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Circuit design help for a 2-wire 500-ohm RTD

Other Parts Discussed in Thread: TMP411, TMP421, TMP451, TMP423, TMP422, TMP461, LMT01, ADS1247

Hi all,

Our customer has decided switch form a thermistor temperature sensor to a 500-ohm RTD in an aircraft environmental control system. They will not run new wiring, so we are stuck with a 2-wire solution rather than the preferred 4-wire.

I found a TI guide for a 4-wire implementation, which has helped me select appropriate op amps and the current source, but I would like help customizing the circuit to our 2-wire implementation. I assume there isn't much point in using a differential amplifier if one end of it is just connected straight to ground (unless that will help cancel out onboard noise?). Here is the 4-wire guide: http://www.ti.com/lit/wp/snoa838/snoa838.pdf

Some details on our system:

-The sensor is connected with 52-feet total (26ft each side) of 22AWG wire.

-They have selected a 500 Ohm RTD, designed to be driven at 0.5mA.

-Our system voltage is 3.3V.

-They require +/- 3degF accuracy over the temperature range from 30-185F.

-I found a resistance table that says for a 500 ohm RTD that is 496-664 ohms.

-I think we'll need to use an op amp to gain up the signal by 7-10x to make the most of our 12-bit ADC.

Any help would be appreciated.

Thanks

  • Hello Glen,

     

    Do you know the reason/motivation why the customer is switching from NTC to PTC?

     

    As you know with a two wire RTD solution, there is no way of getting around the parasitic resistance, unless some type of system calibration is employed. But I doubt this is an option for the customer.

     

    For sure you would want to gain up the signal to utilize the dynamic range of the ADC as you mentioned, I totally agree. But I’m afraid the dominant error occurs before the gain stage, which to some extent defeat the purpose of additional system gain.

     

    I’m going recommend an alternative solution, you can decide if it is viable for your application.

     

    We have a family of remote temperature sensors, which only require two(2) wires running to the remote sensing location. Instead of RTD, the sensing element is an ordinary NPN or PNP diode. Examples of this family include but are not limited to TMP411, TMP421/TMP422/TMP423, TMP451/TMP461. The advantage of the remote temperature sensors is that it is a complete system; you don’t need gain stage and ADC. In addition, the system is not sensitive to parasitic resistance. The accuracy can easily satisfy your requirement.

     

    If you are interested, you can email me or Mayrim, my teammate, for direct and faster support.

     

    g-zhou@ti.com

    mayrim.verdejo@ti.com

     

    Regards,

    Guang-Apps Sensing Products

  • Hi Glen,

    To add onto Guang's post, we recently released a 2-pin digital temperature sensor called the LMT01. It has 0.5C (max) accuracy over the temperature of -20C to 90C and an accuracy of 0.7C (max) over the full temperature range of -50C to 150C.

    -Michael Wong
  • Hm - all interesting suggestions - thanks. We have used dallas 1-wire sensors in military applications in the past with good success.

    The issue here is that my customer's customer just declared that we should switch to the RTD without regard to whether it's really the best solution. I think that's what most of the industry uses for this kind of sensor, so we would really have to push to make a case for going with anything else.

  • Hi Glen,

     

    Please check out the ADS1247 RTD measurement app note:

    http://www.ti.com/lit/an/sbaa180/sbaa180.pdf?DCMP=hpa_contriubted_article&HQS=sbaa180-ca

     

    And here is the data sheet for this family of devices:

    http://www.ti.com/lit/ds/symlink/ads1247.pdf

     

    Regards,

    Guang-Apps Sensing Products