Audioguru
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Hi, I am designing a Data Acquisition System for RTDs, for which i used this application note for analog part. ww1.microchip.com/downloads/en/appnotes/00687c.pdf
I used ADA4004-2 Op Amps instead of MCP609. The application note used 7.47 Gain V/V while i used 8V/V. Moreover, i added another Gain stage(just simple non inverting Gain) after the Filter+Gain stage. The Gain value for next stage is 2.5-5V/V.
Now the issue is that the Op-Amp used in SallenKey+Gain configuration keeps on malfunctioning after some time or is permanently damaged giving saturated value of +15V or -15V. When i replace this OpAmp, new one starts working fine until it faces the fate of the old one.
I would like to add one more thing that i am multiplexing the RTDs and the unconnected ones produce a saturated value at the output, can this be the reason of short life of my OpAmps
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View attachment 116173
This is the image of the Circuit from Application Note
In his first post he said he is multiplexing his RTDs and the unconnected ones cause the opamps to saturate. I think each RTD should be filtered then the outputs of the filters should be multiplexed.
I have never seen an opamp buffer (follower) like from the Microchip application note as shown here. I show three circuits that produce the same output from a low resistance source:
actually the varying input is from the multiplexed channels of RTDs. 16of them. Assume the current be multiplexed. Pots are being used as RTDs, each pot with different value at the moment, each pot is of 500ohm. 1mA sent to each pot where the value of Voltage recieved lies between 100mV to 500mV. The varying Blue signal is the mVolts from the RTDs(Pots). This Blue signal is fed to SallenKey filter Op Amp
your question is sadly lacking details.
Show your MUX input protection, decoupling and layout.
My guess is the switched voltage transient from cable capacitance of your RTD wires and whatever MUX is damaging the Op Amp.
Vin must never exceed the rails, otherwise latch-up failure may occur.
Similarily as @audioguru pointed out, between +/-15V OA's and 5V ADC
I believe your analogy is incorrect. The original buffer has a differential gain that cancels wire resistance DC offset and a common mode gain of +1.
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The problems are in your layout and design of the mux... MORE DETAILS ARE REQUIRED.
Unfortunately the OP didn't manage within six days to post a correct schematic or a problem specification. I fear this is turning out as a helpless case.i already mentionend the filter problem in post#20
and i also mentioned to do the filtering in software (post #11)
this could solve all your problems...
Unfortunately the OP didn't manage within six days to post a correct schematic or a problem specification. I fear this is turning out as a helpless case.
It hasn't be clearly specified, but I presume that the intention is to share a single current source and input amplifier for multiple RTDs.
I can filter out a predictable high frequency noise with moving average filter, I cant remove 50Hz noise or any other low frequency noise with this kind of filtering in software.Hi,
i already mentionend the filter problem in post#20
and i also mentioned to do the filtering in software (post #11)
this could solve all your problems...
Klaus
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