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ADS122C04: Accuracy for low temperature measurements?

Chris van der Aar
Intellectual 570 points
Part Number: ADS122C04
Other Parts Discussed in Thread: ADC-TEMP-SENSOR-FW, TPS734, ADS1248

For a new project we just started we have to measure temperatures between -180C and -160C with an accuracy <0.5C. Therefore we want to use a PT1000 (3- or 4 wire and class A or class  B depending on the price). Probably our sensor supplier can deliver a PT1000 with 0.15C accuracy so we have 0.35C left for the ADC. Therefore some questions:

- Do you think an accuracy <0.35C is possible within this temperature range with the ADS122C04? 

- The resistance of a PT1000 will be around 25 Ohm at -180C. Which reference resistor value would you suggest if we are only interested in accuracy around -180C? And which type of resistor would you recommend? I asume we have to set the IDAC value at max (1.5mA) to get enough voltage at this low sensor resistance value at very low temperature?

_ Do we have to implement the Callendar van Dussen equation in our software or can we just read the ADC value and calculate the temperature with that? I assume the sensor will be linear over such a small range?

- Are there any other things we have to take car for?

- I also found the LTC2986 which is very accurate but the price is very high for that part (and besides that one has SPI). So therefore the ADS122C04 could be an interesting part for us regarding the price.

Best Regards

Chris van der Aar

Sr. Hardware engineer

NTS Group

Eindhoven , The Netherlands

  • 0
    TI__Guru** 113765 points

    Hi Chris,

    25 Ohms for -180 deg C is for a PT100, PT1000 would be 10 times greater or approximately 250 Ohms. A 4-wire would be easier to implement and would require less compensation for lead resistance.  However IDAC chopping would compensate for this when using 2 IDACs for 3-wire RTDs.  If you use a large value of current you could have self-heating of the RTD.

    The measurement itself would be ratiometric as the stability of the reference resistor would be the largest factor in accuracy.  The ratio of the RTD to the Reference resistor is equal to the ADC code result to the total number of codes available for the ADC.

    As far as the calculation you could use polynomial math, or you could simply use a lookup table to make the conversion from resistance to temperature.

    I would suggest looking at A Basic Guide to RTD Measurements.

    You could also take a look at some code examples on how to make the resistance to temperature conversion in this API, ADC-TEMP-SENSOR-FW.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 570 points

    Hi Bob,


    We are working at this moment on our design with the ADS122C04. We will use a 4 wire class A RTD sensor and we want to measure a temperature range between -200 and +50C. We hope to achieve an accuracy of max +/-0.5C in total for sensor + electronics. The resistance range will be between 185 and 1195 Ohm for the PT1000 for these temperatures. I want to use an excitation current of 1mA because I think the self heating will be still minimal with this current.  I will put filters at the sensor and REF inputs with 4k7 and 1nF en 10nF (I don't know what we in fact need but this looks more like the values used on the EVAL board). We do'nt need high speed measurement so 40SPS is fast enough for this because it is a very slow process (takes about 15 minutes to reach the low temperature level).

    Could you please give a suggestion how we have to connect a 4-wire sensor to the ADS122C04. The datasheet shows an example with a 3 wire RTD but there they use 2x IDAC. Do we need 1x IDAC with a 4 wire RTD? And which value for the GAIN and reference resistor would you recommend so we use the complete range of the ADC as much as possible? I will look for a 10ppm or better resistor but I think that will be not a big problem for an acceptable price.

    Best regards,

    Chris van der Aar

    NTS -Group

    Eindhoven, The Netherlands

  • 0 in reply to Bob Benjamin
    Intellectual 570 points

    And another question concerning interfacing the ADS122C04 (please check previous question because we have a very short time frame for PCB design!). We use a USB to I2C bridge for communication (FTDI) and prefer to use +5V I2C levels. Is it possible to use +5V at the Vdd (for matching digital levels) and +3V3 for the analog part? And what would you recommend for the +3V3 supply? We now use T.I. DC DC converters for generating +5V out of +24V (with integrated inductor see https://www.ti.com/lit/ds/symlink/tpsm84209.pdf) and a LDO for generating +3V3 for the logic. Can we use this LDO for the ADS122C04 or would recommend a separate LDO for this? We will now use the TPS73433 with ultra low noise:

    https://www.ti.com/lit/ds/symlink/tps734.pdf

    Best Regards Chris

  • 0 in reply to Chris van der Aar
    TI__Guru** 113765 points

    Hi Chris,

    The ADS122C04 datasheet does discuss other implementations including 4-wire RTD in section 9.2.2.2.1 and demonstrated in figure 76 using a low-side reference.  More combinations and details regarding various connection scenarios are in the A Basic Guide to RTD Measurements.

    The operational voltages are also included in the datasheet in section 6.3 Recommended Operating Conditions.  The analog supply and digital supply can be different voltage levels within the voltage range of 2.3V to 5.5V.

    If the DC-DC converter has reasonable noise, you could use the same 5V for both analog and digital supply, although cleaning up the DC-DC converter with an LDO is preferred.  The TPS734 series is acceptable.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 570 points

    Hi Bob,

    Most things are clear for me now and I want to use a 1K8 reference resistor (0.02% and 5ppm) and 1mA excitation current so we get 1.8V reference voltage. The max RTD resistance is 1184 Ohm at 50 C so we have 1.18V across the RTD so with a gain of 1 this should fit within the ADC range of 3V3,.

    Only one question concerning filtering. In the application sheet of the ADS1248 I see examples with the same filter for the inputs and the REF input so with commond mode and differential mode filters. However in the datasheet of the ADS122C04 I only see a diff mode cap at the REF inputs and no common mode cap to GND. So I was wondering if the common mode cap is not required at the REF inputs of the ADS122C04 and if the differential cap must have the same value as the cap for the RTD input filtering. I am now using 10nF/4k7 and 1nF/4k7


    best regards Chris

  • 0 in reply to Chris van der Aar
    TI__Guru** 113765 points

    Hi Chris,

    Don't forget about the IDAC compliance voltage which is AVDD-0.9V.  For the IDAC to maintain constant current the total voltage drop across the RTD and reference resistor cannot exceed AVDD-0.9V which for 3.3V AVDD is 2.4V.  The values and range you desire to use will total closer to 3V.  In the end you will need to either alter the resistance value for the reference, reduce the IDAC current or use a voltage greater than 3.3V for AVDD.

    As far as the effectiveness for the input filter configuration, this will depend on the ADC being used.  The ADS1248 is a lower noise ADC as compared to the ADS122C04 so what is good practice in theory may not make that much benefit in the conversion result.  For the ADC inputs the most important cap is the differential capacitor.  The common-mode caps could be left out entirely in most cases as the values are such that the cutoff frequency is at least 10 times higher than the filter cutoff for the differential input.  In the case of the reference, the RF4 resistor will help filter ground noise.  If the ground is clean then this resistor can be removed.  If desired you can leave in the RF4 resistor and add common-mode caps, but it is unlikely you will see any improvement in performance using the ADS122C04.

    Consider this option.  If you lower the IDAC current and increase the value of the reference resistor you can potentially add gain to the RTD measurement.  The reason in doing this is the input referred noise lowers with increased gain.  Take a look at the effect of using gain by reviewing the noise tables in the ADS122C04 datasheet beginning with Table 1.

    Best regards,

    Bob B