AD590 Chip......>>>>>> ADC raw data kelvin to degrees Celsius how do I the ma

sam_sam_sam

·Here is the· conversion·· ·1 Kevin = -272.15 degrees Celsius

Here is the data sheet to the AD590 Chip

http://www.analog.com/en/other-products/militaryaerospace/ad590/products/product.html

Thanks to anyone that can help

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Sam

Post Edited (sam_sam_sam) : 4/26/2010 4:14:29 PM GMT

Tracy Allen

Hmmm.
deg2C = deg2K - 27215 ' in units of 1/100 degree

Your puzzle may have more to do with how your ADC returns the data, and what language you are using.

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Tracy Allen
www.emesystems.com

sam_sam_sam

Here is the problem the raw data that I getting is 304 and the temp is about 80*F

I can use / 12 and get 25 wich is a little off it should closer to· 30

I am using a BS2 for this Demo Board


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Sam

Post Edited (sam_sam_sam) : 4/26/2010 4:55:49 PM GMT

Tracy Allen

Well, if the true temperature is "about" 80.0 degF, that is 26.7 degC or 299.8 Kelvin. Call it 300 Kelvin.

Let's assume zero offset.

' value ADC returned here
kelvin = ADC ** 64674 ' calibration: 300 Kelvin @ 304 ADC
celsius = kelvin - 273
fahrenheit = kelvin * 9 / 5 - 460

Do you have a 1kΩ resistor to convert the current from the AD590 into Volts, being read by an ADC at 1mV per bit? If you use a 10k resistor instead, then the reading can be more accurate.

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Tracy Allen
www.emesystems.com

sam_sam_sam

Tracy Allen

I have 1.2 k ohm on there now

Do you have a 1kΩ resistor to convert the current from the AD590 into Volts, being read by an ADC at 1mV per bit?

I try this and see what happen

If you use a 10k resistor instead, then the reading can be more accurate

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Sam

sam_sam_sam

Tracy Allen
·········································································································@·about 80* F
I changed the resister to 10k ohms and now the·result value is 1530·· |
When the resister was 1.2k ohms it was 304···························· ········|··


Is·this what I should get as a value changing the resister to 10k ohms ·········

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Sam

Tracy Allen

Hi Sam,

I believe there must be something wrong electrically with your circuit. Do you have at least 5 volts across the AD590 sensor?

Math for the AD590 is very simple. The sensor produces (nominally) 1 microAmp per Kelvin. So at 300 Kelvin, the voltage across a 1000Ω resistor should be 300 mV, or across 1200Ω it should be 360mV, or across 10000Ω it should be 3000mV. Simple
mV = µA * R /1000 with µA = Kelvin, R in Ohms

300 Kelvin is near 27 Celsius or 80 Fahrenheit.

You report that the result value is 304mV with 1.2 kOhms, which would solve backwards to
Kelvin = µA = mV / 1200 = 304 * 1000 / 1200 = 253 Kelvin
That is -20 degrees Celsius, I don't think so. The result would make much more sense if you had a 1000Ω resistor, which is what I was assuming you had above. The result is even worse with the 10kΩ resistor:
Kelvin = µA = mV * 1000 / 10000 = 1530 * 1000 / 10000 = 153 Kelvin! -120 Celsius!

That bad result could be explained if there is not enough voltage across the AD590. Or maybe I am wrong about the numbers reported by your ADC. Does 304 mean 304 milliVolts?

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Tracy Allen
www.emesystems.com

sam_sam_sam

I have two temp sensors one is

With a 1.2k Ohm· = 1.2 volts @ about 80* F

One is

With a 10k Ohm = 2.345 Volt @ about 80*F

The voltage reading where taken with a volt meter

I hope this helps

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Sam

Tracy Allen

Sam,
Please draw out or describe your circuit, including how you are powering the AD590. Are you measuring those voltages with the MAX186 program you posted above, or with a multimeter?

Those values don't make sense. For example, 1.2V across a 1.2kΩ resistor implies a current of 1 mA, which would be produced by an AD590 at 1000 degrees Fahrenheit (as if it could go that high). Do you see the problem?! At 80 °F, across 1.2kΩ, you should expect near 360mV.

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Tracy Allen
www.emesystems.com

sam_sam_sam

Tracy

I have it the way the data sheet shows it on the top of page 7

·I found the problem why my voltage was so off was that my input ground·was not hook up on the MAX186 Chip

Now I am getting 324.1 millivolts @ about 80*F which is still of by a little I just have to find the right value resistor to get 300 millvolts

Thank for your help

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Sam

Post Edited (sam_sam_sam) : 5/1/2010 4:20:35 PM GMT