I'm trying to use the AD8220 to measure resistance tolerance, here is my circuit and measurements :
Could you please advise the difference in output voltage swing, as a function of: (V+- V-),VS & Rg ?
I am looking into this. I'll get back to you soon.
For the 5V operation, it looks like you are operating outside of the valid input voltage range of the amplifier. From page 6 of the datasheet, the valid input voltage range of the AD8220 only allows input voltages from -0.1 to 3V (for the case of a 5V supply). Looking at your circuit, if you have a 2mA current running through your resistors, Vin+ would already be seeing 4V, resulting in an invalid output. When you extend the supply to 8V, the input voltage range now goes to 6V, which is why you get the correct output at the AD8220.
Is the lower 1k resistor needed? Your circuit will work at 5V if you remove this since the voltage at the input pins will sit within the input voltage range of the part.
I hope this helps!
Thanks for your quick and helpful answer.
But in that case (that my input voltage getting closer to (Vs-2V=3V) Shouldn't I get the Max. Rail voltage on Vout ?
(as saturated amp )
Not necessarily. Operating outside of the input voltage range puts you in an undefined state so your results may be unpredictable. However, I think that in your case, you may also be hitting some limitations due to your input common mode range. You can check this thread for more information.
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