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Design >> Mixed-Signal Design >> Do we need a digital filter following a SAR ADC
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Message started by zrsh001 on Dec 11^th, 2012, 9:16pm

Title: Do we need a digital filter following a SAR ADC
Post by zrsh001 on Dec 11^th, 2012, 9:16pm

I am a freshman for ADC design.
when i use a SAR ADC with sample clock about 2.4 MHz for input signal, whose bandwidth is 100 KHz. We have OSR=12. When finished adc conversion, do we need a digital filter after ADC ?
The quantization noise is spread about(0, 1.2MHz) while signal bandwith is only about 100KHz. If we use a digital filter, we can suppress the noise from 100K to 1.2MHz and SNDR will increase a little bit. Is it correct ?

Title: Re: Do we need a digital filter following a SAR ADC
Post by carlgrace on Jan 4^th, 2013, 3:38pm

Yes if you're oversampling you need a digital filter to gain the benefits of the oversampling. Since you're not doing any noise shaping all you need to do is add up every 12 output data words and divide by 12.

Title: Re: Do we need a digital filter following a SAR ADC
Post by sheldon on Jan 5^th, 2013, 9:28pm

ZRSH001,

Unless you are doing something special with the SAR ADC you do not
need a digital filter. For a SAR ADC with oversampling, you need to
average the outputs. Store the value of out[n] in a register and add it
to the output of the current conversion.
out[n]=out[n-1]+ADCOUT

When you get done keep the top X number of bits that you need,
where X is the number of bits of the ADC.

Unlike an oversampling ADC, oversampling of a Nyquist-Rate ADC
provides limited improvements in the dynamic range. For example,
a 2x oversample rate provides a 3dB improvement, a 4x oversample
rate provides a 6dB improvement.

Note: this behavior is different than the behavior of a Delta-Sigma
ADC.

Best Regards,

Sheldon