Hi Bob, I now use two IDACs for a 3-wire RTD. So I reduce the current from 1mA to 0.5mA and double the gain to 32. This way the circuit should work exactly the same as with a 2-wire RTD. The circuit can be seen below. In case of a 2-wire RTD I make a connection between the input filter of terminal AIN2 and REFP0 and I deactivate IDAC2 (see RTD1 as an example) . The cutoff frequency of the differential voltage is about 230Hz. So I should have -40dB attenuation at about 20kHz. My sampling rate will be <= 20SPS. Input: I choose C_diff=68nF, C_cm=4.7nF, Rin=5.1kOhm f_diff = 1/(2*pi*68nF*(2*5100+158)) = 227Hz f_cm = 1/(2*pi*4.7nF*(5100+158+2490)) = 4.37kHz Output: I choose C_diff=68nF, C_cm=4.7nF, Rin=3.9kOhm f_diff = 1/(2*pi*68nF*(2*3900+2490)) = 227.5Hz f_cm = 1/(2*pi*4.7nF*(3900+2490)) = 5.3kHz (f_cm2=8.68kHz) As filter components I choose resistors with 1% tolerance and ceramic capacitors with 5% tolerance and NP0 dielectric. At the IDAC connectors (AIN0, AIN3, ...) I mount a Schottky diode for protection. Kind regards Cyrill
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