LPF for RTD Measuring

Hi,

I have to aspects for choosing the LPF location:
* if you want to suppress ESD, EMC or some other noise coming from the sonsor or cabling: I´d put the filter in front of the first OPAM. This mainly is to slow down dV/dt and avoid to overdrive the OPAMP´s input stage.
* if you want to suppress general noise, also the noise of the OPAMP, then put the filter after the OPAMP.

Fc depends on what frequency (from sensor) you are intereseted in. lower fc results in better suppression for high frequency noise. But also causes some amplitude drop and phase shift..

CMRR has nothing to do with noise filtering. It´s a measure of how much the output voltage is influenced by changing the common mode input votage. But in your case common mode input voltage is ratherfixed to V_sensor/2.
Do you expect some common mode noise?

Klaus

I don't see a purpose of Wheatstone bridge for RTD measurement. Usual configuration is single ended voltage divider or current source supplied RTD. Differential amplifier is needed for three- or four-wire measurement (elimination of cable resistance), but also different from Wheatstone.

FvM said:

I don't see a purpose of Wheatstone bridge for RTD measurement. Usual configuration is single ended voltage divider or current source supplied RTD. Differential amplifier is needed for three- or four-wire measurement (elimination of cable resistance), but also different from Wheatstone.

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Agreed. A Wheatstone bridge is used for devices with small relative change. Current source works fine.

I’d put the filter right after the sensor. You can also add some gain to the filter so you only need one opamp.