For DC measurements, the cut-off frequency will be usually set much lower.
 

KlausST

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Hi,

Ratiometric neasurement:

Maybe you could connect the higher signal voltage to ADC_VRef and the other signal to the ADC input.
Then the ADC result directly shows the ratio.

Klaus
 

curious_mind

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Hi
If I use simultaneous sampling ADC, can I expect the common mode cancelled then?
 

KlausST

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Hi,

If the noise on both signal is in the correct ratio and if there is no phase shift....
Unlikely.

Klaus
 

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