AD623 and RTD issue with stability (Solved)

Thread Starter

irmanao

Joined Apr 29, 2017
87
I am trying to measure an RTD's (PT1000) resistance via the following circuit. Everything seems to be stable voltage wise, from the -IN pin, +IN pin, vref pin, vcc.
However, when i touch the RTD's wire (plastic part) lightly by hand, the output from the AD623 is erratic, changing up to 200mV while i touch it. Only the output, all other pins are stable when i touch the RTD. For example +IN pin is 1.6V (stable) and -IN pin is 1.55V (stable).
Any ideas for solving this issue?
thanks
 

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Thread Starter

irmanao

Joined Apr 29, 2017
87
Yeap.... i added all the capacitors and it worked. I skipped that part of the d/s.
Isn't the divider i am using low impedance?

thanks
 

crutschow

Joined Mar 14, 2008
26,974
Isn't the divider i am using low impedance?
Not for this application.
The REF output resistance it 100kΩ.
To maintain the amplifier inherent 80dB common-mode rejection, the resistance connected to that node should be less than 1/80dB * 100kΩ =10Ω which is well below the 667Ω you have.
The reduce CMR value from this resistance may be what's causing the sensitivity you notices.

To get such a low impedance you can use a follower op amp to buffer the voltage divider output.
Such an amp has an effective output impedance typically less than a mΩ.
 
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