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.
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.
Hy st,Ohm's law states that 5 mA across 1 mΩ means a voltage of only 5μV !!!
So it must use a good quality chopper stabilized opamp as its front end.
But even then thermoelectric effects between the different materials could create a larger offset. There must be some ingenious nulling ciruits.
Yes it is a good chip, it was 8 quid, offset is in the order of nV, its a auto zero chopper amplifier like you said, at x1000 I get 1v full scale for 100ma through a 10mR resistance.
Yes the reading varies if you blow on it, but when wired to a busbar it works well, biggest problem so far is bannana sockets, contact reistance is naff, moving anything changes the current and affects the reading.
I dont want to end up with dinze sockets on this thing.
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