[SOLVED] Do we need an anti aliasing filter in front of a delta-sigma ADC?

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CHL

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Hello

I have a question about anti-aliasing filter (maybe stupid question)

If the sampling frequency is very high comparing to the input signal frequency, basically there is no aliasing, right?

A delta-sigma ADC is an oversampling ADC, so the sampling frequency is high depending on OSR.

I don't understand why an anti aliasing filter is used in front of the delta-sigma ADC.

Is it because of input noise? or any other reasons?
 

CHL said:
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I don't understand why an anti aliasing filter is used in front of the delta-sigma ADC.

Is it because of input noise? or any other reasons?
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Yes, it's for noise.
Any noise above 1/2 the sample frequency will be aliased into the passband.
But all you generally need is a simple one-pole RC low-pass filter with a corner frequency some above the highest signal frequency to perform the anti-alias function.
 
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    CHL

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In addition, there are two flavor of delta sigma ADC. Discrete time and continuous time.

Continuous time has inherent anti-aliasing.
 
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    CHL

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Hi,

I don't call it noise. For me noise is a random signal with random frequencies.

The anti aliasing filter is for any signal above f_sample/2. Noise or any other signal.
And yes, I always recommend to use an anti aliasing filter.

In your case, where the sampling frequency is far above your signal of interest...it's just a simple RC.

Imagine.
Usually in a delta sigma ADC there is a (digital) low pass filter. It attenuates frequencies between datarate/2 up to f_sample/2.
But a signal above f_sample/2 will be aliased.
It may be real noise, some influence (noise) from an SMPS, influence from bluetooth, cellular phone, WiFi...
--> an anti aliasing filter definitely is useful.

Klaus
 
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