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TI Precision Lab_SAR

Yan Fan
Intellectual 360 points
Other Parts Discussed in Thread: ADS7042

Dear TI Precision lab, 

While I was studying the material regarding Introduction to SAR ADC of TI precision lab SAR, I got a question about the bucket capacitor and Resistor as shown in the attachment. 

I believe this signal chain is after the anti-aliasing filter and the purpose of Cfilt and Rfilt is to overcome the spiky response from the sampling capacitor and better for the settling. But if the calculated RCfilt corner frequency is not the same as the anti-aliasing filter, for example, smaller or larger than that. Whether it will truncate the signal bandwidth or introduce aliasing? 

Thank you very much, 

B.Fan

  • 0
    TI__Mastermind 24288 points

    B.Fan,

    You are correct that the filter (Cfilt and Rfilt) is to overcome the charge kickback (i.e. spiky response) from the ADC.  However, this filter is not an anti-aliasing filter.  Some literature refers to this filter as anti-aliasing, but that is actually incorrect.  The reason that it will not work as an anti-aliasing filter is that in order to get good settling the cutoff of this filter is generally going to be higher than the Nyquist frequency.  In the example you show, the cutoff of the filter is 1.98MHz and the maximum sampling rate for the ADS7042 is 1Msps (Nyquist = 500ksps).  For the filter to be an effective anti-aliasing filter, it needs to have a cutoff lower that the Nyquist frequency.  For example an effective anti-aliasing filter may be a second order filter with a 50kHz cutoff.  This filter will have a 40dB attenuation at 500kHz so any alias signals will be significantly attenuated.  I suggest looking at Anti-aliasing, and Anti-aliasing hands-on videos to understand this better.

    Thanks for your interest.