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PGA309: Noise/Ripple Problem

Mattia Dell'Oca
Prodigy 235 points

Part Number: PGA309

Hi,

I'm testing the PGA309 EVB with a high precision tester and I've found that the Output voltage is not very stable: it doesn't drift but last digit oscillates with an amplitude of 3mV (when signal is 3V). I'm using internal Vref - 4.096V.
Do you have any advice?

Best Regards,

Mattia

  • 0
    TI__Mastermind 27715 points

    Hi Mattia,

    I have two comments on this:

    1. Like any amplifier, the PGA309 has some amount of inherent voltage and current noise. The noise specs are referred to the input of the device, so if the PGA is configured for large gain, as it commonly is, this noise gets amplified and shows up as a significant component of the output signal. Art Kay wrote an excellent app report on how to filter the PGA309 noise, available here.
    2. Are you using an external temperature sensor or the built-in temperature sensor of the PGA309? In either case, is the temperature sensor in a stable thermal environment? Changes in ambient temperature will cause the PGA309 to adjust its fine gain and offset parameters, which can look like noise at the output.

    Best regards,

    Ian Williams
    Applications Engineer
    Precision Amplifiers

  • 0 in reply to Ian Williams
    Prodigy 235 points
    Hi Ian,

    thank you for your answers.
    For your first comment: I've read the application. What I don't understand and I'd like to know if an instability of 0,1% wrt full scale seen on an high precision tester agrees with PGA309 or is too high.
    For your second comment: I'm not using external temperature sensor and the enviroment temperature is very stable. Consider that the oscillation of 0,1% that I'm seeing is very fast, not slow.

    Best Regards,

    Mattia
  • 0 in reply to Mattia Dell'Oca
    TI__Mastermind 27715 points

    Hi Mattia,

    The goal of the total PGA309 system error after calibration is < 0.1% of full scale, so in a good system that should be achievable even factoring in any noise. Are you using any of the suggested methods in the application report, either a capacitor across the summing junction or an output RC filter, to reduce the output noise? Do you have any sort of filtering on your input signal?

    Best regards,

    Ian Williams

  • 0 in reply to Ian Williams
    Prodigy 235 points
    Hi Ian,

    I've tried with a 100nF capacitor across the summing junction and also with a RC output filter (with a cut frequency of 30Hz) but nothing relevant changed.
    I'm using the Ev. Board and I did not change the input filtering.

    Best Regards,

    Mattia
  • 0 in reply to Mattia Dell'Oca
    TI__Mastermind 27715 points

    Hello Mattia,

    Can you show an oscilloscope measurement of your output signal? Also, what is the total gain of your PGA in its current configuration, and are you using the coarse offset function? The amount of noise you're seeing may be the expected result depending on these settings.

    As the test results in the appendix of the application report show, in high gain configurations the measured output noise can be several millivolts RMS even after applying a 100 Hz low pass filter. If the coarse offset is enabled, the noise can be significantly higher.

    Best regards,

    Ian Williams