Anyone built one?
I just prototyped a circuit.
I want to measure the resistance of some cables on a machine, down to milliohms.
This needs to be battery powered so 100ma is the most I want to have test current wise.
As some know offset voltage is an issue for op amps at dc, so amplifying the voltage across a 1mOhm resistance with 100ma through it requires gains of 1000 for a sensible o/p voltage, and with the average op amp offset of 2mv that ends up a large voltage at that gain, so I prototyped a meter using a lmp8358 chopper instrumentation amp with a gain of 1000 found in a thermocouple amplifier circuit, and a lf358 as a current source controlled by a voltage reference, and the other half the lf358 drives a 1ma meter.
Seems reasonably accurate (I only have 5% resistors for the prototype), and stable.
The other idea I had was to pulse the test resistance at 1 or even 10a, and sample/hold.
I just prototyped a circuit.
I want to measure the resistance of some cables on a machine, down to milliohms.
This needs to be battery powered so 100ma is the most I want to have test current wise.
As some know offset voltage is an issue for op amps at dc, so amplifying the voltage across a 1mOhm resistance with 100ma through it requires gains of 1000 for a sensible o/p voltage, and with the average op amp offset of 2mv that ends up a large voltage at that gain, so I prototyped a meter using a lmp8358 chopper instrumentation amp with a gain of 1000 found in a thermocouple amplifier circuit, and a lf358 as a current source controlled by a voltage reference, and the other half the lf358 drives a 1ma meter.
Seems reasonably accurate (I only have 5% resistors for the prototype), and stable.
The other idea I had was to pulse the test resistance at 1 or even 10a, and sample/hold.