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Opamp supply voltage and common mode input range?

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treez

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Hello,
Our external designer wants to use a LPV321 opamp in a circuit where its Vdd would be just 2.5V.
It would be set up as a unity gain buffer. Also, it would have signals up to 2V on its noninverting pin.
Will this be OK?
The datasheet “appears” to say that Vdd is not recommended to go below 2.7V. Also the common mode input range appears to be up to a volt below the Vdd rail…rendering our 2V input “out of bounds”?

LPV321 Opamp
https://www.ti.com/lit/ds/symlink/lpv321-n.pdf
 

Our external designer wants to use a LPV321 opamp in a circuit where its Vdd would be just 2.5V.
Click to expand...
The datasheet “appears” to say that Vdd is not recommended to go below 2.7V
Click to expand...
Did you want to say + - 2.5 V ? (i.e. dual supply) If so, it will be fine.
 
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As this snip from the data sheet shows, it won't work.
Capture.PNG
It plainly says that the (common-mode) input can go not higher than 1.7V with a 2.7V supply (the negative supply voltage has no effect on this).

You need a rail-rail I/O type op amp that will operate from a 2.5V supply.
**broken link removed** an example, which goes for U$0.34 @ Mouser.
 
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Thanks, but page 9 of the datasheet in the top right graph, shows that CMRR goes down to 38dB when the opamp's input goes up to within 0.7V of the Vdd supply rail......that CMRR is probably OK for operation as a unity gain buffer?

Also, page 7 of the datasheet (top left graph), shows the Vdd of the opamp going down to about 1.9V....and this suggests that this is OK...otherwsie they wouldnt have shown anything below 2.7V....?

For opeation as a unity gain buffer i suspect the 2V5 single supply rail and inputs up to 2V will be ok?
 

treez said:
Thanks, but page 9 of the datasheet in the top right graph, shows that CMRR goes down to 38dB when the opamp's input goes up to within 0.7V of the Vdd supply rail......that CMRR is probably OK for operation as a unity gain buffer?

Also, page 7 of the datasheet (top left graph), shows the Vdd of the opamp going down to about 1.9V....and this suggests that this is OK...otherwsie they wouldnt have shown anything below 2.7V....?

For opeation as a unity gain buffer i suspect the 2V5 single supply rail and inputs up to 2V will be ok?
Click to expand...
38dB of CMR means there can be a change of 25mV between the input and output for 0 to 2V of signal in a follower configuration.
If that's acceptable, then that op amp would probably work in your application, but you would be operating right near the margins of the device and that's not a desireable place to be.
 
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The late Bob Pease, of National Semiconductor fame, wrote a column where he chastised those engineers who designed components outside their datasheet limits.
Then months later, those same engineers would come back requesting assistance, because their circuits were behaving erratically or experienced many failures.
 

I recently saw some new TI op amps that have an internal
charge pump feeding the front end pair that lets you apply
over-the-rail (somewhat) inputs. Should not be hard to find,
they seemed pretty proud of it.
 

38dB of CMR means there can be a change of 25mV between the input and output for 0 to 2V of signal in a follower configuration.
Click to expand...
..thanks,
https://e2e.ti.com/blogs_/b/analogw...w-about-cmrr-the-operational-amplifier-part-1

..I see it also gets worse with frequency, so operating outside the common mode range in a noisy environment (which we are) is particularly bad....

The thing is The change in common-mode and input offset voltage is with respect to the nominal value specified in the datasheet, usually mid-supply......btu for a single supply opamp it is not known where the datasheet considers that nominal CMRR is....is it at 0V?
 
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treez said:
.........
The thing is The change in common-mode and input offset voltage is with respect to the nominal value specified in the datasheet, usually mid-supply......btu for a single supply opamp it is not known where the datasheet considers that nominal CMRR is....is it at 0V?
Click to expand...
This from the data sheet for V+=5V and V-=0V shows the CMRR for an input of 0V to 4V:
Capture.PNG

Instead of trying to use an op amp at its marginal limits, I suggest you use one that's not.
 

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