[SOLVED] AD8422 Voltage Follower

[chandlerbing65nm]

Hello folks,

I want help on this voltage follower set-up of AD8422.
Vs = +/-15v
Vin = +/-10v
Rg = 10Meg
Gain = 1

Voltage follower/Unity Gain is supposed to be copying the input voltage with offset. Why am I getting half the input voltage only, -5.006V?

Gain is set to one by using Rg = 10Meg resistor. Here's the gain calculation in the datasheet.

Is there a conflict between the unity gain circuit used and the gain calculation? Both have Gain = 1 configuration. If not, what is my wrong here.

Regards,
Chandler

 

[KlausST]

Hi,

the quoted text clearly states:
"The AD8422 defaults to G=1 when no gain resistor is used".
It seems you want G=1, then why do you add a gain setting resistor?

***

Gain is set to one by using Rg = 10Meg resistor

This is against the given datasheet informations. How do you come to this conclusion?

Klaus

added:
Where do you have this circuit from? Obviously not from the datasheet, because no circuit in the datasheet shows a feedback from output to inverting input. Mind: This is no OPAMP!

My recommendation: read the datasheet and follow it´s informations, then everything will be fine.

****
To me it seems you don´t want/need an instrumentation amplifier at all --then just use a typical OPAMP.

 

[BigBoss]

AD8422 is not an standard OPAMP as you can see..

 

[FvM]

With an instrumentation amplifier, the input voltage has to be connected between both inputs. In this case, connect the "-" input to ground. You don't use external feedback, it's already built inside.

 

[chandlerbing65nm]

I now realized, AD8422 is a InAmp. Thanks!