4

If I have a data sheet or part of it, how can I see if an op-amp is rail-to-rail using only the numbers?

For example, only using the image below, how can I see if input and/or output is rail-to-rail?

Enter image description here

Peter Mortensen
  • 1,676
  • 3
  • 17
  • 23
バカです
  • 53
  • 3

3 Answers3

14

At the top of the chart it says "Vcc +- 15V."

Then it gives the maximum output values Vom as +- 13.5V. So, it does not have rail to rail output.

Then look at Vicr. That's common mode input voltage range. Typical is +-11V when operated on +-15V. So, input is not rail to rail.

JRE
  • 67,678
  • 8
  • 104
  • 179
  • so, if instead of having Vom as +- 13.5V, the Vom was +-17V for example, then it would be Rail to Rail? – バカです Sep 01 '17 at 23:04
  • 3
    If Vom is +-17V for Vcc of +- 15V, then you don't have an opamp. Opamps cannot output a voltage outside of their supply rails. If Vom matches Vcc, the it is rail to rail. – JRE Sep 01 '17 at 23:06
12

enter image description here

Figure 1. Reading the output range.

It's fairly easy to see that the output is not rail to rail. The best guaranteed output at ±15 V supply is ±12 V falling to ±10 V into a 2k load.

Similarly, just above the \$ V_{OM} \$ you can find the \$ V_{ICR} \$ whose guaranteed input is ±11 V at ±15 V supply.

When this op-amp input is not guaranteed to work within 4 V of either supply rail it should be clear that it would be of limited use on a 10 V supply and wouldn't work on a 5 V supply.

Regarding your comment to JRE:

If Vom is +-17V for Vcc of +- 15V, then you don't have an opamp. Opamps cannot output a voltage outside of their supply rails. If Vom matches Vcc, the it is rail to rail.

enter image description here

Figure 2. Internals of the ancient 741 opamp. Source: Wikipedia.

Most opamps will have an output arrangement similar to the push-pull arrangement of the old 741. Others will have FET transistors rather than BJTs. In either case if the top transistor (red oval) is turned on the output will be pulled to positive rail. If the bottom transistor (green oval) is turned on the output will be pulled to negative rail. How close they get depends on the exact output configuration and the driving circuitry.

It should be clear from the above that the best possible outcome would be that the output could go as high as the positive rail or as low as the negative rail but could never exceed either. You can't get ±17 V from an op-amp powered from a ±15 V supply.

Transistor
  • 168,990
  • 12
  • 186
  • 385
-1

Vom (out max) vs. Vcc should be obvious, if not the same. +-15 V is given for Vcc,Vee

To locate economy op-amps (general purpose), Texas Instruments has 154 hits:

Enter image description here

See the box for Out?

Peter Mortensen
  • 1,676
  • 3
  • 17
  • 23
Tony Stewart EE75
  • 1
  • 3
  • 54
  • 182