Opamp questions

[Magnus Danielson]

From: Carlos Vila Deutschbein <si04697 at salleURL.edu>
Subject: Opamp questions
Date: Sun, 19 Dec 1999 17:43:27 +0000 (WET)

> I'm relatively new to analog design so forgive me for my ignorance.. (Oh,
> we youngsters!!)
> I'm using several LM318 Opamps for an ADSR EG. Is this a good choice? I
> thought that high speed was necessary.
> I was tought the theory of Opamps in University but no one explained me
> how to chose one for a certain app.

As allways, theory is nice to figure out the basic stuff, but engineering
takes on not only the theory but also the reality and how that interact to
create the real complexity of the problem.

Now, sure, you can use the LM318, but I don't think you actually need THAT
fast responce unless someone _really_ can explain why... speed is at times
good but at some point you move into over spec and then you migth also run
into new types of problems.

I would use simpler devices.

Also, not that the input resistance is about 3 MOhm which adds to the load
on the capacitor. If you find that this is not a large problem, go ahead.

Also, speed also has to do with the ability to deliver current and the size of
you loads capacitance. Basically, how long does it takes to integrate up the
incomming current to sufficient charge to allow for the requested voltage?

> Secondly, What should I do with the BAL and/or COMP pins, should I leave
> them not connected or should I connect them to GND? Very few schematics
> really make use of these pins, Is the designer supposed to add these
> compensation circuits himself? 

For the LM318 you should just leave them open as far as I can see from the
internal structure.

Cheers,
Magnus