# Differential voltage measurement on shunt resistor

I trying to build a simple differential measurement circuit to feed a analog to digital chip to monitor current consumption of a board with an Arduino. So I decided to go with a "traditional" differential amplifier with op-amp in front to get a nice high impedance for this improvise differential probe. Here's a simple schematic of my circuit: (Op-amp have 12v supply)

To my surprise, the reading on the 5 ohm shunt resistor is way off the real reading (measure with a multimeter). Instead of having 2.02V which correspond to a 404mA consumption (which is what is expected), I got 43mV!? I used this probe (pin 3 & 5) to check voltage around others components on my board and the reading are correct. But on the shunt R1, it's completely off. I tough that since it's a really low resistance that my high-impedance will be ok be it seems not. I look around on the net and the solution seems to go with this approach but that's not working for me right now... Any suggestion?

• You're powering the TLC272s off the same 12V rail? If so you are violating the common mode input range for IC1A and IC1B -- their inputs need to be about 1V below the positive rail, according to the datasheet. – Null Jul 11 '15 at 21:26
• Even worse, you may be powering the 272 from the +5 line, in which case you're lucky the chip is still working - if it is still working. – WhatRoughBeast Jul 11 '15 at 21:33
• @Null, which spec are you talking about? Page 4 : Differential input voltage, VID = +- VDD and Input voltage range, Vi is -0.3 to VDD. ?? Anyway, I made a test and and 2 x 2 diode in series to drop the voltage and it's the same. – Steve S. Jul 11 '15 at 21:47
• @WhatRoughBeast My question say : Op-amp have 12v supply. ?? – Steve S. Jul 11 '15 at 21:48
• $V_{\text{ICR}}$, p. 6. With a 10V supply the common mode input range is up to 9V, so you need about 1V of headroom. – Null Jul 11 '15 at 21:52

The output voltage (to ADC) is trying to go negative, since the left side of $R_1$ is more positive than the right side. Try reversing the connections to $R_1$