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EMI filtering using AD8422 for RTD measurement

Question asked by smnoh on Jan 14, 2018
Latest reply on Jan 15, 2018 by smnoh

I use the AD8422 for RTD sensor measurements and I have problems with EMI filtering. The resistor bridge is used for signal condition of RTD measurement, and this method is described in the application note (Linear Technology AN43). This circuit uses 3 resistors (100, 22k, and 22k) as bridge resistors to measure the RTD resistance in a three-wire manner. The voltage of several tens of mV generated by the RTD resistance is amplified 77 times through the AD8422. The problem occurs when capacitors (C1, C2, C3) are added to the three input lines to suppress EMI emissions. When C values are 22nF, the output voltage of the AD8422 is -12V regardless of the input voltage. When C values are less than 220pF, AD8422 operates normally.

In simulation, the AD8422 output appears to be unaffected by C values of the input signal line, but in practice it is not. Since the capacitance affects only AC characteristics, the effects of C, like the simulation results, are expected to be no effect on DC, but it is not actually. I tried to mitigate this problem by adding C4 to the AD8422, but it did not have much effect.

If high C values such as 22nF are strongly required for EMI filtering, how can I revise the circuit to make the AD8422 operate normally? I attached simulation files for ADIsimPE.