OpAmp slew rate / capacitive load

[Haude Daniel]

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

 

[Rene Tschaggelar]

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?

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

 

[Tim Wescott]

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

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

 

[Haude Daniel]

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

Oopsie! Never mind. Datasheet reaing challenged, me.

--Daniel

 

[Fred Bloggs]

Oopsie! Never mind. Datasheet reaing challenged, me.

--Daniel

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?