Difference between “Single-Supply” and “Rail-to-Rail” Opamps

Thread Starter

mishra87

Joined Jan 17, 2016
648
Could anybody give some basic idea ???

Normal Opamp
Single Supply Opamp
Dual Supply opamp
Rail to rail opamp

How to select these opamps in practical applications ???
Which opamp is best suited where ???
 
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wayneh

Joined Sep 9, 2010
16,281
Could anybody give some basic idea ???
"Dual supply" = this op-amp cannot output to either power rail
"Single supply" = this op-amp can output (near) to one of the power rails
"Rail to rail" = this op-amp can output (near) to both power rails

It's just lingo that is suggestive of how the particular op-amp might be used.
 

crutschow

Joined Mar 14, 2008
24,405
Note that input input common-mode voltage range also generally corresponds to the output voltage range for those three types of op amps (although there are some exceptions).
 

wayneh

Joined Sep 9, 2010
16,281
Note that input input common-mode voltage range also generally corresponds to the output voltage range for those three types of op amps (although there are some exceptions).
Yes, and I'm not sure whether the input or the output range is more often used for classifying the op-amp. I also don't know where the lines are drawn. I mean, an op-amp that can output within a few mV of the rails will be called rail-to-rail, but if it's more than say 100mV away from the rails then I think it would be misleading to use that term.
 

wayneh

Joined Sep 9, 2010
16,281
How to select these opamps in practical applications ???
I'd say that in most designs, you worry first about current and voltage requirements along with frequency of the application. Then you can worry about the which type would simplify your circuit. Why not use rail-to-rail for everything? You certainly could, and it might be a great choice for DIYer to keep a handful of R-R op-amps on hand. But they tend to cost a bit more and often there's no need for it. Personally I have a handful of LM358 on hand because they're cheap, ubiquitous and work well on single supply. I got a couple TLV272IP (R-R) when I needed them for a specific project.
 

dl324

Joined Mar 30, 2015
10,072
How to select these opamps in practical applications ???
It depends on your application. Some require rail-to-rail inputs and/or outputs, some require low noise, some require high bandwidth, some require high slew rate. Pick the one that satisfies your important requirements.

In cases where you think you need rail-to-rail input, you can workaround limitations of an opamp like LM358 (which happens to be my favorite general purpose opamp) by using a voltage divider on the inputs. LM358 common mode input range includes ground. You can use it with split supplies if you connect the output ground to reduce crossover distortion.

EDIT: To clarify. You connect the output to ground through a load resistor; not a direct connection...

Single supply opamps can be used with split supplies, and dual supply opamps can be used with single supplies.
 
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ericgibbs

Joined Jan 29, 2010
9,574
Hi,
The LM324 Quad OPA is a good general purpose amplifier.
One feature is its use as a high side current monitor OPA, due its input design, refer the datasheet.
E
 

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ericgibbs

Joined Jan 29, 2010
9,574
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danadak

Joined Mar 10, 2018
3,912
At low currents there is a pronounced non linearity in OpAmp output,
100 ohm load. With 1k load or 10 ohm load looks pretty linear.

Top curve load V, bottom OpAmp out.

Of course analysis is done with exact R ratios around diff amp,
normal mismatch would generate offsets due to lower CMR due to
R mismatch.

Sweep starts at 2V, below that LM324 model crapped out.



Regards, Dana.
 

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ericgibbs

Joined Jan 29, 2010
9,574
hi OBW,
I know that circuit would accept almost any OPA.
Fellow posters were just naming their 'favourite' RIRO type OPA's, I just prefer the LM324 as it is the type I use in high side current monitors.

I am fully aware of the point made in post #10, ref the input voltage limits versus the supply voltage.
No where have I suggested that the inputs would be connected directly to the same voltage level as the the LM324's supply voltage.

It's only advantage over the LM358, is that its a quad package.

Eric
 
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crutschow

Joined Mar 14, 2008
24,405
No where have I suggested that the inputs would be connected directly to the same voltage level as the the LM324's supply voltage.
Not explicitly.
But no where have you explained the following cryptic comment, which implies it has some special feature for high-side current monitoring.
What in the datasheet are you referring to?
One feature is its use as a high side current monitor OPA, due its input design, refer the datasheet.
 
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