Please, check my calculations (Diff. amp errors referred to input) - Page 1
Forum section: electronics / beginners

author: Kirill V.
author_rank: Regular Contributor
timestamp: October 01, 2019, 06:31:18 pm
content: 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 Rpand Rnterms. What this terms means?https://www.analog.com/media/en/technical-documentation/data-sheets/lt1997-3.pdfI 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 86 times.)

author: Kirill V.
author_rank: Regular Contributor
timestamp: October 03, 2019, 01:42:23 pm
content: Can't anyone help me? After all, this is a simple algebraic analysis of the circuit

author: iMo
author_rank: Super Contributor
timestamp: October 03, 2019, 01:53:23 pm
content: My guess would be the Rn = R1||R2 and Rp = R3||R4 in your doc.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: October 03, 2019, 03:18:25 pm
content: In the datasheet:Note 9: Offset voltage, offset voltage drift and PSRR are defined asreferred 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)) whereRP and RN are the totalresistance 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.

author: Kirill V.
author_rank: Regular Contributor
timestamp: October 03, 2019, 05:13:30 pm
content: Thanks!The question was what did the phrase mean:total resistanceat 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.

author: Kirill V.
author_rank: Regular Contributor
timestamp: October 05, 2019, 11:43:51 am
content: If you don't say anything about it I can conclude that my theory is correct.In any case, thank you all for any help:)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2019, 04:56:46 pm
content: Your math looks good. Cool to see the input resistors drop out leaving bias currents depending on R2 only. Input offset voltage is multiplied only by noise gain, basically by definition; and note that goes for DC offset as well as input noise e_n -- same thing.I would only suggest, in the last steps, splitting the bias current into a common and differential mode, Ib+ = Ibc + Ibd, Ib- = Ibc - Ibd, and working that out. Ibc should cancel out then, and you can be sure only the difference matters. (Actually, as drawn, since the currents are in and out of the amp, it should be the other way around. Alternately, it's enough to prove this separately, and continue with Ibd --> Ib+ = Ib- as shown.)And likewise, the noise currents of Ibd and Ibc may vary; in conventional bipolar amps, Ibc is dominant (base bias current), while in compensated amps (like LT1115) the majority of base current is supplied internally and Ibc is smaller than, or comparable to, Ibd. In FET amps, they are comparable.Also, I guess I should write Ib and Ios instead of Ibc and Ibd.Tim

author: Kirill V.
author_rank: Regular Contributor
timestamp: October 06, 2019, 12:09:22 am
content: Great thanks, Tim!If I understand you correctly, you're suggesting that I consider all the details in this analysis. This is interesting but initially I wanted to show the derivation of the formula from the datasheet.I based it on the classic op-amp with bipolar input stage and which input currents direction into inputs. Not any internal bias cancellation.Later I will try to edit all this and add analysis with unequal internal resistances of signal sources.