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External VREF problem with ADS 129x - signal drift and hold

James Don
Prodigy 80 points
Other Parts Discussed in Thread: ADS1292, OPA350

Hello,


I am having difficulty in setting an external VREF to the ADS129x (ADS1292). When I connect an external VREF of 240 mV (my signal is in the range of 0.1mV-1 mV, so trying to get a large gain by using a low VREF value), with unit gain, the ECG signal constantly drifts and gets stuck at certain arbitrary values every few seconds (the values are much lower than the 2^24 and arbitrary). The signal re-appears after few seconds.

With the above set-up, if I choose to user the internal voltage reference (say the setting for 2.4V), I get the same issue.


The external VREF is generated using an Opamp (OPA350). If this OPAMP is removed, the above system works well in getting ECG using the internal reference. Also, the acquisition is good if a voltage generator (desktop) is connected to VREFP to generate the 240 mV instead of the OPAMP.

Couple of questions:


-- Is it possible to use the internal voltage reference, if the VREFP is connected to 240mV and VREFN is GND ? In this case, what is the voltage reference range of the ADC ? (is it 2.4 V - 240mV ?). If so, why I get this drift and stuck up ?


-- Even if my VREF generated by the OPAMP is noisy, I am not sure why this pattern of drift-stuck-up-and recovery of ECG signal happens. Any ideas ?

I have baseline, notch filters etc in software and the drift-stuck-up seems unrelated to other noise sources.

thanks !

  • 0
    TI__Mastermind 38745 points
    Hey James,

    You should not simultaneously have an external reference connected to the VREFP pin while the internal reference buffer is powered. That will cause a short circuit between the two buffers. In addition, VREFP is measured with respect to VREFN which should always be connected to AVSS. If you're using a single supply, this should also be GND, but I want to be clear on that in case you are using a bipolar supply scheme.

    For me to get a sense of what may be happening when you are using the external reference exclusively, can you please post your schematic and list your full register settings?

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 80 points

    Hi Brian,

    Thanks for the clarification on the internal reference buffer. Here is the schematic (the lower part with OPA350 is generating the 240 mV VREF) and register setting:

    ads1292_writeRegister(0x1, 0x01); //sampling frequency: 250Hz
    ads1292_writeRegister(0x2, 0x80); //Internal Vref disabled
    ads1292_writeRegister(0x3, 0x10); //Default
    ads1292_writeRegister(0x4, 0x60); //CH1SET: Gain=12, normal electrode input// Have also used by 1-12
    ads1292_writeRegister(0x5, 0x60); //CH2SET: Gain=12, normal electrode input
    ads1292_writeRegister(0xA, 0x03); // disable offset calibration - RLDREF is fed internally
    ads1292_writeRegister(0xB, 0x00); // Configure GPIO 1 and 2 as outputs
    regards
  • 0 in reply to James Don
    TI__Mastermind 38745 points
    Hey James,

    I don't see anything wrong with the setup. Given that the system works fine with both the internal reference and a lab power supply generated external reference, I would look into the reference circuit. It looks ok to me on the schematic, but I recommend probing it with a scope and looking for any strange behavior.

    Regards,
    Brian Pisani