[sdiy] input buffer question

[Neil Johnson]

Hi,

David G. Dixon wrote:
> http://www.analog.com/static/imported-files/tutorials/MT-036.pdf

Nice write-up.  Please read it.

> The best diodes to use in this case are Schottky diodes because of
> their lower voltage drop, but I can't see why they would be  
> significantly
> better than regular diodes.

Regular diode drop is about 0.6 to 0.7V.  Schottky about 0.3 to  
0.4V.  So if the AbsMax sates V+ + 0.3 then a regular diode would  
violate that limit and the op-amp would/could/maybe go into phase  
reversal.

> I've also checked the common-mode range of TL07x (a spec I didn't  
> really
> know how to interpret before) and it is -12 to +15V.  Hence, the  
> lower diode
> won't really protect against phase reversal, but how often does one
> encounter a CV lower than -12V?  Also, powering these opamps from  
> +/-12V
> rails would preclude the possibility of phase reversal -- perhaps  
> the one
> good reason to use this voltage level (?).

Depends.  What's the start-up behaviour like?  If the output sits at  
the -ve rail for a while, then you apply +12V input, the slew rate of  
the op-amp will give rise to a violation of the CMRR, and there goes  
phase reversal again.

Neil
--
http://www.njohnson.co.uk