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AD8495 not providing expected performance

Question asked by lmcgill2 on Jun 2, 2011
Latest reply on Jun 3, 2011 by Reemo
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Hello, I am trying to utilise the AD8495 to condition the output of a K-type thermocouple.  My circuit is very similar to figure 33 in the datasheet.  I'm using 10k series resistors, 0.1uF caps on the Tc inputs (IN+ and IN-), a 1uF cap between the Tc inputs, and a 0.1uF cap on Vs.  SENSE and OUT are tied together.

 

Vs = 5.0V

-Vs = 0.0V

Vref = adjustable

 

With the Tc at room temperature, I'm getting ~200 mV (~100 F) out of the AD8495 (note vref measures about 2 mV).  This doesn't make sense to me so I'm looking for help here.  As suggested in the data sheet, I shorted the inputs to ground and used the circuit as an ambient temperature sensor.  With this, I got ~125 mV (~77 F) out of the AD8495.  This is a little more reasonable but my thermostat in the room reads 72F.

 

Attache is my circuit.  You'll see I'm interfacing the AD8495 with and AVR microcontroller.

 

Any Ideas on why I'm getting odd results?  Let me know if there is any information I can provide.  Thanks for any help/advice.  Thanks,

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