This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS122C04: VARIATION OF RTD'S RESISTANCE DEPENDING ON IDAC

Part Number: ADS122C04

To whom may it concern, 

I have a problem with measuring RTD by using the 4-wire method.

The value of RTD's resistance changed depending on the measuring current.(Due to P= I^2 * R) 

Currently, I use normal mode operation, 20SPS(lowest sampling data rate), and single-shot conversion.

1) After doing 20 samplings, would IDAC turn off until another START/SYNC command?

2) shngle-shot conversion mode, 20SPS actual conversion time is 50.01ms. During 50.01ms, would IDAC flow continuously?

3) After the single-shot conversion, ads122c04 gets into the low-power state. Is it different compare to the power-down mode in the perspective of IDAC?

4) Could you advise any other recommendations about this problem?

 

Regards,

JUN HYEOB - LEE

  • Jun,

    1. To turn of IDAC and also put the entire device in a low power state you can issue the power down command (see section 8.4.3.3 and FIGURE 54). Alternatively, you can manually turn on and off your IDAC at the end of your samples.
    2. Once the IDAC is turned on, it will remain on until it is turned off by writing to register 2, or the power down command is issued.
    3. After a single shot conversion the IDAC will not turn off unless you write to register 2 or issue the power down command.
    4. The IDAC has several different current output levels (10uA to 1500uA). The ADS122C04 is a very low noise ADC with an integrated PGA.  You should choose an IDAC value that minimizes the self heating of the RTD.  Of course, using a higher current level will make the input signal level, so the signal to noise ratio may be better with higher current levels, but based on your comments you will be better off using a lower current level and increasing the PGA gain.
    5. A Basic Guide to RTD Measurements is a great resource that covers a lot of the detail on RTD usage and error sources. Section 1.5.2 covers self heating.

    I hope this is helpful to you.  

    Art

  • Thanks der Art key,

    According to your advice, I sent power-down command after readADC().
    The test environment was 25 Celcius in the constant temperature machine. Compare to the previous setting, I mean without power-down command, resistances of RTD increased more. 
    I think it was a weird thing, so I tested it same condition again, and the results were also the same.

    ex) sensor.start();
          sensor.readADC();
             ---------(calculation EQ)
          sensor.powerdown();
              ~~~~ (other code)

    And I gonna try device number 4, and after then, I gonna share how was it. 
    Thanks Art Key

  • Jun,

    I would suggest using an IDAC current level that causes no, or minimal self-heating.  You do NOT want to try and turn on the current and measure quickly to avoid self-heating.  You want to keep the current level low to the point where the current causes minimal self-heating.  This optimal current level will be different depending on your RTD.  If you try and switch on and off higher current level IDACs, your RTD will always be self-heating and cooling depending on if your IDAC is on and off.  Please review section 1.5.2 in A Basic Guide to RTD Measurements.

    Best regards,

    Art