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ADS124S08: Problem with ratiometric measurement using low side reference resistor

Matt Bolsover
Prodigy 10 points

Part Number: ADS124S08
Other Parts Discussed in Thread: ADS1247,

I have taken a fairly simple 3 wire RTD circuit that works well with ADS1247 and applied it to a ADS14S08 where I am now getting a fixed offset error. The IDAC is used to drive the circuit.

The circuit uses a reference resistor below the RTD with the negative connected AGND. REF0 is used as the voltage reference (using resistor), ANI0/1 used to measure voltage across test resistor. With the ADS14S08 I get a fixed offset of +0.8% when calculating test resistances, lead resistances contribute about 0.05% (ish) of the error. Revert same circuit back to ADS1247 and calculated resistance is spot on.

The PGA has been bypassed for all tests.

Adding a bias resistor between the negative of Ref resistor and ground to shift REFN0 up from AVDD has not helped at all.

The only way I have been able to get a sensible answer is to move the reference resistor to above the RTD.



Is there a specific register setting needed to make the low reference work?

  • 0
    TI__Guru** 113765 points

    Hi Matt,

    Welcome to the E2E forum!  I apologize for the delayed response due to US holiday the end of last week.  Unfortunately I cannot see your screen shot as it is being hosted on a server I cannot access.  You can post images directly into the post by drag and drop or by using the Insert drop down and selecting Image/Video/File.

    It would also be helpful for me to know what register settings you are currently using for all of the ADC registers.  You may also find A Basic Guide to RTD Measurements useful.  In particular section 2.3 on page 14.

    What differs significantly with ADS1247 and the ADS124S08 is the nature of the input bias currents on the reference inputs.  One common mistake relates to the use (or incorrect use) of the reference buffers.  For the low-side measurement, the REFP should be enabled, and the REFN buffer disabled when connected to AGND.  If REFN is connected to AGND, then the reference filter resistor should not be used on the REFN input pin.  If you use the bias resistor, then you can use the filter resistor and you should enable the REFN buffer to increase the input impedance.  You may find the information in the FAQ regarding the reference buffer helpful.  The REFN buffer is disabled by default and I suspect that the issue you are seeing relates to a voltage drop across the input filter resistor connected to the REFN input.

    Best regards,

    Bob B