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# Op-Amp Accuracy....How accurate when NTC >= 36000 Ohms?

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#### eem2am

##### Banned
Hi,

The following is an op-amp circuit which reads a potential divider voltage where the upper divider resistance is an NTC thermistor.

http://i54.tinypic.com/2wpqdmf.jpg

LMC6035 OPAMP datasheet

-the thermistor connects between VCC and PL5

-The Vcc = 5V

The temperature measurement is required to be very accurate when it is near zero degrees centigrade.

(this is because it measures water temperature in a shower pipe, and the shower must not be turned on if the pipe water temperature is zero degrees C. -This is because if it was, then the cold pipe may be frozen , and un-mixed hot water would go over the showerer and scald them)

The thing is, when the NTC is at zero degrees C, its value is 36000 Ohms.

This value of resistance gives a very low output voltage reading from the op-amp U4-A.

I am wondering how accurate the op-amp U4-A's output is when it is near ground?

You can recalibrate amp. stage by calculating
voltage divider Rntc and R31

Now input at point U4A pin 3 is 6.2k * 5V / (36k + 6.2k) at zero degrees
= 0.735V

If you select R31 = 33k then 33k * 5V / (36k+33k) = 2.391V input (U4Apin3)

( Program for display needs new coefficient value too )

KAK

The temperature accuracy mainly depends on how much you are willing to afford for a tightly tolerated NTC. Maximum LMC6035 offset voltage of about 5 mV will slightly contribute to overall error. Or you have to perform an individual zero adjustment which is most likely inpracticable for a product.

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