Hello,
When you get down in to resistance values like 1 ohm, 0.5 ohm, and 0.1 ohms and lower like 0.010 ohms, it becomes a problem to measure with an ordinary meter because the voltage drop is so little with a normal test current like 1ma. Most regular meters can go down to about 0.001v but that's it. Going down further means very reduced accuracy in many cases.
Consider a 0.1 ohm resistor with a standard 1ma test current. 0.001 times 0.1 is already down to 100 microvolts, and most standard DVM's can not measure that low very accurately.
So the next step is to find other ways to do this kind of measurement. The first being a more expensive DVM. A more expensive DVM may measure down into the microvolts. That would mean you can use the same 1ma test current and get some usable results. But more expensive meters are, well, more expensive, so that option is usually left for last.
Another cheaper method is to use a higher test current. Since we have been using 1ma, stepping that up to 10ma means we get a voltage reading that is 10 times higher. With the same cheap DVM we now measure 1000 microvolts with the same 0.1 ohm resistor. This gives us a reading of 1.0mv on many cheaper (but not super cheap) meters. That's two digits, which is often good enough.
But why stop there. Increasing the current to 0.1 amps means we now get a reading of 10.0mv, which is far more accurate than before. Now we have three digits when before we only had two.
But increasing the current has it's drawbacks, one being that the resistor under test has to be able to handle the extra current without breakdown. Often this may work, but sometimes it doesnt, and sometimes the resistance is even smaller than 0.1 ohms so we have another idea we can use.
The next idea is to use a stable DC voltage amplifier to amplify the voltage we measure so that the meter reads a higher voltage. We simply scale the reading and that allows us some more flexibility.
For the same 0.1 ohm resistor with a normal test current of 1ma, the voltage drop is again only 100uv, but with a 10x amplifier that gets boosted to 1000uv, and that gives us two digits on the cheap meter which would read 1.0mv.
Similarly, using a 100x amplifier boosts the output to 10.0mv, which is much more accurate to read on the cheap meter.
Just how we build this amplifier though is a little tricky. We need a good input offset spec down in the microvolts or tens of microvolts to get anything decent out of this. Here's where the chopper stabilized op amp comes in really handy, because these devices can have very low DC offsets and so we get decent readings. Without this we'd have to use an input offset null amplifier which means we'd have to adjust it. That works too but is more problematic.
Another idea is to use a resistance bridge. These circuits use the ratio of higher value resistors to determine the value of a lower valued resistor. Low tolerance resistors should be employed here. and connections have to be super solid.
So the choices include:
1. Get a better meter
2. Increase current when possible
3. Use a low input offset op amp to create a 10x or 100x DC amplfier
4. Use a special technique for measuring resistances usually called a "resistance bridge"