Analog interface

S

steve

Guest
Hello all,

Someone has asked me to design a system that can take multiple
"oscilloscope" type isolated differential inputs ranging form 0 to 500volts
AC or DC. In reality they want to be able to hook up to a 3 phase motor or
a 0 to 5 volt signal line, or a current clamp that provides 0-1 volt output.
Frankly I'm a little stumped. The AD converter, multiplexer and automatic
gain selection issues are easy enough to handle, it's the isolation I'm
having trouble with. I can't use a transformer because of DC. I want to
use an opto-isolator, like the Farichild H11F3, but I have the roughly 1
volt drop on the input LED that gives me a problem with the current clamp or
anything in the 0 to about 1.4 volt range. I have considered using the TI
(Burr Brown) INA148, but I don't think it really provides the isolation I
need if I have 3 of them connected differentially across a 3 phase motor.

Anyone got any ideas?

Thanks,

Steve
 
I read in sci.electronics.design that steve <steve9999@SAFe-mail.net>
wrote (in <xmY%d.36612$8D.7106@tornado.texas.rr.com>) about 'Analog
interface', on Tue, 22 Mar 2005:
Hello all,

Someone has asked me to design a system that can take multiple
"oscilloscope" type isolated differential inputs ranging form 0 to 500volts
AC or DC. In reality they want to be able to hook up to a 3 phase motor or
a 0 to 5 volt signal line, or a current clamp that provides 0-1 volt output.
Frankly I'm a little stumped. The AD converter, multiplexer and automatic
gain selection issues are easy enough to handle, it's the isolation I'm
having trouble with. I can't use a transformer because of DC. I want to
use an opto-isolator, like the Farichild H11F3, but I have the roughly 1
volt drop on the input LED that gives me a problem with the current clamp or
anything in the 0 to about 1.4 volt range. I have considered using the TI
(Burr Brown) INA148, but I don't think it really provides the isolation I
need if I have 3 of them connected differentially across a 3 phase motor.

Anyone got any ideas?

Chopper followed by a transformer. Details left to the student. (;-)
Click to expand...
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
Hello John,

Chopper followed by a transformer. Details left to the student. (;-)
Click to expand...
Way to go, especially if the frequency range has to exceed audio. Even
if not I'd do it xfmr/chopper because of cost.

Regards, Joerg

http://www.analogconsultants.com
 
"Joerg" <notthisjoergsch@removethispacbell.net> a écrit dans le message de
news:EZ_%d.14522$C47.3525@newssvr14.news.prodigy.com...
Hello John,

Chopper followed by a transformer. Details left to the student. (;-)

Way to go, especially if the frequency range has to exceed audio. Even
if not I'd do it xfmr/chopper because of cost.
Click to expand...
It may not be totally necessary to have galvanic isolation.
If only high input impedance is possible, then one way is a pair of
inverting amplifiers with common mode feedback, followed by a difference
amplifier.

Much simpler than the chopper and its floating input stage power supply and,
and, and...


--
Thanks,
Fred.
 
In article <xmY%d.36612$8D.7106@tornado.texas.rr.com>, steve9999@SAFe-
mail.net says...
Hello all,

Someone has asked me to design a system that can take multiple
"oscilloscope" type isolated differential inputs ranging form 0 to 500volts
AC or DC. In reality they want to be able to hook up to a 3 phase motor or
a 0 to 5 volt signal line, or a current clamp that provides 0-1 volt output.
Frankly I'm a little stumped. The AD converter, multiplexer and automatic
gain selection issues are easy enough to handle, it's the isolation I'm
having trouble with. I can't use a transformer because of DC. I want to
use an opto-isolator, like the Farichild H11F3, but I have the roughly 1
volt drop on the input LED that gives me a problem with the current clamp or
anything in the 0 to about 1.4 volt range. I have considered using the TI
(Burr Brown) INA148, but I don't think it really provides the isolation I
need if I have 3 of them connected differentially across a 3 phase motor.

Anyone got any ideas?
Click to expand...
There is Analog Devices AD7400 and 7401

Robert
 
In article <xmY%d.36612$8D.7106@tornado.texas.rr.com>,
steve <steve9999@SAFe-mail.net> wrote:
Hello all,

Someone has asked me to design a system that can take multiple
"oscilloscope" type isolated differential inputs ranging form 0
to 500volts AC or DC. In reality they want to be able to hook up
to a 3 phase motor or a 0 to 5 volt signal line, or a current
clamp that provides 0-1 volt output.
Click to expand...
Have a look at the LEM range of Hall Effect current
sensors. The 'instantaneous' type go from DC to
100KHz, small signal. Normal current sensors for
each of the phase currents, and a low-current sensor
(with series resistors) for the voltage pickup.

--
Tony Williams.
 
steve wrote:

Someone has asked me to design a system that can take multiple
"oscilloscope" type isolated differential inputs ranging form 0 to 500volts
AC or DC. In reality they want to be able to hook up to a 3 phase motor or
a 0 to 5 volt signal line, or a current clamp that provides 0-1 volt output.
Frankly I'm a little stumped. The AD converter, multiplexer and automatic
gain selection issues are easy enough to handle, it's the isolation I'm
having trouble with. I can't use a transformer because of DC. I want to
use an opto-isolator
Click to expand...
Google for IL300. You'll have to include an isolated supply to the hot side.

Paul Burke
 
On Tue, 22 Mar 2005 17:13:01 GMT, "steve" <steve9999@SAFe-mail.net>
wrote:

Hello all,

Someone has asked me to design a system that can take multiple
"oscilloscope" type isolated differential inputs ranging form 0 to 500volts
AC or DC. In reality they want to be able to hook up to a 3 phase motor or
a 0 to 5 volt signal line, or a current clamp that provides 0-1 volt output.
Frankly I'm a little stumped. The AD converter, multiplexer and automatic
gain selection issues are easy enough to handle, it's the isolation I'm
having trouble with. I can't use a transformer because of DC. I want to
use an opto-isolator, like the Farichild H11F3, but I have the roughly 1
volt drop on the input LED that gives me a problem with the current clamp or
anything in the 0 to about 1.4 volt range. I have considered using the TI
(Burr Brown) INA148, but I don't think it really provides the isolation I
need if I have 3 of them connected differentially across a 3 phase motor.

Anyone got any ideas?

Thanks,

Steve
Click to expand...
You could go with a pair of high-impedance, high-voltage dividers to
get the signals to within, say, 5 volts of ground, then use a diffamp
to take out the common-mode stuff. You could make the dividers out of
a string of 1% or 0.1% resistors, or buy some nice kilovolt-rated
precision voltage dividers from Caddock or those other guys.

I do this in my electric power meters. I have a die-cast box with the
dividers inside, with some zeners and stuff for added safety.
Electricians can run 480 vac wires into one side of the box, and the
electronics guys wire up to the low-voltage side. There's a metal
divider in the middle, sort of a labor/cultural divide.

John
 
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