At 10:39 PM 7/30/01 -0700, you wrote:
>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

I don't think so.. but as you haven't said whether "volume"
means 5 or 50,000, it's hard to tell. ;-)

>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.

Have you heard of trimpots?  It's often cheaper and
more practical to put an adjustment in rather than
try to make everything fit into your error budget.
Of course, with proper design you can sometimes
use a multiplier stored in EEPROM or whatever, but
there are disadvantages to this.

>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.

Forget it, even optos designed and spec'd for linear
operation have +/-10% or more variation from unit to
unit (--talking about closed loop with an op-amp).
You can buy isolators that will work, but they cost
a lot of money (>> $6 US).
Linearity of +/-0.1% F.S. is achievable, however,.
But you'd still need a floating supply.

P.S. You might get better suggestions if you
disclosed:

1)      Target quantity range
2)      Frequency response range
3)      Accuracy requirements and dynamic range

Of course if you are just seeing what's possible
this may be challenging...

Best regards,


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