Measuring incorrect voltage by multimeter.What could be reason

[Vindhyachal.takniki]

1. I was measuring voltage of thermopile with Fluke multimeter 15B+. Attached is my circuit.  Its party incomplete shown but in actual its complete in hardware.

2. When I measure voltage by only thermopile in hand and not noumted on board, only connect probes of multimeter, I can measure voltage as per its mV vs Celsius graph.

3. When I connect it to board, and don't give 3.3V supply to board and connect probes across J1 connector, then also correct voltage read.

4.But when I give 3.3V to circuit, and connect probes across J1 connector, ideally it should be the same, because I am still measuring across the thermopile only,
but its very off than actual values, which I was reading without giving power. Why is that so?

5. is it high values 1M resistor connected across? But still y probes are connected across thermopile NO?

Earlier when I was reading 10mV, not I am reading around 200mV.

[Cubdriver]

I would assume that it's the meter's input resistance (probably 10M) that is now in series with the (2) 1M resistors connected between your thermopile and the 3.3V and ground rails.  Without the thermopile in the circuit, the voltage divider resulting from the meter and the resistors connected between the 3.3V rail and ground would put about 2.75V across the meter.  Adding the thermopile in parallel greatly reduces the meter's influence on the voltage divider, but between the thermopile generating voltage and its shunt resistance in what is effectively a voltage divider, I'm not surprised that the voltage readings are off.

-Pat

[mikerj]

5. is it high values 1M resistor connected across? But still y probes are connected across thermopile NO?

But you are passing additional current through the thermopile, and it's impedance is not zero so there will be a greater voltage drop across it.  Try changing you biasing scheme so the bias voltage is applied to only one side of the thermopile.

[bdunham7]

What is the purpose of resistors R2 and R5?

[mikerj]

What is the purpose of resistors R2 and R5?

To bias the common mode voltage within range of the ADC.

[bdunham7]

Would you not want to run them both to the same side of the thermopile instead of across it?

[Vindhyachal.takniki]

thanks sorted

[Vindhyachal.takniki]

I am using this thermopile:

Now i am able to measure the thermopile voltage(verified by multimeter also, same reading)  & thermistor votage and then able to convert thermistor voltage into celsisus also.

 

Now I need to convert it into the object temperature. Any mathematical equatin where I can direclty put these values and get the object temperature?  Processing not an issue, since I have send both thermopile and thermistor values to PC, there made a small software. so rather than going by array search better to go with equation.


I downlaoded TE connectivity sheet, they have this one equation(attached also) 
but emisstivity of object also varies, how to keep that into accoount?

 

Most of places I have searched they all do by array search, is there equation method also?

 

[Vindhyachal.takniki]

I am able to measure thermopile(Vtp) mV & NTC(Celsius) temperature correctly as black body reference temperature when measured display correct results.
Took thermopile reading and NTC and did curve fit value.

 
Now when I shifted to human body measurement, results are wrong, due to emmisivity. I took e= 0.95 & 0.97 values and divided Vtp by it & then tried curve fit, but values are all wrong. Can someone suggest what is right way to shift to human body?

1. Currently only dividing Vtp by e. Is it ok?
2. Or need to divide NTC also by e?
3. Or divide final temperature by e?
4.What is right method?