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Analog interface

S

steve

Jan 1, 1970
0
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
 
R

Rob Gaddi

Jan 1, 1970
0
steve said:
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
Optos tend to vary pretty badly in their current transfer ratio from one
part to the next, some of them by 4:1 or so, and are linear in
current, not in voltage. Getting the system accurate will be a pain,
and will most likely require you to supply power to the isolated side to
be able to have some active circuitry over there to help.

That being the case, as long as you're going to have to supply power to
the isolated side, you might as well just move the A/D over there as
well. Digital signals are much more forgiving of trying to push them
across an isolation boundary; a poorly linearized 1 or 0 is still a 1 or 0.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that steve <[email protected]>
wrote (in said:
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. (;-)
 
J

Joerg

Jan 1, 1970
0
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.

Regards, Joerg
 
F

Fred Bartoli

Jan 1, 1970
0
Joerg said:
Hello John,


Way to go, especially if the frequency range has to exceed audio. Even
if not I'd do it xfmr/chopper because of cost.

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...
 
J

John Larkin

Jan 1, 1970
0
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

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
 
R

R Adsett

Jan 1, 1970
0
steve9999@SAFe- said:
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?

There is Analog Devices AD7400 and 7401

Robert
 
T

Tony Williams

Jan 1, 1970
0
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.

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.
 
P

Paul Burke

Jan 1, 1970
0
steve said:
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

Google for IL300. You'll have to include an isolated supply to the hot side.

Paul Burke
 
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