AD8609 Stability issue

Question

Am using AD8609 opamp in Load cell circuit, if the board temperature increase the OP-AMP output voltage is start to decreasing can any one help me to solve this issue, schematic attached below, supply voltage(AVCC) to OPAMP is 4V, single supply.

FormerMember · Accepted Answer

ssk18, 
 A couple of comments/questions. 
 Can you quantify "temperature increases...." 25 deg C to 35C?? 25C to 125C?? 
 "..start to decrease...." How much? From where to where? 10 mV?? 100mV?? 1V?? 
 You said "4V", but schematic says 3.75V?? 
 If yo are using the outpu of U15A to power the load cell, you are asking for 10+mA sourcing. 
 Fig 11 stops at 10 mA, but spec table 1 says at room, with 10 mA sourcing, you will get 
 4.65V or greater at room, and only 4.5V over temperature. 
 If the four resistor difference amp (U15B) has infinite CMRR, then you can run the load 
 cell on an unregulated voltage, i.e., your 4V supply. That gets rid of an op amp. 
 If you need the 3.75V for other things, then it would give you better performance without 
 the additional bridge load. 
 For precision work, I am not a big fan of quads. See the article(s): 
 http://www.eetimes.com/design/analog-design/4210663/Op-amps--to-dual-or-not-to-dual---Part-1-of-2 - 
 http://www.eetimes.com/design/analog-design/4210881/Op-amps--to-dual-or-not-to-dual---Part-2-of-2 - 
 U15C is operated at very high gain. Any offset will be multiplied by 681. CMOS op amps have 
 5-20 uV/C TCVos, so if you have to operate over a wide temperature range, (what range?) you 
 may rail the op amp. 
 Finally, do NOT use C38. It will guarantee oscillation. The noise gain of U15B will go up at 20 dB/decade, 
 and cross the op amp open loop gain going down at 20 dB/decade. C37 is fine. 
 Harry

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