[SOLVED] :: op amp simulating

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tinystorm

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i was simulating this scenario. it is a current sense. 1A = 1v final ratio
as with many sense packages available commercially, they do state that there is linearity problems in the low mA ranges.
so i went to try out some stuff in the old multisim
the question is, by introducing a small negative rail (-1.2v), it seem to correct the op amp offset problem in the low mA sense (50-100mA)
im no expert in op amp, but am i on the right track? originally i simulated only with single supply. or is this just a multisim/simulation anomaly?
i have some actual op amps, but i could not plug those into 0.1" breadboards to try it w/o an adapter.



the neg rail in question comes from V3. the load supply comes from V2 --> probe 1, readout from probe8.
 

One thing about Op-Amps is that their linearity is poor near the supply voltages i.e. near Vss and Vdd voltage levels.
Even for the rail to rail Op-Amps the linearity is not that good near the supply rails.

In your case you have shifted the lower supply rail down from 0V to -1.2V. This would increase the linearity near the 0+V regions.

The curves in the data sheet of the Op-Amp that you have used also show this.

This is why you see an improvement when you introduce the negative rails.
 
To expand on what nitishn explained, the output stage of the LM358 starts to "loose steam" as it approaches the negative rail, around 200mV if I remember correctly.

There are better opamps, but even the best ones will only swing to about 20 mV above the negative rail.

What I've done in the past, is to use a simple and cheap charge pump circuit like the ICL7660 to generate the required negative bias.
 
aha !
ok that resolves my suspicion that the multisim is also putting in these "errors"

but when i pull down the neg supply further beyond -1.2v ... the accuracy or linearity falls apart. what did i miss? an error loop going around neg rails to sense ladder?
 

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