Hey guys.
I build a circuit a few weeks ago and thought I cracked it with little problems, now I've come back to calibrate it and its not behaving as I'd expect it...
I have a couple of op amps which are giving unexpected results, and was hoping somebody could see whats happening.
The sensor part of the circuit consists of two op-amps each with a separate matched cold-junction compensation IC (http://www.linear.com/product/LTK001) to read temps from two K-type thermocouples, powered by a regulated split 12v (-6v to +6v) The result is fed into the ADC of a PIC18F4550.
And here's the problem...when I heat the aluminium with the thermocouples fixed inside, the voltage measured across the thermocouple terminals increases with temperature as expected, but the output, measured against ground sits at about 0.3v until the thermocouple output measures about 2.5mA at 70 degrees. Then the voltage rapidly increases to ~5v, settles and continues to rise fairly linearly with temperature.
Here's the schematic for the sensor board...
http://postimg.org/image/amz6z7xiz/
Any ideas would be welcomed, and thanks for looking
[Edit]...just noticed, the 1025 isn't rail to rail, but I'm powering it off the +6v without any common ground, is this what could be causing my problems?
I build a circuit a few weeks ago and thought I cracked it with little problems, now I've come back to calibrate it and its not behaving as I'd expect it...
I have a couple of op amps which are giving unexpected results, and was hoping somebody could see whats happening.
The sensor part of the circuit consists of two op-amps each with a separate matched cold-junction compensation IC (http://www.linear.com/product/LTK001) to read temps from two K-type thermocouples, powered by a regulated split 12v (-6v to +6v) The result is fed into the ADC of a PIC18F4550.
And here's the problem...when I heat the aluminium with the thermocouples fixed inside, the voltage measured across the thermocouple terminals increases with temperature as expected, but the output, measured against ground sits at about 0.3v until the thermocouple output measures about 2.5mA at 70 degrees. Then the voltage rapidly increases to ~5v, settles and continues to rise fairly linearly with temperature.
Here's the schematic for the sensor board...
http://postimg.org/image/amz6z7xiz/
Any ideas would be welcomed, and thanks for looking
[Edit]...just noticed, the 1025 isn't rail to rail, but I'm powering it off the +6v without any common ground, is this what could be causing my problems?
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