Amplifying a Small Voltage

Resistor10k said:

Hello All,

Here is the issue. I need to amplify in the simplest and cheapest way a voltage drop across a presicion resistor of 5mΩ. There are different current levels that can run across that precision resisitor that I want be able to distinguish from each other: 0amps, 1.5amps, and 3.5 amps. So, they will give me 0volts, 7.5mV and 17.5mV repectively. Extra details: I have a single supply font of 12V. So My circuit is made of:
Bat(12V)---------(precision Res 5m)---------Load------GND.

I wanted to use two opamps to subtract/amplify (lm358) in the configuration attached as shown in the datasheet. But I ran into two problems.
Problem 1. ( guys, correct if i am wrong ) - Since i was feeding both the lm358 and my circuit with 12volts, the opamp wasn't biased in a linear region so i couldn't get anything to work. To solve that I try to things: 1. add a voltage dividor on both ends of the precison resistor, but my voltage drop ( which is already very small ) was divided by two; 2. added a resistor and a 5.1 zener to subtract the voltage, but the circuit wasn't fast enough.
Problem 2. the voltage drop is too small, very close to the offset of the lm358.

Is there some way to do that precisely?

Thanks to all in advance.

Click to expand...

ronv said:

If you really need that small of sense resistor and some accuracy with a high side monitor you will need to go with something like https://www.electro-tech-online.com/custompdfs/2010/10/ina138-1.pdf.

Click to expand...

Resistor10k said:

Hello All,

Here is the issue. I need to amplify in the simplest and cheapest way a voltage drop across a presicion resistor of 5mΩ. There are different current levels that can run across that precision resisitor that I want be able to distinguish from each other: 0amps, 1.5amps, and 3.5 amps. So, they will give me 0volts, 7.5mV and 17.5mV repectively. Extra details: I have a single supply font of 12V. So My circuit is made of:
Bat(12V)---------(precision Res 5m)---------Load------GND.

I wanted to use two opamps to subtract/amplify (lm358) in the configuration attached as shown in the datasheet. But I ran into two problems.
Problem 1. ( guys, correct if i am wrong ) - Since i was feeding both the lm358 and my circuit with 12volts, the opamp wasn't biased in a linear region so i couldn't get anything to work. To solve that I try to things: 1. add a voltage dividor on both ends of the precison resistor, but my voltage drop ( which is already very small ) was divided by two; 2. added a resistor and a 5.1 zener to subtract the voltage, but the circuit wasn't fast enough.
Problem 2. the voltage drop is too small, very close to the offset of the lm358.

Is there some way to do that precisely?

Thanks to all in advance.

Click to expand...

ronv said:

Nice op amp but it won't run on 12 volts. "Since i was feeding both the lm358 and my circuit with 12volts"

Click to expand...

ronv said:

Resistor,
He is right about 1 thing: A larger sense resistor would reduce your error from a total of 5% at the low current.

Click to expand...