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ADS1262: External Reference Best Usage

Neal Gallagher16
Prodigy 110 points
Part Number: ADS1262

Hello!  Me again.

I have a circuit based on the ADS1262 where I am measuring temperature using a Wheatstone bridge (sample circuit attached).  I am configuring the circuit to be ratiometric (VinP-VinN)/(VrefP-VrefN).  My question is: now that I would like to use the internal 2.5V reference (REFOUT in the schematic) to excite the bridge, what is the best way to supply Vref?  As shown, I have a filtering cap on the reference input and I take traces from the top (Vref) and bottom (GND) of the bridge to the ADS1262's AIN5 and AIN6 pins.  Is this necessary? I know that it is possible to just configure the reference multiplexer to use the internal reference (REFMUX to 0) but I worry that by not directly sampling Vref in situ (i.e. - across the bridge), some fidelity in the conversion may be lost.  Any thoughts from the pros on this?

Thanks!

Neal

  • 0
    TI__Mastermind 44680 points

    Hi Neal,

    Welcome back!

    You can use the internal reference as the excitation source as long as you are pulling less than 10 mA (bridge impedance needs to be > 250 Ohms). If you need to source additional current, then you'll need to consider another source with a higher output.

    Since this measurement is ratiometic (i.e. the measurement is more immune to noise from the excitation source), you might consider using the analog supply as the excitation source. Increasing the excitation to 5V will provide you with a larger output signal from the bridge and may improve the overall resolution.

     

    Whatever you do end up using as the bridge excitation source, I would recommend routing the excitation voltage (from the bridge) to the reference input pin pairs (AIN0/1, AIN2/3, or AIN4/5). Note that AIN5 and AIN6 are not able to be used as reference inputs:

      

    As you said, the excitation voltage may encounter voltage drops between the excitation source and the bridge. By sensing the excitation voltage at the bridge and routing the excitation voltage back to the ADC, you'll get the best ratiometric matching.

    Does that answer you question?

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 110 points

    Hello Chris,

    Thank you very much for your reply!!  In total, the bridge will draw ~350uA, so we're good there.  While you are correct that increasing the bridge excitation voltage to the +5V in would halve the resolution (in °C), it also increases self heating error by a factor of 4.  The resolution will be easily recouped by increasing the gain (I do not have a wide measurement range).

    Thanks for the reminder about the pins available for reference!

    Regarding the measurement of excitation source, yes, perfect!  Thanks for your answer.

    Cheers,

    Neal

  • 0 in reply to Neal Gallagher16
    TI__Mastermind 44680 points

    Hi Neal,

    You're welcome! Let me know if have additional questions I can help with.

    Best regards,
    Chris