Current clamp with Opamp Common mode input range?

[cupoftea]

Hi,
The attached LTspice and jpeg is the well known opamp/PNP current clamp seen all over the internet..please imagine the opamp used is TL084.
Would you agree that TL084x would not be useful for this?….only “TL084H” would be good for this service?
Page 10 of the TL084 datasheet tells that TL084x cannot handle inputs above 15V…..(but “TL084H” is capable of handling such inputs?)
Also, would you agree that LM324 would not be useful here?
Page 6 of the LM324 datasheet tells that common mode input range extends only up to Vcc-2V for LM324.

TL084 datasheet
https://www.ti.com/lit/ds/symlink/t...l=https%3A%2F%2Fwww.ti.com%2Fproduct%2FTL084H

LM324 datasheet

https://www.ti.com/lit/ds/symlink/lm324.pdf?ts=1692172262927&ref_url=https%253A%252F%252Fwww.ti.com.cn%252Fproduct%252Fzh-cn%252FLM324%253FkeyMatch%253DLM324%2526tisearch%253Dsearch-everything%2526usecase%253DGPN-ALT

 

[FvM]

Page 10 of the TL084 datasheet tells that TL084x cannot handle inputs above 15V…..(but “TL084H” is capable of handling such inputs?)

Page 10 specifies absolute maximum ratings, not normal operation conditions. It tells nothing about usable input voltage range. Although datasheet specifies typical input common range up to Vcc, the guaranteed (minimal) range is 4 V less. Neither TL084 nor TL084H can be safely used in this circuit. Use a typical IO rail-to-rail OP instead.

 

[danadakk]

You are regulating current thru 15 ohm ?

And with a 324 :

Regards, Dana.

 

[cupoftea]

Although datasheet specifies typical input common range up to Vcc, the guaranteed (minimal) range is 4 V less

Thanks, but i didnt see that common mode input range was Vcc-4V in the TL084H datasheet? (above).
Page 13 of the above datasheet says that Common mode input range for TL084H is right up to Vcc+.

The "TL084H" does appear to be a lot better than the other TL084's, eg the TL084I, TL084C, TL084A, etc etc.

You are regulating current thru 15 ohm ?

Thanks, yes, and its with the "TL084H" specifically...the others wont be able to do it....they all share the same datasheet though.

 

[KlausST]

Hi,

There is the abs max common mode input voltage range (beyond may lead to destructiin)

There is the recommended common mode input voltage range (reliable and clean operation, VccNeg +4V .... VccPos-4V, page 11)

And there is the "operable" common mode input voltage range (operable but maybe at the edge....) page 13


Klaus

 

[cupoftea]

There is the recommended common mode input voltage range (reliable and clean operation, VccNeg +4V .... VccPos-4V, page 11)

Thanks, but that doesnt concern the TL084H.

And there is the "operable" common mode input voltage range (operable but maybe at the edge....) page 13

Thanks, but as you know, page 13 says that TL084H has 80dB of CMRR when the input is up at Vcc+......thats a really good amount for a general 80mA current clamp such as we want.
80dB means, as you know, that the differential mode gain, is 10000 times greater than the common mode gain.

The main point is, would you agree that only the TL084H can support input voltage signals above 15V?
The TL085C cannot, neither the TL084A or TL084I?

TL084H can indeed have a VCC+ of 20V, and an input voltage of 20V?

 

[FvM]

As stated before, I didn't yet see a specification of minimal TL084H common mode input voltage better than Vcc-4V.

Please show the specification!

--- Updated Aug 18, 2023 ---

O.K., problem is that the link in post #1 doesn't show the most recent datasheet. https://www.ti.com/lit/gpn/tl084h

The latest version has separate specifications for TL084H with guaranteed common mode input range.

 

[cupoftea]

Thanks, though the datasheets seem to be the same...though on ti.com the title of the datasheet link is indeed different.
I am wondering if all these TL084's (A,B,C,I,H, etc), are actually exactly the same part, its just that the "TL084H" part has been qualification tested to the better spec?

 

[cupoftea]

Page 10 specifies absolute maximum ratings, not normal operation conditions. It tells nothing about usable input voltage range. Although datasheet specifies typical input common range up to Vcc, the guaranteed (minimal) range is 4 V less. Neither TL084 nor TL084H can be safely used in this circuit. Use a typical IO rail-to-rail OP instead.

Thanks, this is always the situation, when someting is outside of limits, and what then happens......regarding the NON TL084H versions, ie the TL084CDT version...this is what a "Guru" on the offficial Texas instruments forum says...

QUOTE >>>>>>
The datasheet says that the common-mode input voltage range typically goes up to V+. So I'd expect the worst that would happen is that the electrical characteristics are not met, i.e., that the offset voltage could be somewhat larger than 6 mV.

Just use the TL084H (or TL081H for a single channel) instead, which is guaranteed to go up to V+ (and is cheaper, and has a smaller offset voltage).
<<<<<< UNQUOTE

So, now with this extra info...if we dont use the TL084H, but other TL084x then what would be the worst that coudl happen may i ask?

Its in the BOM, production will go nuts if we just change something without excellent reason...even then they will hit the roof.....so can you gaurantee that TL084x WHERE "x is not equal to H", will go badly wrong?......we just need a +/-10% current regulation type thing.

 

[KlausST]

Hi,

My opinon:
If you go beyond specified limits then you can´t rely on the specified values. As long as the datasheet does not tell otherwise.
So you don´t know whether it is 0.1% off, 5% off, it will start to oscillate, increase supply current...

I wouldn't even trust a TI guru. In the event of a malfunction, you can neither blame TI nor the Guru for the problem.
It will be your resposibility.

Klaus

 

[cupoftea]

Thanks, yes it is the case that the non TL084H version is a disaster waiting to happen.
As you know, the typical solution to this is a TLV431 into a NPN...but if youve got loads of spare opamps then its then worth using one as in the top post. (as long as TL084H)

 

[cupoftea]

From what i can see, this TL084C by st.com would be able to handle input voltages tight up to the vcc+
(Our Vcc+ is +20V, our Vcc- is 0V)
Would you agree?

https://www.st.com/resource/en/datasheet/tl084c.pdf

Current clamp in jpeg and LTspice attached

 

[cupoftea]

Thanks,
So anyway, we need an opamp with common mode input range which includes VCC+.
Also, it must be able to take a 0-20V supply voltage.
Since it must perform the attached rough current clamp function.
However, the cheapest opamp that can do this is LT1498, which is $4.

Our customer cannot afford this, and they wish to use a cheap jelly bean opamp instead, the TL084CDT,
which is $0.3
Their circuit , which does the attached, works fine.
They wonder why they should pay $4 when their $0.3 circuit works just fine.
Their volumes are a few thousane a year or so.

LT1498 ($4)

https://www.analog.com/media/en/technical-documentation/data-sheets/14989fg.pdf

TL084CDT ($0.3)

https://www.futureelectronics.com/p/semiconductors--analog--amplifiers--general-purpose/tl084cdt-stmicroelectronics-4593950?msclkid=0426691494681f3ba6230ca37a437b06&utm_source=bing&utm_medium=cpc&utm_campaign=A-MPN%20EMEA%20STMICROELECTRONICS&utm_term=TL084CDT&utm_content=Adg%20-%20TL084CDT

 

[D.A.(Tony)Stewart]

On a similar question with the same issue, what am I missing here? It's only $0.17 40V rated and works Vcm to Vcc+ and only $0.10 at Digipart's source.

Opamps and 'common mode input range' ambiguities

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...

www.edaboard.com

Digipart.

 

[cupoftea]

Thanks, i got pulled in by that one aswell....its the marketeers at work...scroll down to page 17 and note that CM range for TL074 is only up to Vcc+ -4V

TL074

https://www.ti.com/lit/ds/symlink/tl074.pdf?ts=1706599111612&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTL074

 

[KlausST]

Thanks, i got pulled in by that one aswell....its the marketeers at work

Read carefully

"The TL07xH (TL071H, TL072H, and TL074H) family
of devices are the next-generation versions of the
industry-standard TL07x ..... and
common-mode input to the positive supply. ..."

--> Mind the "H" suffix
And read the according specification table on page15.

While the "H" types are not sold by every distributor .. they are available at mouser for about 0.12€ @ 1000pcs. for example

But I agree to the "marketing point"...

Klaus