Non Inverting OP amp

[brodin]

I need to amlify a low-voltage.
I have a contactless temp sensor (SMTIR-9901).
http://www.smartec.nl/infrared_sensor.htm

It gives a few mV output (+- 0-20mV depending of air temperature).
It have two output-pins + and -.

I am planning to use a LM358 OP-AMP. Can i wire up a Non-Inverting circuit like this?


But there is still a problem if this works. The output voltage is depending on outside temp of the sensor. If the air-temp is warmer than the sufrace it is meassuring the sensor will make a negative voltage. What will happen then? Will the amplifier amplify a negative voltage?

 

[Styx]

yep but why using two gains of x11 - ie total gain of x121
why not just a single gain of x121 (resistors 120k and 1k)

also OPAMP's are capable of giving negative outputs ONLY if it has negative supply to it. so give yr OPAMP some negative volts

 

[brodin]

Ok, so there is no problem using large resistors? If i wanted to amplify x1000 would i be able to use a 1Mohm and i 1K?

My OP amp is a DIL-8 with 2 amplifyers. I thought it might be better amplifying twice, but there is maybe no difference?

 

[Roff]

You are implying that the indicated temperature is relative to the case temperature. I don't see any indication of that in the datasheet. How did you come to this conclusion?

 

[brodin]

The documentation is a bit unclear, so i called to smartec and talked to them. They said that the internal termistor in the device would be used to meassure the temperature, then the thermocouples gives an output depending to the difference in temperature between the air temp and the radiation from the surface or something.

I don't really understand how it works, i just know how to use it.

 

[Roff]

I think the thermistor needs to be used to compensate for the drift in the thermopile caused by ambient temperature. If done correctly, I'll bet the output will never go negative.

 

[brodin]

Well, look at this powerpoint presentation smartec sent me. As i understand it i need to use the thermistor to get a reference temperature, and the voltage output is the difference in temperature between sensor temp and surface temp.

 

[Roff]

I'd like to look at it, but I can't open a .rar file. Is that a compression format? If so, can you post it as a .zip file? I think I do have a Powerpoint viewer.

 

[Styx]

*.rar files are opened with "winrar" a program that is soo much better then winzip.

Wirar opens and makes zip files but also you can use the RAR algorithm that is alot better.

download it from www.rarsoft.com

 

[bmcculla]

Thermocouples generate a voltage that depends on the temperature difference between its two juctions (reference and sense junctions). Thermopiles are just a bunch of thermocouple stuck together. So its quite possible for the thermopile to generate a negative voltage. The thermistor provides the temperature of the reference junction so you can get an absolute temp. If the Thermistor reads say 20 degrees C and the Thermopile reads a difference of +5 degrees C your total temp is 25 C. If you do a google search for thermopile amp you should come up with an analog circuit to do this addition.

Hope this helps
Brent