We are using the LMV358 in the voltage follower configuration - but are seeing strange results. The part clearly mentions that it is a rail to rail output - but for one of the opamp circuits never reaches the rail voltage while the other output does.
Has anyone seen anything like this, for a supply for 3.3V (VCC) and an input of 5V to both Opamp circuits in the voltage follower circuit - output of the first circuit is 3.3V but the second one max is 2.75V.
The exact part number is LMV358ID. The OpAmp output is directly fed to a microcontroller ADC channel input - even if I disconnect it from the Micro the output does not change.
Thank you for your help.
The LMV358 is a R-R output, but it is NOT a R-R input.
In the follower configuration, Vin = Vout, which means Vout is equal to the input common mode voltage.
For the LMV358, the input voltage range is -0.2V to 1V below V+, or 2.3V max input for a 3.3V supply. This is specified on page 5 of the LMV358 datasheet by the "Common Mode Input Voltage Range" line.
The chain is only as good as the weakest link, and in this case the weakest link is the input. it is the input that cannot go above 2.5V.
You have "lucked out" with one of the amplifers as it seems to be functioning at the higher common mode level - but that operation cannot be guaranteed.
You need to use a Rail to Rail input device, like a LMV932MA or similar...
More concerning is your statement: "for a supply for 3.3V (VCC) and an input of 5V to both Opamp circuits in the voltage follower circuit "
Are you saying the op-amps supply is 3.3V and your input voltage is 5V?
Obviously, you will not get a 5V output with a 3.3V supply - even with a R-R input device!
And you are grossly violating the abs max input common mode range. Placing a voltage on the input higer than the supply voltage will cause the ESD diode to conduct current into the V+ line. This is not good and can damage the device.
SVA Precision Applications
Integrated Signal Chain Applications, SVA
Thank you for your reply, I think made some basic mistakes in writing my post. I am not trying to get 5V output from a 3.3V VCC supply OpAmp.
But I completely understand the CMIVR comment - I was testing the part and increased the input all the way to 5V during test and posted the same results.
I now get that I was making a mistake.
Thank you for your detailed answer.
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