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Please, check my calculations (Diff. amp errors referred to input)

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Kirill V.:
Hi every one!
Some time ago, on other forum was asked question:

In datasheet of diff. amplifier LT1997 says about output offset formula (page 6, Note 9). In this formula uses Rp and Rn terms. What this terms means?
https://www.analog.com/media/en/technical-documentation/data-sheets/lt1997-3.pdf

I decided to make my own calculations to get a similar expression. Now I am interested, are my results correct? Please, check it!
Please, note that I uses IEC symbols and much mathematics:)
 Diff. amp calculation.pdf (840.52 kB - downloaded 78 times.)



Kirill V.:
Can't anyone help me? After all, this is a simple algebraic analysis of the circuit

iMo:
My guess would be the Rn = R1||R2 and Rp = R3||R4 in your doc.

RandallMcRee:
In the datasheet:

Note 9: Offset voltage, offset voltage drift and PSRR are defined as
referred to the internal op amp. You can calculate output offset as follows.
In the case of balanced source resistance, VOS,OUT = (VOS • NOISEGAIN)
+ (IOS • 22.5k) + (IB • 22.5k • (1– RP/RN)) where RP and RN are the total
resistance at the op amp positive and negative terminal, respectively.

In cases like this, where doubts arise, you might want to create a simple LTSPICE model and verify your calculations.

Kirill V.:
Thanks!
The question was what did the phrase mean: total resistance at the op amp positive and negative terminal, respectively. In other words, this is addition of resistances or parallel value of resistances. I think I've been able to prove mathematically that it's a parallel connection R1||R2 and R3||R4 respectively. In datasheet not says about this details directly.
LT1997 provides the user with the ability to select different internal resistors which can cause output offset. Standard differential amplifier has unchangeable internal resistor networks, therefore R1||R2=R3||R4 in this case and this calculation do not need.
I would like you to appreciate the accuracy and consistency of my theoretical calculations. I've already tested it with the simulator and that's right, seem to me.

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