fet op amps, max differential input

[gearhead]

Looking at datasheets for op amps that can take a large differential
input.
TLC271 says +/-Vdd
TL071 says +/- 30volts
does this mean Tl071 input can go beyond the rail but TLC271 can't?

 

[Chris]

On Dec 27, 7:35 pm, gearhead <nos...@billburg.com> wrote:

Looking at datasheets for op amps that can take a large differential
input.
TLC271 says +/-Vdd
TL071 says +/- 30volts
does this mean Tl071 input can go beyond the rail but TLC271 can't?

OK -- here's the story. Absolute maximum ratings are like Yul Brynner
in "The Ten Commandments" saying, "Exceed these and you shall surely
die". Very important, which is why they usually put 'em first, but
not exactly what you want. You're looking for the working electrical
characteristics, which are usually below in the fine print.

You want to look at the input common mode voltage range, listed inside
the data sheet.

The TL071 definitely isn't what you're looking for -- it lists V(ICR)
(min) as +/-11V for a +/-15V supply, meaning that to be absolutely
sure, you can't let the input signal get within 4V of the power supply
rails.

The TLC271 is a bit of an odd beast, because it has a bias select pin
which basically makes it three different user-selectable op amps (with
three similar data sheets). But looking at all three (high, meduim
and low bias), the input characteristic in question is the same. At
25C, you can actually go 0.2V below the negative rail, and must stay
within 0.8V of the positive rail. For any and all conditions, you
have to stay within 1.5V of the positive rail.

You may have experienced a little irritation from other posts about
your question. You've more than likely got the data sheets in front
of you in another window or tab. Why not copy & paste a link, like
this?

http://focus.ti.com/lit/ds/symlink/tl071.pdf
http://focus.ti.com/lit/ds/symlink/tlc271.pdf

Cheers
Chris

 

[gearhead]

On Dec 28, 3:55 am, Chris <cfoley1...@yahoo.com> wrote:

On Dec 27, 7:35 pm, gearhead <nos...@billburg.com> wrote:

Looking at datasheets for op amps that can take a large differential
input.
TLC271 says +/-Vdd
TL071 says +/- 30volts
does this mean Tl071 input can go beyond the rail but TLC271 can't?

OK -- here's the story.  Absolute maximum ratings are like Yul Brynner
in "The Ten Commandments" saying, "Exceed these and you shall surely
die".  Very important, which is why they usually put 'em first, but
not exactly what you want.  You're looking for the working electrical
characteristics, which are usually below in the fine print.

You want to look at the input common mode voltage range, listed inside
the data sheet.

The TL071 definitely isn't what you're looking for -- it lists V(ICR)
(min) as +/-11V for a +/-15V supply, meaning that to be absolutely
sure, you can't let the input signal get within 4V of the power supply
rails.

The TLC271 is a bit of an odd beast, because it has a bias select pin
which basically makes it three different user-selectable op amps (with
three similar data sheets).  But looking at all three (high, meduim
and low bias), the input characteristic in question is the same.  At
25C, you can actually go 0.2V below the negative rail, and must stay
within 0.8V of the positive rail.  For any and all conditions, you
have to stay within 1.5V of the positive rail.

You may have experienced a little irritation from other posts about
your question.  You've more than likely got the data sheets in front
of you in another window or tab.  Why not copy & paste a link, like
this?

http://focus.ti.com/lit/ds/symlink/tl071.pdfhttp://focus.ti.com/lit/ds/symlink/tlc271.pdf

Cheers
Chris

Yeah, I was in a hurry to click send and get out the door.
Thanks for the response.