# Difference between input and output offset voltage

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#### Junus2012

Dear friends,

What is the difference between the input and output offset voltage in operational amplifier, how to simulate both of them ?

Thank you

#### deep_sea

##### Full Member level 1
Hi. Assume you have the differential input is zero (and common mode input voltage is zero for the moment). You opamp will produce ideally a zero output voltage. In reality, it will produce the "output offset" voltage. Now if you want the output to be zero, you have to apply the "input offset" voltage to cancel the non-ideal offset effect.
Vin(offset) = Vout(osset) / open loop gain

Junus2012

### Junus2012

Points: 2

#### asdf44

Having seen your other thread I also assume here you're referring to inst amps.

Inst amps are usually 2 stages. Input offset is the input referred offset of the first stage. Output offset is the offset of the final stage.

Total offset should be Vos_in*gain+Vos_out

Or referred back to the input it should be Vos_in+Vos_out/gain

Points: 2

### Junus2012

Points: 2

#### deep_sea

##### Full Member level 1
Adding to asdf44, the offset is better understood by Vos_in so that you can assess the non-ideal voltage characteristics of the opamp (inst amp) independent on the gain.

Junus2012

### Junus2012

Points: 2

#### Junus2012

Having seen your other thread I also assume here you're referring to inst amps.

Inst amps are usually 2 stages. Input offset is the input referred offset of the first stage. Output offset is the offset of the final stage.

Total offset should be Vos_in*gain+Vos_out

Or referred back to the input it should be Vos_in+Vos_out/gain

Dear friends,

How would you know I mean about Instrumentation amplifier ? does it mean that op-amp only characterized by input offset voltage ?

#### asdf44

Correct, you'll only see one offset number in an opamp datasheet.

#### Junus2012

Dear asd

Thank you alot for your help,

Still it is not clear for me how to simulate the input and output offset voltage seperately ?

Thank you

#### asdf44

Draw the standard 3 op-amps instrumentation amplifier circuit and add offsets to each amplifier. The net offset of the input amplifiers is the input offset. The net offset of the output amplifier is the output offset (it will include Vos and Ios*R).

In the LTSpice the ideal opamp model has a Vos parameter you can manipulate. Alternatively you can insert a series voltage source at the opamp input.

Junus2012

Points: 2

Points: 2