Amplifier for current sensing shunt - problems...

[Odysseas]

Hello there,

I have built an amplifier, which I want to use to amplify the voltage drop across a current sensing shunt.

Here's the schematic: **broken link removed**

The left part is a simple charge pump, which creates a negative supply voltage for the OP07 in the right part. I have already constructed the curcuit and it works - almost.

The OP07 in the right part is configured as a basic differential amplifier - in theory it should amplify any voltage difference between the two pads marked Shunt High and Shunt Low with a factor of 390. It does, however, only when the voltage on the input is referred to ground.

However, my current sensing resistor is on the high-side, i.e. Shunt High is practically tied to +12V. Now, even when there is no current flowing through the shunt (i.e. no voltage difference between Shunt High and Low), the amplifier outputs close to 12V. But when I disconnect the shunt from +12V and let it float, the OP07 outputs close to 0V.

It would seem that the basic idea behind the circuit is flawed. Can anyone please help me with this?

Thanks!!

 

[MikeMl]

To get the inputs within the Common Mode Range of the opamp, you either have to operate the opamp on a voltage higher than 12V, or configure the resistors so that they act as a voltage divider to bring the inputs within the CMR, which will make it hard to get an amplification ratio much above ~2. Usually, it takes a two opamp diff-amp, a three opamp diff-amp, or an instrumentation amp to do what you are trying to do (high CMR and high gain simultaneously).

Get a high-side monitor like a ZXCT1009.

 

[Odysseas]

Alright, I get what my problem is. In fact, the input of the amp is even higher than the supply voltage, since the supply of the amp is decoupled by a diode, and the shunt is not. Thanks for your help!!

 

[audioguru]

A TL081 opamp has inputs with a common-mode range that includes the positive supply voltage.