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Original question:

ADS122S14: Using a TI ADS122S14 (or alternative) as a precision resistance meter

ADS122S14: Design assistance with ADS122S14

James Winter
Prodigy 20 points

Part Number: ADS122S14

I posted a question a couple of months ago regarding using the ADS122S14 to measure resistance.

ADS122S14: Using a TI ADS122S14 (or alternative) as a precision resistance meter - Data converters forum - Data converters - TI E2E support forums

 

I have now purchased the development kit - but if possible could do with a little more help with the design of the schematic to prove the concept of this if possible please.  Do you have any example circuits with 4 wire ratiometric resistance measurement circuits connected up to an ADS122S14?   Whilst I've been an electronics designer for 30 years - I've never needed to design a resistance meter before, so it's a bit like a return to University! (but 30 years after I had to tackle the math"!)  It's more a case of a little bit of assistance getting my head around how the idea and the theory works, so I can then actually crack on with implementing it.  My plan is to "lash up" the test circuit on veroboard - connected to the Dev kit, and then take it down to our customers site to see how it copes with measuring the resistances needed.

Thanks

 

James

 

  • 0
    TI__Guru 50025 points

    Hi James,

    Thank you for evaluating TI's ADCs.

    The RTD measurement circuit is fundamentally a resistance measurement circuit — RTDs are just resistors that vary with temperature. The same topology can be used to measure any unknown resistance. As I recommended in your post before, refer to the Four-Wire RTD Measurement Low-Side Reference (section 2.6) in the application note A Basic Guide to RTD Measurements or Four-Wire PT100 RTD Measurement Circuit With Low-Side Reference. You can also see the circuit block diagram in figure 2-5 in ADS122S14EVM-PDK and ADS122C14EVM-PDK Evaluation Module User's Guide. A 4-wire or Kelvin measurement technique is most accurate and it eliminates the effects of test equipment lead and probe (lead) resistance. A known current from the ADC's internal current source (IDAC) is forced through the test resistance (labeled as RTD in the application note and EVM user guide) and then through a precision reference resistor, generating a stable reference voltage. These elements form a ratiometric resistance measurement circuit.

    Key design considerations for resistance meter use:

    1. IDAC current selection: For higher resistance values, reduce IDAC current to avoid saturating the ADC input.
    2. Reference resistor: Must be larger than the maximum resistance to be measured when using a single current source. Use a high-accuracy, low-drift precision resistor.
    3. Calibration: Small offsets due to PCB trace/cable resistance can be removed via system calibration.

    Let me know if you have any questions.

    BR,

    Dale