ADS1248: how many 4-wire RTDs measurement possible with ADS1248?

Abolfazl,


Yes the basic topology is correct to measure two 4-wire RTDs. However, I would change the reference resistance to a larger value.

If you use a low reference resistor, you get a small reference and you need a larger current to get the reference higher. If the current is larger, then you may have more self heating in the RTD measurement. Also the reference voltage shifts the RTD voltage up, giving the RTD measurement a higher common-mode voltage. Ideally, you want to set the input measurement at mid-supply, to allow for the greatest range for the PGA. By using a larger reference voltage, you shift the input measurement closer to mid-supply.

I presume that you've chosen the reference to be 422Ω to be able to measure a maximum of 400Ω for the RTD. If this is the case, I would change the reference resistance to near 1.68kΩ for the reference and use a 1mA for the IDAC current, while setting the PGA gain to 4. This will set the common-mode input to about 1.9V with a maximum RTD value, likely centering the input near mid-supply.

Verify that the RTD voltage is within the input range of the PGA and that the voltage at IEXC1 and IEXC2 do not go high enough to reduce the output current of the IDAC. If the IDAC output voltage goes too high, the current source loses compliance.

If you look at the ADS1248 datasheet there is an example application, with calculations for the design. This design is for a 3-wire RTD measurement. While the calculations are slightly different, the methodology is similar.

Also, if you need to more channels than available in the ADS1248, consider using the ADS124S08. It is a similar device, that we consider the next generation of the ADS1248.

If you have any other questions, please follow up with another post.


Joseph Wu