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ahgan84

About my ADC measurement settings.

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I have set up a NTC thermistor to interface with my ADC12 for measurement. I'm using a voltage divider settings. But I have encounter some problem.

 

I've uploaded a picture for my settings. Sorry I just want to be quick so instead of using schematic software, I just use hand drawing.

 

The top one is a voltage divider using a resistance box. As you can see, the voltage values that I measure using multimeter for 1k resistor and the 10k resistance box is the same with when I use calculation using voltage divider formula. So, the values are correct.

 

But as for the bottom settings which I use a Thermistor the values started to get weird. The resistance for the Thermistor is also 10k like the resistance box but the voltage I get is 2.08V instead of 2.28V. I get 0.42V for the 1k resistor instead of 0.22V. Why is it this happening? Is there a problem with my thermistor?

post-5260-135135528734_thumb.jpg

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It seems that you have it wired correctly. You should be getting different readings with the 10k Thermistor vs a 10k resistor. The Thermistor should only provide 10k ohms at it's nominal temperature which is usually 25

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Ohm's law.

 

In a thermistor, the resistance will be changing with temperature.

 

I know but with 2.08V, the resistance I should get after my calculation is 4.95k ohm. But when i measure the thermistor using my multimeter, I still get 10k resistance. Besides, 4.95k ohm of temperature is around 40 degree C. That's impossible... I'm in a 25 degree C room.

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Your power supply is stable? Measure the power supply when connected resistor and thermistor. If you use small battery then 2,2mA can cause a voltage drop.

 

In practice upper (fixed) resistor should be selected approx. 10~100kOhm for 10kOhm thermistor.

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Your power supply is stable? Measure the power supply when connected resistor and thermistor. If you use small battery then 2,2mA can cause a voltage drop.

 

In practice upper (fixed) resistor should be selected approx. 10~100kOhm for 10kOhm thermistor.

My voltage is a stable one. I measured already, I don't experience voltage drop.

Yup. You are right. I've change the upper resistor to 30k Ohm and it give me a correct voltage value. But do you know why we should use a larger value for the upper resistor?

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I've found the below circuit which can measure resistance using ADC12. But what I don't understand is what is the 1M resistor in the middle for? We must put it or can we omit it?

post-5260-135135530004_thumb.png

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Our schematics is "voltage follower" - it's impedance converter (high input impedance and low output inpedance). 1M resistor is not necessary, input impedance of good FET opamp is much higher. But is almost necessary using rail-to-rail opamp.

IMHO use opamp for this is overkill. Put capacitor (100nF is enough) paralel into analog input and throw away opamp.

 

Opamp is necessary if you measure something like vacuum tube photocell or other high impedance device (Mohms to Gohms). For measuring at the ~30kOhm divider is unnecessary.

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