Current sensing circuitry

[Jelmer]

Hi, I am working on developing a new motor driver using ST motor control.

I am trying to understand current sensing circuitry (three shunt resistors) but I don't understand the offset circuitry as shown in this picture: 

As I currently understand it this R1/R2/R3 circuitry is there to add a voltage offset so that when there is reverse current the shunt resistor generating negative voltage. But what I don't understand is that this offset voltage(in the reference designs I have looked at) that offset voltage is very high (2 volts) because it is still being amplified by the Opamp resulting in a way to high ADC voltage??

What am I missing here?

[MasterT]

Pls, provide a link to reference design.

You are right, offset V is gonna be amplified the same way as signal across shunt sensor. So it works if DC offset voltage at non-inverting input is small, let say 50 mV, than x Gain (32 ?) = 1.6V at the OPA output.

Choosing small value for R3 ( <1 kOhm) and adjusting R1/ R2 should be easy, elementary school math

[STUser34 · Accepted solution]

Check the below calculations for the current sensing circuit attached in pdf

I hope you are referring to the schematic i have attached.

[Jelmer]

Ah oke so i deduct from your calculation that the offset is actually 0.129V which is then amplified 16 times. The weird thing to me is that in the motor control  workbench there is a value filled in called polarization offset which is 2.064V instead of 0.129V  which to me seems to make more sense.  Anyway its clear now thanks a lot!