Analog Interface

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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?



Re: Analog Interface

here's some ideas.

Thats why the bits you want are expensive to buy.

The 500 volt sensor could be done with a high value resistor and a LED, if
you can get the current - light -current  transfer the signal accurately.

Isolating with low voltage will require a non-isolated powered unit doing
something like PWM  through a LED.

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Re: Analog Interface

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The modern way to achieve the isolation you want is to use isolated DC/DC
converters (e.g. the ones from Newport with 1.5 - 3 kV isolation) and put
separate serial A/D converters on each isolated channel, then stick the
digital I/O of these through optos.


Re: Analog Interface

One vital piece of information is missing - your signal bandwidth.
Presuming it is fairly low - try a voltage to frequency converter on the
isolated side of a fast optocoupler. Some V-Fs are linear to a couple of MHz
over many orders of magnitude. Of course an isolated or safe battery supply
is required for the V-F and conditioning cctry.

On the nonisolated side either F-V or microcontroller with capture input
could decode the freq output. This can be pretty crude and still give a good
result for low bandwidth signals

-Andrew M

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