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AD8429 as thermocouple amplifier

Question asked by neeviour on Mar 25, 2013
Latest reply on Apr 2, 2013 by Gustavo.Castro

Trying to build a thermocouple amplifier with AD8421/8429 in-amp.

 

The thermocouple is 0.0005" diameter type K which will be working in non-linear range, and that's why we could not use any existing thermocouple amp. It has a resistance around 1300ohm: 800ohm on positive arm, and 500ohm on the negative side. Only the joint potion of the thermocouple will be subjected to temperature change.

 

The proposed solution is to use AD8421/8429 in-amp with a gain of 100. then the output is captured by NI X-6366 simutaneous DAQ differential analog inputs, which have another set of in-amps. Desired sampling rate will be 50kHz+. For mobility reason the amp is battery-powered, and the reference voltage is set as the mid point of the battery packs (two independent regulated battery packs, +/-5V).

 

While let the thermocouple floating or connect the IN- pole directly to the reference, the amp is working but accuracy might be questionable considering the bias current. Multisim said so and I have not tested it out. By connecting either one or both of the IN-/+ poles to reference with 10Mohm resistor (as stated in the data sheet and other sources) will disable the amp. The output does not change from zero.

 

Q1\ Considering the amp part, what's the right way to create current return parth in this case? Or, may the error appear to be a constant zero-drift? Then it can be processed later in software.

 

Q2\ For RFI suppression, I would use the resistance of the thermocouple as part of the single stage low pass filtering. Thus only capacitors are needed. Is this correct? Is there any guidelines I should take in choosing the type of these capacitors (~200pF)?

 

Q3\ Considering the DAQ side, AI+ connects to AD8429 OUT, AI- connects to AD8429 REF. Then AI+ and AI- are connected to AGND with a 10Mohm resistor, respectively. Is this setup correct? Or should I ground the AD8429 REF? Or should I take the AD8429 as a referenced source? I tried all setup and observed no difference in output.

 

Q4\ For the focus of tracking fast temperature change, which one among the AD8421/AD8429 appears to be a more suitable unit in this case? May the low bias current of AD8421 minimize the problem in Q1?

 

Thanks for your help!

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