How to figure out the maximum current flowing into your op-amp

Thread Starter

MikeJacobs

Joined Dec 7, 2019
201
So i am trying to figure out how to understand what the maximum current flowing into and or out of depending on construction of the non inverting and inverting terminals of an op amp

I know its some combination of input bias current with some addtion of input offset current.

Can someone explain a bit more?
Thanks
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
201
I understand that but its not a futile effort when trying to understand the error of an op-amp
that total current multiplied by a very high input impedance gives way to significant voltage error
 

crutschow

Joined Mar 14, 2008
27,218
The voltage error is the op amp input offset voltage multiplied by the non-inverting gain of the circuit (even if it's configured as an inverting amp).
The bias voltage error is equal to the input bias current times the feedback resistance.

The op amp input bias current is unrelated to the input voltage offset.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
201
The voltage error is the op amp input offset voltage multiplied by the non-inverting gain of the circuit (even if it's configured as an inverting amp).
The bias voltage error is equal to the input bias current times the feedback resistance.

The op amp input bias current is unrelated to the input voltage offset.

I am completely dumbfounded as to why you would say the voltage offset has nothing to do with the input bias current.
Case in point the voltage offset error is caused by the input bias current multiplied by the input impedance.
I know you have been around a long time but are you sure you did not just mis speak there?
Thanks
 

Ian0

Joined Aug 7, 2020
2,226
The voltage error due to the bias current is the bias current multiplied by the resistance to ground, not by the input impedance. This voltage error is added to the offset voltage of the op-amp, and the error at the output is the sum of the two multiplied by the DC voltage gain of the circuit.
The maximum input bias current is quoted in the datasheet. Use this figure to calculate the maximum error. Measuring it for an individual op-amp and using that figure for calculations will lead to problems if the op-amp is ever replaced.
 

Thread Starter

MikeJacobs

Joined Dec 7, 2019
201
Appreciate all the answers.
Can someone answer this.

What is the total current into the terminal?

Is it Input bias + Input offset?
 

crutschow

Joined Mar 14, 2008
27,218
I am completely dumbfounded as to why you would say the voltage offset has nothing to do with the input bias current.
I'm referring to the bias current at the input terminals to the op amp, not the current flowing into any of the bias resistors.
The op amp bias is basically unaffected by any other parameters.
 
Last edited:

dl324

Joined Mar 30, 2015
12,871
So i am trying to figure out how to understand what the maximum current flowing into and or out of depending on construction of the non inverting and inverting terminals of an op amp
It's in the datasheets.

Normally we don't worry about input bias currents. Normal design guidelines tell you to match the impedances on the inputs so the effects of bias currents will cancel.

This is from a 1979 Signets Analog Applications Handbook:
clipimage.jpg
 

MrChips

Joined Oct 2, 2009
23,531
Generally speaking, op-amps are used to amplify DC or AC.
If you are working with AC signals then you can remove the DC offset.
If you are working at 0Hz then you have at lot more to have worry about.
 

crutschow

Joined Mar 14, 2008
27,218
Let me see if i can find the equation. Its basically a certain combination of the input offset current and the input bias current.
Input bias current is the current into (or out-of) each op amp input terminal.
Offset bias current is just the difference between the two (as they are typically not quite identical).
You can reduce the voltage offset from the bias current by making the equivalent impedance on each input equal.
Then the voltage offset is just the offset current times that impedance.
 
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