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ADS7809 oscillation, "CAP" pin capacitor, low ESR?

Kenneth Aquino1
Intellectual 280 points
Other Parts Discussed in Thread: ADS7809, ADS8509

I see on the oscilloscope oscillation (6 MHz, 0.5 volts peak-to-peak) on the analog input (the signal to be digitized is connected to this pin) to an ADS7809 16 bit ADC.  For troubleshooting purposes, the input to the ADC was isolated from its driving circuitry by removing a series resistor between it and the driving circuitry and the ADC input was tied to ground.  The oscillation was still there.  

I believe that the oscillation is due to instability in the internal amplifier that buffers the external reference (please see either the ADS7809 or its cousing the ADS8509 data sheet).  Specifically, there's a recommended 2.2 uF tantalum cap that i placed between the "CAP" pin and ground.  The block diagram shows the buffer amplifier driving that cap directly without an isolation resistor.  I believe that the ESR of the capacitor may be too low resulting in instability in the buffer amplifier.  I replaced the capacitor and the oscillation went away. 

Would anyone please comment on this?  Why does the data sheet specifically recommend a tantalum capacitor?  Is it because there needs to be a minimum value ESR that's necessary to keep the buffer amplifier stable (the alternative type of capacitor, the ceramic, has very low ESR).

Ken

  • 0
    TI__Expert 5705 points

    Hi Ken,

    You are correct. The inherently higher ESR of the Tantalum cap works like an isolation resistor and helps keep the internal opamp stable. If you're using a ceramic cap instead, a 1ohm series resistor is typically enough to prevent the opamp from oscillating.

    Thanks,
    Harsha

  • 0 in reply to Harsha Munikoti
    Intellectual 280 points

    Hi Harsha,

    thanks for replying.

    We use a tantalum cap, 2.2uF,  as recommended in the data sheet.

    However, as I wrote earlier, I see on the oscilloscope signs of oscillation.

    My theory is that the ESR is unusually low for this cap.  Recall that we replaced the 

    cap and the oscillation went away.

    Does any of TI's design notes for this part (published or unpublished) specify a minimum ESR?

  • 0 in reply to Kenneth Aquino1
    TI__Expert 5705 points

    Hi Ken,

    Unfortunately ESR vs. ref buffer stability is not something that has been characterized on this device. Based on empirical evidence a value in the 1 to 2 ohm range should be sufficient. 

    Thanks,

    Harsha