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# How does the AD8422 calculate the output voltage correctly?

Sorry I'm using an instrumentation amplifier for the first time and I'm a novice
I have two sensors and the output voltage is both referenced to 0.9V. The two sensor signals differ by 180 degrees. My signal is between (0.9V+0.4V=1.3v) to (0.9V-0.4V=0.5V).
Gain is set to 2 gain
Set REF to 2.5V
Vin+=5V
Vin-=gnd (single power supply)
According to the calculation, the final differentially amplified voltage should be [(+-0.4V*2)+2.5]=+3.3V to +1.7V.
Why are the results I simulated using the simulator different?
And LTspice's analog waveform is severely distorted.
The Instrumentation Amplifier Diamond Plot Tool also says there is an error.
Is it because a negative voltage must be supplied to Vin-?

2gain
[edited by: yaro-0916 at 5:48 AM (GMT -4) on 15 Apr 2022]
• Hi,

We are looking into this and will get back to you soon.

Thanks!

Kristine

• 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.

[(+-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