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ADS7066: RC(kickback) filter selection

Mark Philip Felipe
Genius 10850 points
Part Number: ADS7066

Hi Team, seeking some support.

Using the ADS7066 for a data acquisition application. I have a MAX41400 amplifier for 10uV-10mV(5Hz-5KHz) signal amplification and this goes to the RC(kickback) filter before the input of the ADC. the value of the RC filter is chosen with the help of Analog Engineer's Calculator app from ti and the values used are 51ohm and 620pF.


I am using two inputs of the ADC; channel 3 and channel 4. And I removed the RC on channel 3 and the signal is much better than on the channel with the RC.

I have attached images from a custom GUI which plots the data(10Hz) and the first image is without the RC filter and the second image is with the RC filter circuit.
As you can see the input to the channel without RC filter is much cleaner than the one with RC. it gets worse as I increase the frequency.
Do you know why this happens? Is it because of the values I have chosen? Or do you think it's better not to use the RC filter or do you think using the right RC filter might again improve the signal being recorded?

Thank you.

-Mark

  • 0
    TI__Expert 3785 points

    Hi Mark,

    I see that the amplifier has 28kHz bandwidth. This means the amplifier’s output impedance will be comparable to the external resistor at >28kHz. I could not find the output impedance vs frequency curve in the amplifier datasheet.

    You can try one of these two things:

    1. Reduce external R while ensuring the amplifier is stable. 
    2. Reduce the sampling rate (speed) of the ADC to allow the amplifier time to settle through the RC filter.

    Regards,

    Rahul

  • 0 in reply to RahulVK
    TI__Genius 10850 points

    HI Rahul, 

    I tried the following and didn't make the signal better.
    1. Changed the 51 ohm to 33 ohm
    2. decreased the sampling rate to 10.4ksps (which is the min)
    The signal is the same as before.
  • 0 in reply to Mark Philip Felipe
    TI__Expert 3785 points

    Hi Felipe,

    I recommended reducing external R and/or reducing speed as removing the external RC external filter resulted in better readings from the ADC.

    The calculator for ADC drive circuit assumes that the amplifier has higher unity-gain-bandwidth as compared to the RC filter corner. This is not the case here as the amplifier has 28kHz bandwidth whereas the RC filter corner is at 8.06 MHz (R = 33 and C = 620pF).

    Can you verify that the amplifier is stable with driving the 620pF capacitor?


     Regards,

     Rahul