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ADS1247/8 IDAC accuracy when multiplexing

Martin Drinnan
Prodigy 20 points
Other Parts Discussed in Thread: ADS1247, ADS1248, ADS1248EVM-PDK

Hello.

The ADS1247/8 data sheet lists the IDAC initial current error (min=-6%, typ=1%, max=6%) characteristics on page 4 of the datasheet. Does this error apply only to turning an IDAC on initially, or does it also apply whenever the IDACs are multiplexed to different AINs? The IDAC current would remain at 50uA. For the application, it would be important for there to be a minimum of excitation current error after multiplexing to a different output pin.

Thanks for any thoughts.

  • 0
    TI__Guru* 97465 points
    Martin,


    The 6% error refers to the absolute error that you would see for the IDAC for use in any of the possible outputs. It's not meant to indicate a current error at initial start-up or routing from pin-to-pin. In ratiometric measurements, the absolute error is generally not of great concern.

    Generally, there isn't any starting delay routing the current through the multiplexer from pin to pin. However if you're measuring voltages, capacitance on any of the given pins may cause some settling time the current is routed to resistance.

    Out of curiosity, what is your application? There may be more too add depending on how you want to set up your system.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Hello Joseph,

    Thanks for the quick response. I think I understand what you're saying here, but the datasheet does say "initial error". So that metric is actually referring to absolute error? That seems like something which might warrant a small revision of the datasheet.

    I'm an intern at our company, and this layout was quickly adapted from our previous layout, so please excuse the crossed wires.

    We are using 4-wire measurement of a thermistor for temperature sensing. Pins 9 & 10 are used as excitation sources. I was hoping to upgrade to the ADS1248 and sample two thermistors at once, using all 8 of the AINs. The IDACs would have to be muxed to separate pins to measure the second thermistor voltage, hence my initial question.

    Thanks!

    Martin

  • 0 in reply to Martin Drinnan
    TI__Guru* 97465 points
    Martin,


    Thanks, we'll consider that change in the datasheet. Normally, changes to the datasheet can be pretty difficult depending on what we need to change, but this particular datasheet is slated for an update anyway.

    What you're describing is a pretty common implementation. You would switch from one thermistor to another thermistor for the excitation current and the measurement.

    There was a recent post about making measurements with this device, and I wrote a list of application notes for this part. You can find this post here:

    e2e.ti.com/.../1488932

    The first three application notes discuss four-wire RTDs and how to set up the measurement.

    One other thing you could do would be to get the ADS1248EVM-PDK and try some sample measurements yourself. With a four-wire implementation, you shouldn't have to worry about the filter resistors on the input causing additional error from the excitation current.


    Joseph Wu
  • 0 in reply to Joseph Wu
    TI__Guru* 97465 points
    Martin,


    Re-reading through my reply, you do have to consider any capacitance slowing down the reading in the RTD. This would include the input filter capacitance and how that affects the voltage settling in the reading.

    Often there's a capacitance at the input of the reference in parallel with the reference resistor. You'll have to consider that settling as well.


    Joseph Wu