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Issues using ADS1118 and 3 wire RTD, PT1000, in wheatstone configuration

juan fernandez
Prodigy 80 points
Other Parts Discussed in Thread: ADS1118

I am having an issue  using ADS1118 with a 3 wire RTD, PT1000, in wheatstone configuration - please see figure 41 in the link below:

www.omega.com/.../rtd-measurement-and-theory.html

We are using AIN3 (pin 7) for the reference voltage, AIN2 (pin 6) for the RTD voltage, and AN0 (pin 4) to read Vcc 

We have a few prototypes working perfectly and a few reading slightly off but consistently off - for example, reading 3 degrees low in both room temp and ice bath tests.  As mentioned we are using PT1000 RTDs which all test perfect out of circuit and we are using 1K 0.02% resistors which all test perfect out of circuit as well (using a recently calibrated HP 34401A). If we replace the RTDs with 1K 0.02%, to replicate a stable 0 degree celsius condition, the readings match up (very close to 0V differential). Again, some units are working great, some not.  So, what can cause a perfectly working RTD (1k in icewater, 0 degree celsius) to consistently appear to be a few ohms low when in circuit under the same conditions but when replaced with a precision 1k resistor the readings produce 0 celsius?

Please ask any and all questions needed if I need to clarify or go more into detail

  • 0
    TI__Intellectual 1610 points

    Juan,

    Thank you for your query. If I understand correctly, all the conditions remain unchanged except that the prototype boards are changing. In other words, if we use the same RTD, precision resistors, HP34401A but only change the prototype, some prototypes measure a slightly lower RTD resistance than the others. Is that a correct statement?

    Also, is there a reason why you have 3-wire RTD in a Wheatstone's bridge configuration? Alternatively, can you use a 3-wire RTD as shown below? ADS1118 can be configured as below. In the case below, the ADC multiplexer can be configured to measure all the voltages differentially. Also, you will need one less precision resistor.


    Regards,
    Krunal