Thanks everyone for all of your advice.
Unfortunately for cost reasons, I must stick with
using a low-ohm resistor as a current sensor. A
hall-effect current sensor, a magnetoresistive device
and
a reasonably accurate current transformer, cost $6 and
above in volume. It looks like these are my only
options:

1. Use a 1:1 transformer to isolate the voltage across
the low-ohm resistor (current sensor), from the
interfacing analog circuitry. On the secondary side,
the voltage can be referenced to ground, making it
quite easy to get reasonable accuracy out of an
inverting op-amp circuit with .1% resistors.
 This will work, since I am only concerned with AC
current. The problem is finding a 1:1 transformer with
truly accurate winding ratios (within 1%). Or else, I
will have to perform some type of testing in each unit
during manufacturing and come up with a normalization
look-up table in software.

2. Use some type of optocoupler. Again the same
problems as solution #1. I am wondering how difficult
it would be to get an monolithic optocoupler
that has consistant voltage/current transfer ratios
between device samples.

Perhaps some of you can comment on these potential
solutions. Thanks for all of your help.





--- Sanjay Punjab <piclistguy@YAHOO.COM> wrote:
> I have an application where I need to monitor and
> digitize the current waveform output of an audio
> power
> amplifier into a subwoofer (speaker).
> Previously, I used a hall-effect based sensor, but
> for
> cost purposes (consumer product), it is impractical.
> So instead, I have decided to use a 1 milli-ohm
> current sense resistor. The problem now is creating
> an
> electrical interface to convert the instantaneous
> current (0 - 50 amps) into a proportional
> instantaneous voltage (0-2.5v). Since the audio
> output
> is not referrenced to ground, some type of
> differential circuit must be used. But although the
> voltage across the resistor will always be small,
> the
> voltage at each of the 2 resistor taps, with respect
> to ground can swing as high as much as +/-100 volts.
> Unfortunately most instrumentation op-amps that can
> handle such a high common-mode input voltage, are
> also
> quite expensive. In addition, a textbook
> differential
> op-amp circuit doesn't provide the needed accuracy,
> even with .1% resistors, since any component
> tollerance is multiplied by the gain of the circuit
> (50). I though it would be easy finding an interface
> circuit that would solve my problem, especially
> since
> almost every DMM must use something similar for DC
> current measurement. But I have had no luck. I would
> appreciate some advice, even better a schematic.
> Thanks
>
>
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