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ADS1263EVM-PDK: Negative PGA rail issue while measuring a floating sensor

Olivier Bergeron25
Prodigy 130 points
Part Number: ADS1263EVM-PDK
Other Parts Discussed in Thread: ADCPRO, ADS1262, ADS1263

Hi,

I am having an issue with the evaluation board ADS1263EVM-PDK and the ADCPro software. The evaluation card is us to make a proof of concept for a medical device. There is three electrodes that are connected to IN2, IN3 and GND terminals.

Each time I use the electrod and sampling some data like my heart beat, I am always getting a PGA negative rail fault even if the data is between the PGA rails.

The evaluation card is supply in bipolar (±5V) and the internal reference is used.

The ADCPro is setup for sampling AIN2 and AIN3. The PGA Gain is set to 1V/V.

The sampling data magnitudes are between 3.9mV and 4.7mV.

If I use a signal source, I am able to sample a DC signal from -2.2V to +2.2V without the negative rail error.

I have attached the sampled heart beat signal exported to an excel file.

Any idea?Heart beat.xlsx

Thanks

Olivier Bergeron

  • 0
    TI__Mastermind 44600 points

    Hi Olivier,

    Take a look at the ADS1262 and ADS1263 Excel calculator tool here for checking if your input signal is within the PGA's common-mode range: http://www.ti.com/tool/ads126x-calc-tool

    If you have bipolar supplies available (+/-5V), are you supplying AVDD and AVSS with +2.5V and -2.5V, respectively?
    If AVSS is 0V, then your input voltages will need to beat least >0.3V to prevent the PGA outputs fro saturating into the rails. Using a +/- 2.5V bipolar analog supply might resolve this issue.

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 130 points

    Hi Christopher,

    AVDD is +2.5V and AVSS is -2.5V. 

    When I measured the ADC inputs signal, I got around -30mV on each terminal versus GND which is within the rails.

    But I did some sampling test with my multimeter connected between an ADC input and the GND, I did not get the error message. The add of the multemeter in the circuit is enough to correct the problem.

    So, probably my electrodes impedance is too high. I got two electrodes to capture the differential signal and a third one that is used as a reference. The reference is connected to ADS1263EVM ground. If the electrodes impedance is too high, probably it is not sufficient to bring the analog inputs around 0V. This is probably why it gets out of the rails with the electrodes and not with a low impedance signal source.

    I measured an electrode impedance of 300kR with a multimeter.

    Olivier Bergeron

  • 0 in reply to Olivier Bergeron25
    TI__Mastermind 44600 points

    Hi Olivier,

    It sounds like your electrode voltages are floating with respect to the ADC ground. Adding the DMM to the circuit is placing a large resistance between your signal and ground; however, it is enough to keep your signal from floating outside of the PGA's input range.

    I'm not too familiar with ECG applications, but as I understand it, there is typically a driving source somewhere to bias the patient/electrodes to some common-mode voltage. Here are some references:

    http://www.ti.com/lit/pdf/sbaa196

    https://e2e.ti.com/support/data-converters/f/73/t/314891

    https://e2e.ti.com/support/data-converters/f/73/p/681161/2523108#2523108

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 130 points

    Hi Christopher,

    Thanks for the reply.

    I have a question on a different topic, but still about the ADS1263EVM. I'm having three inputs (IN4 to IN6) on my evaluation card that are stick to -2.5V.

    These voltage levels appear at power up without an ADCPro configuration. Even if a configuration is programmed to the evaluation card (without DAC, IDAC or GPIO activated), these levels are still at -2.5V. These three inputs are actually not connected to any external circuit. The inputs should be bring to the GND level by the two commun mode capacitors on the EVM input circuitry.

    I didn't see anything in the schematic that could explain these voltage levels.

    Could these inputs be damaged?

    Thanks

    Olivier Bergeron

  • 0 in reply to Olivier Bergeron25
    TI__Mastermind 44600 points

    Hi Oliver,

    Do you have the JP1 and JP3 jumpers populated?

    The IN4-6 inputs have some additional circuitry connected to them to allow for an easy connection to an RTD:

    • When JP3 is connected, the IN5 input will be pulled down in AVSS (which I assume you have configured for -2.5V).
    • IN4 is connected to IN5 through R17, so IN4 will get pulled low as well.
    • When JP1 is connected, the IN6 input ill be connected to IN4.

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 130 points
    Hi Christopher,

    You are right, the jumpers are populated. I assumed there were not populated by default.

    Thanks

    Olivier