AD8495: Output drift with power supply

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Medfood
 
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AD8495: Output drift with power supply

Post by Medfood »

Hello,
I see an unexpected problem: the output shifts with power supply.
I don't understand why?
REF (pin2-AD8495) is stable then it is not responsible of drift.
By moving supply from 5V to 18V the output drift is 40mV that corresponds to 8 degrees.
Someone knows how to solve this issue?
I thank in advance for your cooperation.
Regards
Carlo

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adafruit_support_mike
 
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Re: AD8495: Output drift with power supply

Post by adafruit_support_mike »

That's not too surprising. The AD8495 datasheet specifies a power supply rejection figure of 0.5C per Volt of change in supply voltage, with a specified supply limit of 2.7V to 5V (page 3 of the datasheet, last line of the INPUTS section, near the middle of the page).

The fact that they don't spec a wider range of supply voltages is a strong hint that the supply rejection is worse than 0.5C/V outside that 2.7V-to-5V range. Even at the 0.5C/V limit, a 13V change would be expected to cause up to 6.5C of variation.

That's pretty much inevitable for any kind of precision circuit. There's always some relationship between the supply voltage and the output, and when you're measuring something as delicate as thermocouple voltage, even small variations are on nearly the same scale as the signal of interest.

Calibrate the sensor under the expected operating conditions, and take reasonable measures to keep any non-thermocouple voltage or current connected to the sensor constant. If you have to change the supply voltage that radically during normal operation, you'll have to characterize the supply rejection for the voltage changes you expect to see.

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Medfood
 
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Re: AD8495: Output drift with power supply

Post by Medfood »

I appreciated your answer.
Thank so much.
C.

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