Hey guys
I'm having some difficulty with my sensor circuit...I posted a question a couple of days ago to no avail, so I thought I'd try and simplify things somewhat
The sensor part of the circuit (http://s23.postimg.org/ej7btm6ij/bre...19_06_2013.png) 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 a block of aluminium with the thermocouples fixed inside, the voltage measured across the thermocouple terminals increases with temperature as expected, but the output, measured against common 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.
The only thing I notice is the splitter circuit is not earthed in anyway...would this be an issue, and how would I get around it?
Thanks for looking
I'm having some difficulty with my sensor circuit...I posted a question a couple of days ago to no avail, so I thought I'd try and simplify things somewhat
The sensor part of the circuit (http://s23.postimg.org/ej7btm6ij/bre...19_06_2013.png) 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 a block of aluminium with the thermocouples fixed inside, the voltage measured across the thermocouple terminals increases with temperature as expected, but the output, measured against common 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.
The only thing I notice is the splitter circuit is not earthed in anyway...would this be an issue, and how would I get around it?
Thanks for looking