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

 

hi ir,
This clip is from the AD623 d/s
BTW: I guess you know that the resistive divider on the Vref will effect the balance of the AD623 amp.?
Ideally the Vref source should be a low impedance source.

E

 

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

 

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.

 

It's working relatively fine as of now but i'll keep that in mind.
thanks a lot guys