# Rtd Sensing Problem for 1mA accurate source

1. ## Rtd Sensing Problem for 1mA accurate source

hi guys,

Til now i am sensing the 3 wire rtd with my cypress ic CY8C2845-24PVXI with internal Idac.
This Idac give me 5V and 1mA accurate source for sensing rtd element. but this ic has less memory so i need to move on other ic for other application but other ic has not a idac internally.

now i need to generate external 1mA source with same 5V load for same application.i Tried the attached circuit with 5V reference and lM324 IC up to 1 mA source generation level after that i connect my circuit.
i got only 3.38 V on out put instead of 5V.
i am also little bit confused about power for LM324 opamp . how can i decide the power level of opamp?

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2. ## Re: Rtd Sensing Problem for 1mA accurate source

Hi,

What's the power supply voltage for the Opamps?

Did you read the Opamp's datasheet?
It is no Rail-to-rail Opamp. Datasheet mentiones several times:
• Large Output Voltage Swing 0 V to V+ − 1.5 V

Klaus

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3. ## Re: Rtd Sensing Problem for 1mA accurate source

i got only 3.38 V on out put instead of 5V.
Should be well sufficient for Pt100. Most critical point is minimal low output voltage of LM324, but it's probably O.K. for this circuit.

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4. ## Re: Rtd Sensing Problem for 1mA accurate source

Hi,

I agree that 3.38V are sufficient.

But for true resistor calculation you need to know the actual current. I´m not convinced that the circuit gives exact 1mA .. I rather think the OPAMP is saturated at it´s rail limit and the current is somwhere below 1.00mA.
--> Then the resistor calculation may be wrong.
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A simpler method is to just use an about appropriate resistor and measeure the voltage drop across this known resistor. Then you may calculate the true current.
Measure the voltage drop acrross the PT100 and calculate the exact resistance.

Klaus

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