I have designed a board with AD8495 and AD693 to measure temperature with a K-Type thermocouple,within the range of 0~400 degrees centigrade.When I use ICE bath to test this circuit,the results a totally frustrated me, cause when I change the input of ad8495 as 16.397mv ,(signal is output from YOKOGAWA CA150 ,and through K-Type thermocouple then get into AD8495 ),K-Type thermocouple' cold junction is in the ice bath,
Vref=-11.xxx mV( OP1177 OR AD4077-2 is used to provide the voltage for the AD8495 reference pin.)
I found that the output of AD8495 which should be 2.01V is keep increase from 2.007XX V to 2.03xxxV ,and this increase let the output of AD693 which should be 20mA,now it also increase from 19.987mA to 20.03XXXmA, in another words, the output of 4~20mA have about 50uA fluctuation and this is too large for me ,
keep the input of AD8495 at 16.397mv for about 30mins, 20.3XXXmA will be stabilized. then turn the input of AD8495 to 0 mV, AD693's output increased about 50uA either ! Then keep 0mV for about 30mins ,50uA will be decreased to the normal.And then turn the 0mV to 16.397mV, keep 30 mins ，about 50uA will be increased again! And then....
So my circuit's stability is not as good as I expected, and I can not find what's wrong with it.
AD8495 and AD693 are connected through a resistive divider of 2200R/670R.