Low res_ohmmeter circuit

Hi,
I am designing a low res measurement circuit with these specs; 10mOhms(0.1V) to 100mOhms, <70mA supply current, +/-5V supply rails, and under 2% output error. Below is what I tried on Proteus;



Is this practical enough to build? I will add a reference 100milli Ohm 0.1% and switch to calibrate current source.

I tried to add an offset nulling pot but there was no effect on the simulation; probably it is not implemented in the spice model.

I also thought of introducing a reference voltage to calibrate the amplifier. What do you suggest?

thanks

The circuit looks as though it should work. Be careful about supply decoupling. How are you going to "test" a 10m Ohm resistance, contact resistance can easily exceed 3 mOhm. Will it be a 4 terminal measurement?
Frank

I've done a lot of this kind of measurement and I doubt your method will work. You do not state where the measurment will be taken but you also have to take into account that you will have 5V across open probes and the resistance between the current source and voltage measurement points will almost certainly be much higher than the resistance you are trying to measure. If used on circuits containing low voltage capacitors or semiconductors it will likely cause damage to them.

A better method is to use a four wire measurement system. Pass a lower current at lower voltage (<0.5V) through one set of wires and measure through a different set of wires. The only place the source and sense wires should meet is at the point of measurement. This will eliminate almost all the residual circuit effects. For even greater accuracy you might need an additional 'guard' probe. This is a probe carrying a buffered copy of the voltage at the measurement probe, you connect it at the far side of low value components attached to the measurement probe so the voltage across them becomes zero and hence no current flows through them.

Brian.

Here are Two Low Ohm Circuit:

http://chemelec.com/Projects/Low-Ohm/Low-Ohm.htm