Author Topic: OpAmp Rail Voltage Margin  (Read 2330 times)

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Offline beaker353Topic starter

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OpAmp Rail Voltage Margin
« on: March 15, 2014, 04:24:56 pm »
I have a project that I am working on that will use an opamp to buffer a DC signal to a number of adjustable power regulators. The opamp circuit will be arranged as a voltage follower with with the power supply rails across a pot, pot wiper tied to the opamp input. I know that most (if not all) opamps are not able to drive the full voltage of their power supply rails. From my understanding, even rail-to-rail opamps have some margin. I don't think I have a need for a rail-to-rail opamp, but I am having a hard time determining from the datasheet how much margin is required to stay in spec on parts I'm considering. Do I just not know the name spec I am looking for? Do I need to derive this number from more than one datasheet spec? Thanks!

-EM
 

Offline kg4arn

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Re: OpAmp Rail Voltage Margin
« Reply #1 on: March 15, 2014, 07:05:45 pm »
Take a look at the LM741 data sheet
http://www.ti.com/lit/ds/symlink/lm741.pdf

I am not suggesting this op amp, just an example.
Look at the spec called "Output Voltage Swing."  It tells you that the LM741 has a minimum output swing of +/-12V
with a +/-15V supply and a 10K load.  You have to extrapolate ( and test) for supply and load conditions that are not specified.

Another consideration is the common mode input voltage.  If you are using a non-inverting configuration like a follower.  On this data sheet it is the "Input Voltage Range" spec and it is specified for supply voltage of +/-15V.  The minimum for the LM741 (not the A or C versions) is +/-12V.  So if either of the inputs gets outside this range, the op amp behavior is not specified.  Might work, might not.  Might work for one unit and not another.  So you have to keep the input voltage on the + pin of your follower inside this voltage range.
 

Offline w2aew

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Re: OpAmp Rail Voltage Margin
« Reply #2 on: March 15, 2014, 08:18:59 pm »
I have a project that I am working on that will use an opamp to buffer a DC signal to a number of adjustable power regulators. The opamp circuit will be arranged as a voltage follower with with the power supply rails across a pot, pot wiper tied to the opamp input. I know that most (if not all) opamps are not able to drive the full voltage of their power supply rails. From my understanding, even rail-to-rail opamps have some margin. I don't think I have a need for a rail-to-rail opamp, but I am having a hard time determining from the datasheet how much margin is required to stay in spec on parts I'm considering. Do I just not know the name spec I am looking for? Do I need to derive this number from more than one datasheet spec? Thanks!

-EM

Most data sheets will include some graphs that indicate the output saturation or output voltage swing capability vs. load current.  From this, you should be able to see if a particular device will meet your drive requirement.
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