zero problem on MAX 4167esa op amp

[TTman]

I'm looking at a voltage follower that has minus 0.002 V on the input , but
the o/p is+0.007 V
Is this normal for the o/p ?
At the top end, for 2.450 i/p I get 2.450 o/p, as expected.
Supply is 3.3V; traight forward cct ,neg i/p connected to o/p, o/p goes to
a12 bit single ended ADC...
It's screwing the bottom end linearity of the ADC

 

[Spehro Pefhany]

I'm looking at a voltage follower that has minus 0.002 V on the input , but
the o/p is+0.007 V
Is this normal for the o/p ?

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.

At the top end, for 2.450 i/p I get 2.450 o/p, as expected.
Supply is 3.3V; traight forward cct ,neg i/p connected to o/p, o/p goes to
a12 bit single ended ADC...
It's screwing the bottom end linearity of the ADC

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.

 

[TTman]

Do you have a negative power supply for the opamp?

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

John

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

 

[TTman]

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.

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

 

[[email protected]]

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

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 <75uV. For
about a buck a pair, it's a pretty nice device.