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ADS1261EVM: Sensor-excitation current source

Adrian Unkeles1
Prodigy 210 points
Part Number: ADS1261EVM
Other Parts Discussed in Thread: ADS1261

Hi,

In the ADS1261 EVM datasheet (section 9.3.6) it specifies using the IMUX1 and IMUX2 registers to connect the corresponding current sources to the analog inputs, and then specifies using IMAG1 and IMAG2 to set the corresponding current magnitude. Immediately thereafter it says "Enable the internal reference for current source operation".

I've looked through the registers (particularly the REF register) and don't see how to change the internal reference functionality. I'm testing the sensor-excitation current source with a 100Ohm resistor across AIN1 and AIN0 (with IMUX1 set to AIN1 and IMAG1 set to 100uA). I expect to see 10mV on scope probes that I place across the resistor, yet I see nothing. How do I configure the internal reference for current source operation?

Regards,

-Adrian

  • 0
    Prodigy 210 points

    Tried enabling the REFENB field on the REF register, this looks to be the register I was looking for...

    -Adrian

  • 0 in reply to Adrian Unkeles1
    TI__Mastermind 35520 points

    Hi Adrian,

    Thanks for quick update. Yes, the REFENB bit in Reference Configuration (REF) Register can be used to enable or disable the internal reference, the default condition for interface reference is disabled (REFENB = 0).

    Also, the ADC requires to enable the internal reference for current source (IDAC1 and IDAC2) operation.

    Please let me know if you need further support, thanks.

    Regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 210 points

    Hi Dale,

    The burnout current source is limited to a maximum output of 10uA (set by BCSMAG), while the sensor excitation current (IDAC) source is limited to 3000uA (set by IMAG1). Is it possible to use the IDAC source to drive an analog pin while simultaneously reading the voltage across that pin (setting MUXP to AIN1)?

    The setup I have is a 100 Ohm resistor across AIN0/AIN1, and driving 1000uA on IDAC1 into AIN1. On my oscilloscope I can see the expected 100mV signal across the resistor, but the ADS1261 fails to read the same. Notably, for the various current output settings, the ADS1261 consistently records ~20-25 mV less than I see on the oscilloscope. Can you help explain this behavior?

    Regards,

    -Adrian

  • 0 in reply to Adrian Unkeles1
    TI__Mastermind 35520 points

    Hi Adrian,

    When you use same IDAC current source for different circuity simultaneously, the circuits may interfere each other. I'm not sure which pin you are driving with IDAC. Can you provide your schematic? Also, please let me know what you set for current output and what you measured. 

    Thanks&regards,

    Dale  

  • 0 in reply to Dale Li
    Prodigy 210 points

    Hi Dale,

    Let me rephrase:

    ADS1261 register settings:

    Driving the AIN1 pin with IDAC1 (as set by IMUX1 register).

    Trying to sense the differential voltage across AIN1 and AIN0  (by setting registers MUXP to AIN1 and MUXN to AIN0).

    Driving 1000uA of current on IDAC1 (as set by IMAG1 register)

    I had enabled the internal reference (as set by the REFENB register)

    As for the schematic:

    All I have is a 100 Ohm resistor across AIN1 and AIN0.

    I am seeing the expected 100mV across AIN1 and AIN0 when I measure with an oscilloscope, but only 76.5mV is reported by the ADC. Where is this discrepancy coming from? For each of the other current settings (well, for all the settings above 100uA) I see a similar ~20-25mV loss between what the oscilloscope measures and what the ADS1261 reports. Thank you for helping diagnose this issue.

    -Adrian

  • 0 in reply to Adrian Unkeles1
    Prodigy 210 points

    All other registers were left at their default values. One exception: DR had been increased to 1,200 SPS in order to get sufficient readings for printout within a reasonable time (~2 seconds, with points/sample set at 2048 in Data Analysis tab).

    I hope this paints a complete picture.

    -Adrian

  • 0 in reply to Adrian Unkeles1
    TI__Mastermind 35520 points

    Hi Adrian,

    I apologized for late response. It's not easy to understand your whole circuit and test without a schematic. Is there any resistance between 100 ohm resistor and ADC's input? for example, RC filter or long lead wire? Are you doing RTD measurement or other circuits? Is there any specific reason you want to save one pin dedicated for IDAC current? usually a dedicated pin is designed for IDAC, the following circuit is an example for RTD measurement. Otherwise, any resistance on the input will lead to an unbalanced voltage drop.

    Regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 210 points

    Apologies for the delayed response on my end. The issue turned out to be due to incorrect grounding practices. Thanks for your responses.

    -Adrian