What would you recommend to detect thermocouple opens, shorts with this circuit?  I am considering current injection using the on-chip current sources buffered by a transistor.

Hi Dan,

We did not implement hardware detection in the circuit for open circuit because it was easily detected in software.

Could you explain in more detail how you were going to check for short circuits?

Regards,

Rob

Hi Rob,

The circuit I am looking at is below.  I was thinking that I could detect an open, shorted, and normal TC by injecting current into it.  The thinking is - when I inject current via Q1, if the TC is normal, I should see a small increase in voltage, if it is open, I should get a large increase in voltage, and if it is shorted, I should not see voltage change.

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Regards,

Dan

Hi Rob,

Actually, I can think of some reasons NOT to do the detection the way I described.  1)  If CM TC voltage is too high, Q1 will not turn on.  2) If TC CM goes too negative WRT micro ground, Q1 will not turn off.  3) Q1 leakage may distort the measurement.

Basically, it does not work.  What were you considering doing in software?

Regards,

Dan

Hi Dan,

You could use the Burnout Currents on the channels and make a judgement call in the software.

Page 25 of the datasheet explains how an open circuit and short circuit may be detected on thermocouples.

Set bit 13 in the configuration register and monitor bit 6 in the status may point to open circuit and short circuit with flags at all 1's and 0's.

http://www.analog.com/static/imported-files/data_sheets/AD7792_7793.pdf

Regards,

Rob