Opamps and 'common mode input range' ambiguities

[cupoftea]

Hi,
Why is it that not all opamp datasheets are as clear as LM324B about the common mode range, and the situations
surrounding it?
The LM324B datasheet makes it extremely clear that your common mode range will always be from (VCC-) to (VCC+) -2V.

Also, on page 23, sec 7.3.3. the LM324B datasheet helpfully states the exact conditions which cause output phase reversal
when the input(s) go outside the common mode range.

Why cant other opamp datasheets do this?

The TL084C datasheet leaves us guessing as to the input common mode range. The front page says its all the way up to (VCC+), but then
page 5 tells us that common mode range might only go up to (VCC+)-4 Volts.

The TL084C datasheet tells us nothing about what would or could happen if either or both inputs go outside the common mode range.

Also, the TL084C datasheet for some unknown reason, insists on giving every voltage (except differential voltages) with respect to the voltage
value that is half way between VCC- and VCC+.
...On page 2, the TL084C datasheet says

QUOTE...
1. All voltage values, except differential voltage, are with respect to the zero reference level (ground) of the
supply voltages where the zero reference level is the midpoint between VCC+ and VCC-
...UNQUOTE

This seems very odd and unecessary.

LM324B datasheet

https://www.ti.com/lit/ds/symlink/lm324.pdf?ts=1706254776508&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM324

TL084C datasheet:

https://www.st.com/en/amplifiers-and-comparators/tl084c.html

 

[FvM]

Using TL07x and TL08x since more than 40 years, I never noticed that TL07x has guaranteed CM voltage range, only selected it for better noise spec. Thanks.

 

[KlausST]

Hi,

At first you complain about how datasheets are written.
Then about the common mode range
you don´t accept a design change
you don´t accept suitable OPAMPs because of price
then your fear about running out of stock

Are you sure you are looking for a solution ... or maybe just for arguments against your customer?

****
The current clamp you show in the other thread .. just calls for problems.
Indeed I don´t see it working as a true current clamp because it "expects the supply voltage to be perfectly 20.000v" as well as the reference voltage. In reality they are not perfect.
If it should work as a true current clamp you need to measure the voltage across the 15 Ohms.
* And for this we have perfectly suited "high side current measusrement amplifiers".
* or you could just simply pull down the inputs of the OPAMPs a little bit ... by the expense of two simple resistors
* or you could use low side current sensing

So there are plenty of simple and reliably working, cheap solutions ...

***
We don´t have all your specifications and requirements.... But I see rather good solutions even without an OPAMP.

And I don´t think it´s a good idea to change from one topic to the other in one thread ... in a way that the headline loses it´s meaning.

***
So in case you really want a solution, then start a new thread for exactly this topic ... and by giving all your specifications and requirements in your first post.

Klaus

 

[cupoftea]

Thanks, the take away point which was golden, and with the excellence from all above excellent contributors, was written in the doc sent by Danadak, which prooved that no opamp in the world can assure a user that its output won't go into phase reversal if one or both of its inputs go outside the input common mode range..........unless of course, the opamps's datasheet states that it wont go into phase reversal in such a case. (i am not speaking of an input going outside the supply rails here).

So the great moral of the story was , if you want to definitely avoid phase reversal, never go outside the input common mode range unless the datasheet specifically states that you can avoid phase reversal by doing same.

 

[KlausST]

Hi,

I´d like to more generalize it:

So the great moral of the story..

.. is to read and keep on datasheet informations.

--> as long as you are within the specified common mode operating range of any OPAMP there will be no phase reversal.

Klaus