I am hoping you can help me.

I am trying to design an op amp circuit that can read negative 2ma-20ma. We are trying to read a thermocouple and we have tried many variations of a simple circuit design with no luck. Can any one please help. I have attached our current circuit design.

Important details:

-Single Supply Voltage (battery)

-Negative DC mV input

-Invert input and amplify for ADC reading.

 

I am hoping you can help me.

I am trying to design an op amp circuit that can read negative 2ma-20ma. We are trying to read a thermocouple and we have tried many variations of a simple circuit design with no luck. Can any one please help. I have attached our current circuit design.

Important details:

-Single Supply Voltage (battery)

-Negative DC mV input

-Invert input and amplify for ADC reading.

I don't see your attachment.

 

Sorry about that.

I have attached it.

Thank you.

 

Sorry about that.

I have attached it.

Thank you.View attachment 132489

Since your circuit is AC coupled on both the input and the output, I don't see how you can expect the DC thermocouple signal to propagate through the amplifier.
Can you explain why you think the circuit should work, please?

 

You have the thermocouple signal input and output capacitively coupled.
Does that mean it is an AC signal (since capacitors don't pass DC)?
If so, what is its frequency?

 

questions;
what is the open voltage(ov) at the -2mA=>-20mA.
you feed this current into a resistor. ( resistor x 20mA max voltage) value resistor is linked to ov and output impedance 2-50mA device.
(resistor to high current will not flow. Low should be no problem output current should be limited )

Your opamp has two inputs one used for offset and one to feed a dc signal ( no cap (dc block) at input).
The resistors you are using 47Ohm are far to low use 47KOhm.
Assume an input resistor of 50Ohm @ 20mA it should produce 50 x 20 =1000 mV and @2mA 100mV.

so R5/R2 = 2 =>> 200mV to 2V scale r5 =100K r2=47K correct/ calibrate after ADC.

Picbuster

 

Thermocouples produce very tiny voltage, not current like 20mA. What is the temperature sensor that you are actually using?

 

Even if your circuit were DC coupled, it would not work. The circuit has a DC gain of over 200. 20 mA into 47 ohms is almost 1 V, so the circuit would try to make an output of 200 V. Where did you get this circuit, and why do you think it will work?

This opamp has a common mode input range that is spec'd down to 0 V and typ down to -0.2 V below the negative rail. Not great, but good enough to delete R6 and R7, and tie the + input to GND. This will give a + output voltage for a - input current. As a test, change both R2 and R5 to 100 ohms. The output won't like it, but you should see it vary with the input.

As noted above, a thermocouple typically is a voltage-mode device. What are you using that can generate 20 mA? Also, there are analog parts designed specifically to condition thermocouple signals.

What is the output voltage range you need?

ak