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ADS124S06: ADC Channel Over-reading

Steve Kynaston
Prodigy 10 points
Part Number: ADS124S06
Other Parts Discussed in Thread: ADS124S08

Hi

I have seen this problem with similar circuits using both the ADS124S06 and ADS124S08.

I am using the device with a single 3v3 supply. I believe the supply is OK and I have the recommended decoupling in place.

I am reading 'housekeeping' temperatures using PT1000 RTD sensors by having the RTD connected from the analogue input to ground and configuring the ADC to drive a bias current through the device while it measures the voltage against the internal 2V5 reference in a single ended measurement. I have one channel connected to a high tolerance 1K resistor to measure the ADC bias current for some basic correction. My controller switches between channels every 100ms and I believe the single conversion SPS etc I have selected means each measurement will be over after about 60ms.

What I see is sensors attached to certain channels appear to read OK while the same sensor type in the same environment on one of the other channels seem to overread by up to 10%. If I scope the voltage across the sensors I see a rectangular wave when the bias current is applied. It looks like the voltage being generated across each sensor is very similar. I have tried polling the channels in a different order and the problem seems to stay with the physical input.

This 10% difference results in a temperature reading of about 50oC compared to the other sensor which is reading 25oC - these sensors are very close together in the same environment. A measurement with a DVM shows both sensors reading the same resistance.

 Here are the values I am loading to registers 2 to 8 for Ain0 - The other channels have the same setup with the upper nibble of INPMUX and both nibbles of IDACMUX changed accordingly.

0C INPMUX
00 PGA
34 DATARATE
3A REF
06 IDACMAG   -- set for 750uA
00 IDACMUX   -- both bias currents routed to the sensor channel
00 VBIAS
11 SYS

I have checked that all of the channels are presenting < 2.5V ( I have seen issues before if one of the channels is higher than the ref)

Is this setup sensible - have I missed something about the correct use of the current sources / internal reference / single ended measurement?

Are my register settings sensible for this setup?  (I have tried experimenting with various settings but cannot change this behaviour)

Thanks in advance

  • 0
    TI__Guru** 113765 points

    Hi Steve,

    Welcome to the E2E forum!  Generally it is recommended to make a ratiometric RTD measurement where the excitation source is used for both the reference and the RTD.  A very good discussion on RTD measurements along with various circuits can be found in A Basic Guide to RTD Measurements.

    As to the measurement you are conducting, it is best not to send out current using the same pins as the measurement.  In other words, use one pin for AIN0 for the measurement and a different pin for the IDAC current where the IDAC pin is connected outside of the measurement and any input filters.  This eliminates any measurement error with respect to the very complicated input mux.  Keep in mind that if you drive current out of an input pin, any resistance in that path will be added to the conversion measurement.  Also, this resistance can vary over temperature so it is not easily calibrated out of the system.

    Best regards,

    Bob B