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TIDA-01526: Anti-aliasing filter question

Nikos Agianniotis
Prodigy 120 points
Part Number: TIDA-01526
Other Parts Discussed in Thread: ADS122U04

I am very confused regarding the selection of the anti-aliasing filter cut-off frequency. I see there are 100 Ohm series resistors and 4.7 nF CM capacitors. That creates a CM filter with cut-off frequency of 338.6 kHz. In my opinion, that is way too high. The modulation frequency of the ADC is 256 kHz at normal mode. Even if we assume the ADC works at its maximum sampling rate of 2 KSPS, the 338.6 kHz is much higher than what is recommended in the datasheet. There it is mentioned that the anti-aliasing filter should be set at around the sampling rate or 10 times higher.

  • 0
    TI__Mastermind 34901 points

    Hi Nikos,

    For choosing the RC filter components, the following are some general recommendations:

    Select the smallest possible R value that will limit your input current to 10mA at the maximum expected overvoltage. You want some margin of course. If you want to protect for voltages up to 40V, you would for example use a 4.7kOhm or larger series resistor. If overvoltage protection is not important then you would start with 1kOhm or 470Ohm. Keep in mind that the larger the filter resistors, the more offset errors will be introduced due to the input leakage currents of the ADC.

    Select a differential cap value to get a corner frequency at least 1x decade below the modulator frequency (fMOD =  256kHz for ADS122U04, as you have noted) so that you get at least 20dB attenuation at the modulator frequency from the RC filter. The lower the corner frequency of the RC filter, the longer you may have to wait for the input signals to settle when you change the Mux channel. Means for fast channel scanning you should use a higher corner frequency in general.

    The cutoff frequency of the CM filter is generally set to 10x the differential filter cutoff. So if you set fc(diff) = fMOD/10 = 25.6 kHz, then fc(CM) = fc(diff)*10 = 256 kHz

    I am not sure why those specific values were chosen for the reference design in question, but you can see an example of the recommendations above in the ADS122U04 EVM image below. Note that the yellow are the input filter resistors, the red circle is for the differential capacitors, and the blue circle is for the CM caps.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 120 points

    Hi Bryan,

    thank you for taking the time to answer my question and for your recommendations. I will for sure follow them.

    Best regards,
    Nikos