Question about Op-Amp MAX4238ASA

[m.abdelwanis]

Can I use this circuit designed to measure temperature from a thermocouple with a resistance of 1.2 ohms at 25°C to measure temperature from a PTC sensor with a resistance of 50 ohms at 25°C? If so, what adjustments need to be made to the resistor network?

[beenai2018]

The gain of this circuit  is R2/R1 +1  = 56000/100  = approximately  560

You will have to adjust R1 ( make it higher to reduce the gain). The R13 could be changed also , but I show a method for calculating R1.

Check how much the resistance varies in the new  ptc ,from the datasheet , for your temperature range  of interest.

Using this ,calculate the maximum and minimum voltage at the positive  node of opamp . (The ptc resistance and the 1M (R13) form a simple resistive divider) .

Use the maximum and minimum voltages at the  input to calculate the opamp gain . Remember the output should not go  near the maximum supply voltage of 5 and minimum voltage of 0 .



For ex . If maximum opamp input  is 200mv and minimum is 10 mv  ,your gain can  be about 20 ( so the opamp  maximum output voltage is 200mv * 20  = 4v ,and the minimum output is 10mV*20 = 200mV)

Thus R1 = approx 2.5K

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