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ADS1262: IDAC current sharing

Simon Steinegger
Prodigy 170 points
Part Number: ADS1262


Hi there,

For our Pt-100 application we need to find a way to get an increased measurement current of 5mA. Using an on-chip IDAC is most convenient, but a single IDAC is limited to 3mA. So we would like to use both IDACs in parallel at 2.5mA. Does anyone have experiences in doing so? Are current sharing series resistors or even diodes mandatory? What value would be recommended?

Best regards

Simon

  • 0
    TI__Mastermind 44720 points

    Hi Simon,

    I haven't come across any circuits that use the ADS1262 IDACs in this way, but I'd don't see any reason why you couldn't do this.

    • I have seen that the absolute error of the IDACs may increase if both IDACs are operating at high current values. However, as long as you are able to measure your RTD ratiometrically, then this error shouldn't have much effect on the temperature measurement.

    • As long as you have analog inputs to spare, I'd probably recommend externally tying two of the analog pins together (instead of internally selecting the same analog pin for both IDACs) to reduce the power dissipation inside of the IC.

    • I don't think series resistors or diodes are mandatory, but they are always a good idea for protecting the analog input pins from electrical over-stress (EOS). If you use these in your circuit, just be sure to size them appropriately such that you still have plenty of headroom to satisfy the compliance voltage requirements for each IDAC.

    • The other thing I would take care to look out for is the power dissipation in the RTD and the reference resistors. If there is not good thermal conductivity to dissipate this heat energy, then you might see start to see some self-heating effects (i.e. resistance drift) which can degrade the measurement accuracy.

    I hope that helps.

  • 0 in reply to Christopher Hall
    Prodigy 170 points

    Hi Chris,

    Thanks for your reply. We'll follow your recommendations regarding external connections and take small (probably 22ohms) series resistors, which should be sufficient for the expected mismatch < 1% and not waste too much of our short headroom.The rest - we will see. Luckily we can modify an existing PCB quite easily.

    Thanks and best regards

    Simon