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'[OT] Instrumentation amplifiers Zap Bosses'
1999\03\12@110018 by Lawrence Lile

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OK I've got this oldtime scope.  I ask my boss, Mr. Miserly "Midas"
Pinchpenny, for a new HP mixed signal oscilliscope, and he says forget it.

I go into the lab, hook my scope's ground lead to a live 120 volt circuit,
with the expected result.  Once I cleaned the black stuff off my face and
replaced the fried ground lead, I proceeded to power the scope through a
nice isolation transformer.  Now it's safe to hook the ground lead of the
scope to a live 120V circuit (which is all I ever work on - nonisolated
resistor-zener power supplies for PICS  see it is on topic!)

So the scope's ground is hooked into 120V.  I'm fine with that, I know it
and don't touch the scope.  Along comes my boss, Mr. Miserly "Fingerpoken
und fiddlefarten" Pinchpenny, and sets his elbow squarely on the case of the
scope and his other hand on a nice grounded equipment casing.  KaZAAP!

So he's not only cheap, hes also mad.  I need to build a circuit which will
amplify signals in my circuit, but isolate them from the scope's ground so I
can stop zapping my boss.  I'm grumpy about the scope's lousy low input
impedance loading my circuits, so I think the instrumentation amp could help
this problem too.   I've looked into a couple of instrumentation amp
circuits like the Analog Devices AD524.  I can imagine a nice high impedance
circuit with a differential input that can float from real earth ground, but
I can't see how  it can really isolate.  It can only handle 36 volts on an
input pin.

I considered using a 10:1 resistor ladder on the input,  having maybe a 10
meg - 2 meg - 10 meg in series and tapping the differential input across the
center one.  This would give me effectively  360 volts isolation and a 22
meg input impedance - I think??? The instrumentation amp could then amplify
by 1:10 and I'd have the same signal out as in.  I'd lose a little very low
amplitude resolution, but I really don't care.

Anybody got any better ideas?

1999\03\12@111038 by Quentin

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Lawrence Lile wrote:
>

> Anybody got any better ideas?

Fire your Boss. I did, five years ago.

TGIF!
Quentin

1999\03\12@112921 by Scott Newell

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>So the scope's ground is hooked into 120V.  I'm fine with that, I know it
>and don't touch the scope.  Along comes my boss, Mr. Miserly "Fingerpoken
>und fiddlefarten" Pinchpenny, and sets his elbow squarely on the case of the
>scope and his other hand on a nice grounded equipment casing.  KaZAAP!

Use that isolation transformer to power the device under test, not the scope.


>circuits like the Analog Devices AD524.  I can imagine a nice high impedance
>circuit with a differential input that can float from real earth ground, but
>I can't see how  it can really isolate.  It can only handle 36 volts on an
>input pin.

So use something that can.  Burr Brown has an entire chapter of their
databook devoted to isolation products.  Optical, capacitive, digital
(ADC-DAC), and transformer coupled.  ISO100 looks good, if you can live
with the bandwidth (60 kHz).  Some of their devices even have an isolated
power supply.


>Anybody got any better ideas?

I know that there are some complete solutions in the Tektronix catalog, but
they're not likely to be cheap enough.


newell

1999\03\12@113546 by Scott Dattalo

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On Fri, 12 Mar 1999, Lawrence Lile wrote:

> Anybody got any better ideas?

For about $500 you can get a Tek differential probe. It's good for about
20Mhz BW and 1500V isolation.

1999\03\12@114616 by Wagner Lipnharski

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> > Anybody got any better ideas?
No better idea, zapping the boss is the best one. :)
By the way they are trainned, they would enjoy one or two
zapps a day. This would recharge their negative side. :)
Wagner

1999\03\12@115443 by Dwayne Reid

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Hi there, Lawrence.

You are taking the wrong approach, me thinks.  You should use an iso
transformer, but it needs to power the circuit under test, NOT the scope.  I
realise that you tend to work with largish heater loads - just make sure you
iso transformer can handle your largest load.  I use a variety of iso
transformers, depending what I am working on.  My largest is 5000 VA (Topaz
ultra-isolators) - I use 3 at a time if I am doing 3 phase stuff.

Floating your scope ground (which is all you are doing when you power the
scope with an iso) is dangerous and should NOT be your normal setup.

If you absolutely have to measure something live, look at differential scope
probes from Preamble (division of LeCroy).  Very nice stuff, good bandwidth.
Tek also makes one that seems pretty good.

One last thing about using an iso transormer on your circuit:  you HAVE to
do this if you want to use an in-circuit emulator.  Otherwise, your whole
computer setup is also at line potential (the ground lead of the pic is
eventually connected to the computer case).

Topaz is only one of many people who make good isolation trasnformers but
they can be expensive.  You are talking about several hundred dollars for a
decent 2000 VA unit.  Don't scrimp here - used properly, they will outlive
you.  Spending the money for a good transformer up front pays off big time
over the long term.

dwayne


Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice          (403) 487-6397 fax

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1999\03\12@122332 by Harold Hallikainen

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       Check out the Burr Brown INA117.  It is a unity gain differential
amplifier with a common mode input range of +/- 200V.

Harold


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Hallikainen & Friends, Inc.
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1999\03\12@125859 by Gerhard Fiedler

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At 09:53 03/12/99 -0700, Dwayne Reid wrote:
>you.  Spending the money for a good transformer up front pays off big time
>over the long term.

yep -- didn't we have that lengthy thread just a =little= while ago about
scopes, DUTs and isolation transformers? and why it's better to use one =on
the DUT=? :))

ge

1999\03\12@153724 by Lawrence Lile

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-----Original Message-----
From: Dwayne Reid <dwaynerspamKILLspamPLANET.EON.NET>
To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Date: Friday, March 12, 1999 10:54 AM
Subject: Re: [OT] Instrumentation amplifiers Zap Bosses


>Hi there, Lawrence.
>
>You are taking the wrong approach, me thinks.  You should use an iso
>transformer, but it needs to power the circuit under test, NOT the scope.

Agreed.  Maybe, given the sucky bandwith of isolation amplifiers, I'm
barking up the wrong tree.

So I'll ground my scope, plug the load into the iso transformer, and make an
instrumentation amplifier that has at least as much bandwidth as my scope
and about 100X the input impedance.  That way at least I'll know I'm not
loading my circuit.   I'll probably look into a FET input instumentation amp
with at least 25 - 50mhz bandwidth.

>
>Floating your scope ground (which is all you are doing when you power the
>scope with an iso) is dangerous and should NOT be your normal setup.


No not at all - the other way 'round - float your scope ground at 8000V,
invite your boss in, and POOF! no boss! Leaves nothing but a pile of ash -
GREAT for fooling pesky detectives too! ; )


>
>If you absolutely have to measure something live, look at differential
scope
>probes from Preamble (division of LeCroy).  Very nice stuff, good
bandwidth.
>Tek also makes one that seems pretty good.
>


Wish I could afford one.

1999\03\12@175356 by Lawrence Lile

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\>
>yep -- didn't we have that lengthy thread just a =little= while ago about
>scopes, DUTs and isolation transformers? and why it's better to use one =on
>the DUT=? :))
>
>ge


Sorry - I was asleep or something.  Yes you are right - use the iso
transformer on the DUT unless otherwise noted.

1999\03\14@152435 by Russell McMahon

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>Topaz is only one of many people who make good isolation
trasnformers but
>they can be expensive.  You are talking about several hundred
dollars for a
>decent 2000 VA unit.  Don't scrimp here - used properly, they will
outlive
>you.

Used improperly they will; DEFINITELY outlive you :-)



           Russell McMahon



Spending the money for a good transformer up front pays off big time
>over the long term.

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