ADS1120: ADS1120

Hi Julian,

The reference resistor serves 2 purposes.  One is the obvious which is to set the reference voltage value so a ratiometric measurement can be made.  The second is to place (or bias) the RTD measurement within the correct common-mode for the ADS1120.  You must also make sure that the total voltage drop across the RTD and Rref meets the compliance voltage (the voltage relative to AVDD where the constant current IDAC source can remain constant).  The compliance voltage is AVDD-0.9V for the ADS1120.  So you need to calculate the highest RTD value you expect to measure.  Then you need to consider the IDAC current you wish to use so that the Rref value meets both the desired reference voltage and common-mode for the desired PGA setting.

One easy starting point possibility is to use the value of 2*(Rref*10) from the PT100 3-wire RTD case from the datasheet and divide the IDAC current by 10 (500uA to 50uA).  The value for Rref was initially calculated so that the common-mode would be set to 1.65V (1/2 of the 3.3V AVDD supply voltage.)  In the 3-wire case, 2 IDACs are used for lead cancellation.  However only 1 IDAC is used for RTD excitation.  In the 2-wire case only one IDAC will be used for both Rref and RTD excitation so the resistance of Rref needs to be 2 times the value used in the 3-wire RTD case.  So instead of 1.65k Ohms it would be 3.3k Ohms.  But as the current is 10 times less, the resistance should be 10 times greater for the same voltage drop.  So Rref now becomes 33k Ohms.

Best regards,

Bob B