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ADS1262: IDAC ROTATION ENABLED-NOISE COMING FROM INTERNAL CURRENT SOURCE

Part Number: ADS1262

Hi Team,

I designed a set of circuit by referring to the 3-wire RTD circuit in the data sheet of ADS1262. I want to use ADS1262 channel AIN4 and AIN5 to measure RTD, and I want to use AIN1 and AIN6 to internal current source 250uA and internal reference using 2.5v. 

My application requirement is RTD-pt 100 (-200.000 to 850.000)deg C 0.001 resolution. So the maximum input voltage 100mV  (250uA*400 ohms = 100mv). I applied Gain16. possible to read 0.001 resoltion?

the problem is i measured internal current source with IDAC rotation enabled  (247.8uA to 248.8uA)  jumping. so the total ADC count is jumping. the  measuring device FLUKE 5522A used. i attached the circuit.ADS-1262-UPI-dkd.pdf

  • Hi Kumar S,

    The RTD code for an ADC is approximately given by the RTD resistance, R_RTD, divided by the reference resistor, R_REF. In your case, the maximum RTD resistance is ~400 ohms, while the reference resistor is 3900 ohms. Therefore the maximum gain you could theoretically use is R_REF / R_RTD (max) = 3900 / 400 = 9.75. Using the ADS1262, you would need to round down to a gain of 8. So no, you cannot use a gain of 16 with this configuration.

    I would also encourage you to consider using one IDAC and two measurements instead of two IDACs + IDAC chop. Using one IDAC and two measurements will help improve the system accuracy, and you will not have to worry about switching the IDACs (which can lead to analog settling errors. The two different options are shown in the image below

    For you system, you would use AIN6 as a measurement input as opposed to an IDAC output. The rest of the circuit would stay the same, though you would need to add an RC filter on AIN6 (and remove D2).

    How fast are you measuring the RTDs? This gives you a first-order approximation of the resolution you can achieve, assuming the total system noise is purely from the ADC. In other words, if you have external noise sources, the resolution could be lower.

    -Bryan