Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

opamp buffer fed input outside common mode range

Status
Not open for further replies.
T

treez

Guest
Hello,
We wish to supply LPV358 opamp with 5V and use it as a unity gain buffer.
We wish to input 5V (from its supply voltage) to its non-inv input.
This is outside the common mode input range.
Is this OK?
Will it simply give 5V at the output?
After all, it is only being used as a buffer.


LPV358 opamp datasheet
https://www.ti.com/lit/ds/symlink/lpv358-n.pdf

We cant use an opamp with rail-to-rail inputs, as they take more bias current than LPV358
 

Hi,

This is outside the common mode input range.
Is this OK?
Nobody of us can say "it is OK" when the datasheet says it is off specifications.

Klaus
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
It will be OK
Differential input voltage ±Supply voltage

The common mode voltage is to meet the common mode rejection.
your output will be slightly less then the 5V rail.

Is this for some kind of current limit on a load connected to the output?

Not sure why you would buffer the rail voltage.
 

"unity gain buffer" implies inverting input tied to OP output. I don't see a data sheet specification for the exact output voltage when the input exceeds the common mode range. For the positive excursion, it can be somewhere between CMR limit and positive rail.
 

Hi,

Either the input stage, the output stage or both saturate.
Using positive supply as unity gain buffer input makes no sense to me.
It just makes signal quality worse.
Less output voltage, less stable, more noise....

Where do you see a benefit?

Klaus
 

Thanks,
Ill post you a schematic if i may.
Its a led light.
We have a dimming pin (ADIM) into which we must feed a voltage >2V.....however, we put an RC delay of 10meg/10uF before it (for long softstart), and the unfortunate current out of the ADIM pin drops an unfortunate voltage across the 10meg resistor...therefore we buffer the RC on its way to the ADIM pin..............we have a 5V rail and since 5v>2v, we can feed that to the ADIM pin.......but as discussed, we must buffer it on the way to the ADIM pin......the opamp's input is the 5v.

We could divide the 5v down and then feed it to the buffer, but we are extremely space constrained, and struggle to fit in even two 0402 divider resistors.

Thanks Klaus you kindly explain the saturation situation...i am not sure if that matters for us, ...as long as the opamp output goes above 2V and stays there then we're ok.

- - - Updated - - -

...ummm, ive just realised that what ive said doesnt "quite" make great sense.
....What i didnt say was that we actually have fitted the divider (into the opamp noninv input) just in case we wish to feed a dimming voltage <2v to the ADIM pin....(we fitted it but top resistor is zero ohms and bottom is not fitted.
...it gives us the option to dim it in future.

....when dimmed below 2v then the drop across the 10meg does matter.
 

Hi,

Dimming with an RC has some drawback:
The voltage rise is steep at the beginning, then flat at the end.
But for "nice" dimming you want it the other way round. First flat, then steep. More like a square function.

What's your ADIM input current?
And what is the ADIM_voltage to brightness relationship? 0V - 0% .... 2V - 100%?. Linear?

Awaiting your circuit.

Klaus
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top