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Application questions for ADS1247 - accurate resistance measurement

Charles Allen
Prodigy 40 points
Other Parts Discussed in Thread: ADS1247

Hi,

I'd appreciate any advice on the suitability of the ADS1247 for an application I have in mind. The application is skin resistance measurement - measured by applying a maximum of 0.5V across two electrodes. The resistance range I need to cover is 20KOms to 1MOhm. I envisage using the reference voltage to supply a 0.5V (via series resistors) to the measurement contacts.

Would the ADS1247 be appropriate for this? I was envisaging using a modified version of the 2-wire RTD application from here: http://www.ti.com/lit/an/sbaa180/sbaa180.pdf

Thanks for your advice,

Charles.

  • 0
    TI__Guru** 113765 points

    Charles,

    Welcome to the forum!  You can use 0.5V for the reference, but it is not clear to me how you want to connect this up.  The 2-wire RTD application uses a current source to generate the voltage for both the reference and the measurement of voltage dropped across the RTD.  This works as the RTD is much less in resistance than the Rbias resistor that provides the reference.  You will not be able to use the IDAC current sources to drive 20K to 1M resistances as the smallest IDAC current is 50uA.

    You will also need to be careful of the common mode input range restriction, so using 0.5V generated by Rbias will most likely be a problem if the reading is close to ground, or if you use any gain.

    So, if you could give us a schematic of how you would like to connect the sensor, we may be able to help more.  Applying a voltage to a varying resistor will always be the applied voltage, but the current will vary.  The ADS1247 measures voltage.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    Thanks for the quick response Bob. You're right - I hadn't fully thought through how the circuit would work and was thinking of using the IDAC to supply the current like in the RTD application. I see now that this won't work.

    I'll work up a schematic based, I think, on a wheatstone bridge and post that here for your feedback.

    Thanks,

    Charles.

  • 0 in reply to Charles Allen
    Prodigy 40 points

    Hi,

    Would using a simple circuit like the attached work OK with the ADS1247?

    Charles

  • 0 in reply to Charles Allen
    TI__Guru** 113765 points

    Charles,

    The PGA has a high impedance input, especially at the lower data rates, so the configuration should be ok.  With the high values of resistance you are using, you might be pretty susceptible to noise, like 50/60 Hz, EMI/RFI, etc..  You may have to add some filtering to eliminate possible noise.  When adding any filtering using capacitors, you will need time to settle, and with the large resistances it could take a while.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 40 points

    Thanks Bob - very helpful.

    In terms of filtering, would a 10uF (or larger?) capacitor across the inputs to the PGA be sufficient?

    Charles.

  • 0 in reply to Charles Allen
    TI__Guru** 113765 points

    Charles,

    10uF will have a large time constant with 100k resistors, so that will take a long time to settle.  You might try something around 10nF.

    Best regards,

    Bob B