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Old 22nd June 2010, 08:07 PM   #1
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Default Bit OT, thermocouple amplifier

Hi Folks,

This is a bit OT, but maybe there is someone out there that knows about this. I am trying to design an op-amp circuit to amplify the difference between two thermocouples, I basically have an iron common and two constantan wires in different locations, I don't care what the absolute temperature is, but I need to amplify the difference up to about 1 volt so I can feed this into a microcontroller.
I think the delta temperature between the two is about 10-20C at ambient.

Does anyone have a suitable circuit?

Oh and preferably I need something that can work from a single ended supply, IE +12 volts and ground, not +12v -12v etc.

Thanks!

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Old 22nd June 2010, 10:31 PM   #2
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Go to the web sites of a few companies that make op amps. National Semiconductor, Analog Devices, Linear Technology, Texas Instruments. They all address this problem and have devices that will work for you. Their application notes are treasure troves of information.
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Old 23rd June 2010, 06:43 PM   #3
star882 is offline star882  United States
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Just use a low offset opamp.
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Old 23rd June 2010, 07:14 PM   #4
tomchr is offline tomchr  United States
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Low offset, low 1/f noise op-amp wired as a differential amp or instrumentation amp would do the trick. I suggest looking at the LMP2021 or LMP2022 for this purpose. 0.4 uV nominal offset, 5 uV max over temperature. No 1/f component to the noise. 40 nV/deg C TC(Vos).

I wonder if you can exacerbate the difference voltage by using a Wien bridge where the thermocouples form two of the legs. Just a thought...

Another option would be to use one of the dedicated thermocouple interface chips with cold junction compensation. I think Maxim makes some. You'd then get the two absolute temperatures and do the math either in a micro controller or with a differential amp.

Thermocouples can be tricky. Remember that every time you have two dissimilar metals joined together you have a thermocouple. The little bead at the end of the thermocouple wire is obvious, but the "cold junctions" i.e. where the ends of the thermocouple wire connects to your circuit are also thermocouples.

~Tom
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Old 23rd June 2010, 07:21 PM   #5
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Just like bob91343 said:
http://www.analog.com/static/importe.../AD594_595.pdf
http://www.analog.com/static/importe...61261AN369.pdf
http://www.analog.com/static/importe...82741AN274.pdf

This will give you all the background you need.
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Old 24th June 2010, 03:20 AM   #6
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Thanks guys, this helps a lot
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Old 29th June 2010, 04:09 AM   #7
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My application has changed a bit, I'm going to use a self heated platinum RTD in a bridge and measure the voltage output, I think it will work better.

There are lots of circuits online for wheatstone bridge's but most of them try and avoid heating of the RTD element, I actually want to self heat my RTD and measure the cooling.

Can I just use low resistance values to drop some large currents through the system to heat the RTD? problem is the other resistors in the system also has large powers going through them...
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Old 29th June 2010, 04:48 AM   #8
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The temperature transfer function for most thermocouples/thermistors/RTDs is highly nonlinear. If you want to compute a temperature difference it is important to understand that the signal difference between the two elements will not be constant with temperature. For example, a +5 degree difference at 40 degrees will yield a different result than a +5 degree difference at 70 degrees. The correct way of solving this problem is to linearize each device separately and then take the difference between the two. Omega makes a line of differential thermocouple meters that do this, and I have used them with good success.
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Old 29th June 2010, 04:32 PM   #9
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Plug RTD into the search engine at Linear Tech --

AN11 - Designing Linear Circuits for 5V Single Supply Operation
... Linearized RTD Signal Conditioner Figure 1 shows a circuit which provides complete,
linear- ized signal conditioning for a platinum RTD. ...
cds.linear.com/docs/Application%20Note/an11f.pdf - 2010-03-27

AN93 - Instrumentation Applications for a Monolithic Oscillator
... Platinum RTD Digitizer A platinum RTD, used for R SET in Figure 5, results
in a highly predictable O1 output period vs temperature. ...
cds.linear.com/docs/Application%20Note/an93f.pdf - 2008-01-01

AN23 - Micropower Circuits for Signal Conditioning
... circuit techniques. Platinum RTD Signal Conditioner Figure 1 shows a simple
circuit for signal conditioning a platinum RTD. Correction ...
cds.linear.com/docs/Application%20Note/an23f.pdf - 2010-06-08

AN52 - Linear Technology Magazine Circuit Collection, Volume 1
... Temperature-Measurement System The circuit in Figure 1 shows how a transducer output,
such as a platinum RTD bridge, can be digitized with one op amp. ...
cds.linear.com/docs/Application%20Note/an52.pdf - 2008-01-01

AN78 - A Collection of Differential to Single-Ended Signal ...
... Rail-to-Rail Input Common Mode Range and is “Live at Zero” LTC2400 Bonus Circuits
#1: An Extremely High Resolution LTC2400-P t RTD Temperature Digitizer ...
cds.linear.com/docs/Application%20Note/an78fs.pdf - 2008-01-01

AN25 - Switching Regulators for Poets
Page 1. Application Note 25 AN25-1 an25fa Switching Regulators for Poets A
Gentle Guide for the Trepidatious Jim Williams September 1987 ...
cds.linear.com/docs/Application%20Note/an25fa.pdf - 2008-01-01

An62 - Data Acquisition Circuit Collection
... AN62-10 ..... LTC1091/2/3/4 DS Digitally Linearized Platinum
RTD Signal Conditioner ..... Figure 14 ..... ...
cds.linear.com/docs/Application%20Note/an62.pdf - 2008-01-01
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