Help - Amplifier for Thermocouple

Hi all,
I'm doing a project of reading a temperature with an IR sensor (Calex EL serie) with an output of 1-50mV for a 0-250ºC. This output must be amplified in order to use a ADC MCP3201 (12 bits) and a PIC 16F876a. After that the read values must be sending by an RF module (easy radio ERx00TRS02) to a PC.
Starting for the beginning: I need an instrumental amplifier with a gain=20 adjustable by digital POT by the PIC. I’m thinking about using the LTC1051 that have a low offset (5:mu: V) and chopper stability. Which circuit do you suggest to use for this amplifier? I’m thinking to use the one suggest in the Data-Sheet of the LTC1051 page 1 (High Performance Low Cost Instrumentation Amplifier).
If you have any suggestion please reply here or to my email: ana.filipa.matos@sapo.pt

Thanks in advance

AFMatos said:

Hi all,
I'm doing a project of reading a temperature with an IR sensor (Calex EL serie) with an output of 1-50mV for a 0-250ºC. This output must be amplified in order to use a ADC MCP3201 (12 bits) and a PIC 16F876a. After that the read values must be sending by an RF module (easy radio ERx00TRS02) to a PC.
Starting for the beginning: I need an instrumental amplifier with a gain=20 adjustable by digital POT by the PIC. I’m thinking about using the LTC1051 that have a low offset (5:mu: V) and chopper stability. Which circuit do you suggest to use for this amplifier? I’m thinking to use the one suggest in the Data-Sheet of the LTC1051 page 1 (High Performance Low Cost Instrumentation Amplifier).
If you have any suggestion please reply here or to my email: ana.filipa.matos@sapo.pt

Thanks in advance

Click to expand...

The LTC1051 is a good choice for this application. The LTC1051 is not an instrumentation amplifier by the way. You can however, connect a couple of these together to _make_ an instrumentation amplifier. You can see an example in the datasheet you posted. Note the tight resistor matching to maintain a high CMRR. If you really need an INAMP, then CMRR is important to you so don't overlook this aspect of the design.

Also, your circuit layout will be critical. You will have to take care not to form "parasitic" thermocouple junctions with the components and copper tracks/wires. You can easily destroy the 5uV offset voltage spec of the '1051 if you are not careful.

You didnt mention accuracy for your temperature readings so a crummy CMRR & offset might not be a problem to you - I dont know. I do know that achieving < 1 deg C accuracy spec can be a challenging design.

Post your design here for review when you get it worked out.

Hi,
thanks for your fast answer.

Optikon said:

Note the tight resistor matching to maintain a high CMRR. If you really need an INAMP, then CMRR is important to you so don't overlook this aspect of the design.

Click to expand...

Sorry my bad english, what do you mean about tight resistor?

Tx,

Look at the typical 2 opamp INAMP circuit on first page of your datasheet.
I do not know if this is how you will wire up your circuit. If you do, notice the precision resistors needed.

Optikon said:

Look at the typical 2 opamp INAMP circuit on first page of your datasheet.
I do not know if this is how you will wire up your circuit. If you do, notice the precision resistors needed.

Click to expand...

My intend is to use that circuit (what do you think about that?) with the digital POT here, controlled by the PIC. I don’t know yet if I have to use in series a precision resistor to achieve a better precision. What do you think about this implementation?

In your first post you talk about accuracy of the temperature readings, the CMRR and offset. The only imposition that I have now is to have a precision in the output of the circuit of the thermocouple+INAMP of 0-1.000V. So if the maximum of the offset of the LTC1051 is 5uv with a gain of 20 we have 0.0001V that are in the range that I have to have in the output of this block thermocouple+INAMP. I know that is very difficult to achieve because of noise. How can I turn around this problem? Can you give me some ideas?

Thanks in advance
Ana

AFMatos said:

My intend is to use that circuit (what do you think about that?) with the digital POT here, controlled by the PIC. I don’t know yet if I have to use in series a precision resistor to achieve a better precision. What do you think about this implementation?

In your first post you talk about accuracy of the temperature readings, the CMRR and offset. The only imposition that I have now is to have a precision in the output of the circuit of the thermocouple+INAMP of 0-1.000V. So if the maximum of the offset of the LTC1051 is 5uv with a gain of 20 we have 0.0001V that are in the range that I have to have in the output of this block thermocouple+INAMP. I know that is very difficult to achieve because of noise. How can I turn around this problem? Can you give me some ideas?

Thanks in advance
Ana

Click to expand...

The CMRR of that circuit will depend on the resistors. If your CMRR is too low, then you will have to model it's error term as an offset voltage. This may be too high for you (I dont know). If these resistors are digital pot types, the tolerance mismatch _may_ cause too low of a CMRR for you. I dont know, you have to do analysis.


Also,

You need to setup an error budget to see if you will get the accuracy you need. Start with the Temperature reading accuracy. What kind of thermocouple and what accuracy (+/- 1 deg C??? 10 deg C???)

... then translate this into a voltage accuracy spec. Now, with a voltage accuracy spec you can determine how much total error you can tolerate in your amplifier circuit. (ITS-90 tables for standard thermocouple types)

How can you even know that the circuit shown will provide the accuracy unless you do this error budget analysis?