OpAmp slew rate / capacitive load

Sep 12, 2006 4 Replies
H

Hello,



Say I'd like to drive a capacitive (25nF) load with a slew rate of 20V/us and an amplitude of +/- 175V. Being lazy, and wanting to not having to buy a commercial unit for some $5000, I thought I'd look at some of Apex's HV Op Amps. There is, for instance, the PA92 with 50V/us and plenty of oomph:



4A max. output current (I need 0.5).

But the data sheet shows that the slew rate craps out to 3V/uS with only



300pF of load capacitance. In other words, this amp cannot deliver more than 9mA into a 300pF cap.

SOA considerations can not be the cause of this; the PA92 can dump about



100mA into a continuous dead short to ground (at ~175 V supply), and .6A pulsed.

What's the reason for this?


--Daniel


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R

This is a usual piezo application ? You will never get the slew rate you expect with a sensible current. On the other hand with a lot of current you damage the piezo. The amp of choice could also be the PA140.

Rene

Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net
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T

That's not a load capacitance, that's the external compensation capacitor. They're different things.

Granted, you may well have to compensate the thing pretty heavily if you try to run 25nF straight off of the output, so you may not see your full slew without playing other tricks, but first you have to get past your capacitor confusion.

Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
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H

Oopsie! Never mind. Datasheet reaing challenged, me.

--Daniel

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F

And I'm sure the lesson is not so much to not ignore possibly critical adjectival modifiers so much as 2 check and see iff'n thar talkin' compnzation or load capasitor, eh?

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