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# calculation of errors by common mode voltage and reference voltage

Category: Datasheet/Specs

I'm trying to calculate errors by common mode voltage and reference voltage.

Output voltage error, ΔVo by common mode voltage should be calculated by

ΔVo=G*1/10^(CMRR/20)*ΔVcm

ΔVcm:=Vcm - Vcmo

Specsheet says CMRR (f=dc to 10kHz) = 86dB, but what is Vcmo?

Output voltage error by reference voltage should be calculated by

ΔVo2 = δ * ΔVref

ΔVref:=Vref - Vrefo

δ is accuracy and I'm going to apply the same volatage to Vref1 and Vref2, so δ = +/-1. What is Vrefo?

• Can someone support my question? I need to calculate total error to know the accuracy of my current sense circuit.

• Hi ,

I tried to look at your question and respond but I didn't find any attachements / files / images that'd make it easier to understand what you're asking.

Regards,

Hooman

• Hi,

I don't know where are these equations from, but I think Vcmo (isn't it Vcm0?) and Vrefo (Vref0?) are just initial values where the changes are measured from. For example, if the common mode voltage changes from 10V to 11V, then Vcm=11V and Vcmo is 10V. It may also help to read this reference about CMRR calculations and measurements: www.analog.com/.../MT-042.pdf

• This calculation comes from the datasheet of TI's INA240. https://www.digikey.com/en/products/detail/texas-instruments/INA240A1D/8571542. See sec.8.4.4.2. It's for total error calculation.

I'm going to use Vcm=13.6 [V] (typical), but I think Vcm0 is needed to calculate the error.

• My circuit is attached below. U4 is AD8418A. I want to calculate output error of U4. The common mode voltage and reference voltage should makes small error on output of U4, so I want to know how to calculate the error. IN- of U4 is typ. 13.6V and REF1 and REF2 is  typ. 1.65V.

• I think you should know what is your common mode range. You can take the expected 13.6V as Vcm0, it is the initial value. Then you can calculate how much the output will be shifted, if the common mode voltage moves from this value. Alternatively, you can estimate the minimum common mode voltage and take it as Vcm0, and Vcm can be the maximum. Then you can calculate the maximum output error over this range.