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ADS114S08: ADS114S08 - Circuit for 6 RTDs (PT1000) in 2 wires mode with 13 channel ADC

William Fried
Prodigy 20 points
Part Number: ADS114S08

Tool/software:

Hi,

in your Application Note "A Basic Guide to RTD Measurements" in section 2.9.1, a circuit is shown for 4 RTDs needing a 12 channel ADC.
 As AIN2, AIN5, AIN8 and AIN11 are basically shorted together at the RTD inputs, would it be possible to modify the circuit to combine them using just 1 AIN?

=> RTD1 is measured between AIN1 and AIN11 and RTD2 is between AIN4 and AIN11, etc.

Here is a picture with the mentioned modifications (A and B are all connected together):

We want to measure slow signals from 6 PT1000s (max. 1°C/s), would this modified circuit work?

Do we have to add a delay for the settling time of the RC-filters, as all the capacitors add up?

Or can you provide documents, etc. why it would not work.

Thank you very much in advance!

Best Regards

  • 0
    TI__Mastermind 44451 points

    Hi William Fried,

    Answers to your questions

    William Fried said:
    We want to measure slow signals from 6 PT1000s (max. 1°C/s), would this modified circuit work?

    I have not personally tried this configuration, but it should certainly work. It will be challenging to place the differential capacitor from all of the measurement channels to AIN11 (in your example), so you might just forgo this component. You will not get the benefit of the differential filtering, but since you are measuring 2-wire RTDs this might not be an issue because the accuracy is the lowest (compared to 3- or 4-wire RTD measurements). 

    William Fried said:
    Do we have to add a delay for the settling time of the RC-filters, as all the capacitors add up?

    You might, that really depends on the components you choose. There will be some settling time when you switch the IDAC from one RTD to the next, as the next RTD will go from an unbiased to a biased state. The ADS114S08 has a programmable delay time that you can use for this purpose, which adds a delay between when you trigger a START condition and when the ADC actually starts the conversion process. This allows time for those external analog circuits to settle

    -Bryan