zero problem on MAX 4167esa op amp

Do you have a negative power supply for the opamp?

It can't output -2 mv without a negative supply.

John

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Even the best rail-to-rail op amp output won't get within about 100mV of the power rails.

Reading the op amp data sheet is always a good idea.

-- Bill Sloman, Nijmegen

Is that +/-3.3V? Or +3.3V/Gnd (on Vcc/Vee)?

In the former case it looks to me like it isn't normal (the offset voltage of that op-amp is generally better than 1mV, so that's not it). In the later case it's very much normal, as the data sheet says the part can typically only get within 10mV of its lower rail.

Hmm... I'm thinking you're talking +3.3V/Gnd, since +/-3.3V would slightly exceed the maximum supply range of the part anyway. Hence, it's normal.

---Joel

An op-amp can't ever quite swing all the way to either supply rail, particularly when it has to sink/source current to the opposite rail, plus there is an input offset voltage of as much as +/-850uV depending on grade for this type.

Either give it a couple hundred mV of negative voltage on the minus supply pin or deal with it by offsetting the input somehow.

It should not affect the linearity between, say, +100mV and +2.5V ("independent linearity"), but it will affect the terminal linearity.

With a rather light (100K) load to Vdd/2, this op-amp is only guaranteed to get within 25/30mV of the supplies- see "Output Voltage Swing" specs. You should avoid getting too close to even that, since the gain will tail off close to saturation, and you may even see issues with stability very close to the supply rails.

It's 3V3 and 0V, thanks for the other replies...

Thanks, I guess that's resolved the issue. ) 0V in = few millivolts out until it gets going.

The Maxim can get down to around 10 millivolts.

Exactly.

John

You could put a voltage follower on the output to get the output out of the dirt at the negative end. Of course it limits how close you can get to the positive rail. IOS is still a problem, though. The AD8606 Vios

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He could always pull down the output to a small negative voltage too-- that takes hardly any current. I think it was Pease who showed an optoisolator doing that, using the phototransistor's b-e junction as a crummy current source.

-- Cheers, James Arthur