High voltage, large bandwidth instrumentation amplifier

Le Thu, 16 Aug 2007 15:47:08 -0700, Bob a écrit:

Bob,

I'm currently working on such a thing (but up to 4kV and with a 1:2000 ratio) :-) and it's not for the fainted heart (lots of minute details will get in the way). 200V will ease things a bit, but still you've got to be very, very, cautious. There are two ways of doing this: xformers and the 'classic' dual dividers way. I proposed the best one, my client choosed the worse (which will not necessarily be the same for you, given your lower voltage level). A few 'nice' things among others: dividers compensation isn't as simple as it seems because of caps dielectric behavior vs frequency (hey JL!) varying enough from one cap to another to compromise not only CMRR, but frequency response too. And of course, the caps from the high and low branches have pretty different values, (hence construction) which magnifies the pb. Varying PCB parasitics with temp, humidity and frequency is another. Tight geometry control is yet another (that was the last catch). I hope that's really the last one.

In our case, we have to 'manually' tune each board (a handful of poles and zeros) to trim the frequency response and CMRR. I'm currently working on the automatic values extraction because there's too much interaction bewteen the adjustments to make 'scratch head tuning' really possible.

You can contact me off list if you're interested. My email address have to be unmangled.

Tektronix did things like this in their differential-input plugins, all the way from the tube types (Z, W plugins) through the 7A13 for the 7000 series scopes, which had phenomenal common-mode and overload recovery to 100 MHz bandwidth. They were basically diff pairs of tubes or fets, heavily cascode bootstrapped.

I've seen nice bootstrapped opamp circuits, where the opamp power supplies are driven to track the opamp output or common-mode voltages.

The Tek isolated-input scopes (like TPS2024) have insane specs. I think they do transformer-coupled gigahertz FM coupling or something.

Air has pretty low dielectric absorption.

John

I saw your article is very good, I like it very much. I will continue to pay attention to your article, the following are the points I hope that I have similar concerns.

"faint of heart".

Cheers! Rich Grise, Insufferable Pedant ;-)

Le Fri, 17 Aug 2007 22:11:37 +0000, Rich Grise a écrit:

At least I didn't get the apostrophe wrong :-)

It also occurred to me, I might have been thinking of "faint _at_ heart"; either way, it's a pretty easy mistake to make. :-)

I once saw a guy writing up a dorm-room inventory form, and one of his items was "Chester drawers." ;-)

Cheers! Rich

Ah, that's also true! I guess when stuff is right, I just let it be right and hardly notice it. :-)

Cheers! Rich

Bob nimby snipped-for-privacy@roadrunner.com posted to sci.electronics.design:

I do not speak for anyone but myself, but the application description screams out for a tube solution.