How does the AD8422 calculate the output voltage correctly?

Hi yaro-0916,

Determining the headroom of an InAmp is different from an OpAmp. Aside from headrooms, InAmp also uses diamond plots to determine the headroom. You can read more on the diamond plot in this article: Instrumentation Amplifier Common-Mode Range: The Diamond Plot. 

For example, AD8422's diamond plot is shown below for your requirement 5V single supply, 0.4V differential inputs and gain of 2.

The white space inside the diamond represents the area of your operation. For a supply of 5V, your common mode voltage should be at least around2V.

if the sensor's reference can't be modified, suggestion is to make AD8422's operation in dual supply.

The waveform in LTSpice becomes distorted due to the violation in the diamond plot.

yaro-0916 said:

[(+-0.4V*2)+2.5]=+3.3V to +1.7V.

By the way, your output voltage calculation should be (0.4V-(-0.4V) x 2 + 2.5V. Differential voltage is 0.8V hence the Vdiff input signal in the diamond plot tool.

Let me know if you have more questions.

Best regards,

Kristine

First of all thank you for your help~
Yes, I can't change the common mode voltage of the sensor, so I may need a dual supply of +-5V.
But I really can't find a way to provide negative voltage.
Since I need the final output noise to be less than 40uV I will go with a voltage reference IC, but the problem is an IC without a negative voltage reference
. So I am confused.
Can you give me some suggestions?

Feel sorry!. I found out that your Instrumentation Amplifier Diamond Plot Tool has an auto-suggestion feature. So I have no more questions thank you

Feel sorry!. I found out that your Instrumentation Amplifier Diamond Plot Tool has an auto-suggestion feature. So I have no more questions thank you