percussion ribbon controller AGAIN!!!!

Caloroso, Michael E CalorosoME at
Mon Nov 15 23:35:02 CET 1999

> > You can deal with low resistance by using a bridge connection and a
> differential (instrumentation) amplifier. Strain Gauge and RTD people do
> this all the time... but it is a lot of circuit....
> >
> A couple of months ago I was working on a problem that involved measuring
> very low impedances, namely electrical conectors between the card and
> backplane of a telecoms rack system (typically 1mOhm).  I found a nice easy
> and inexpensive way of doing this.  I passed a amp through the connectors
> from a constant current source and measure the voltage across it using a
> differential amplifier circuit using a gain of 100 and an OP177 (nice low
> offset) as the opamp (with a 10k resistor as the load).  This worked
> perfectly without the need for instrumentation amps or extra buffers, strain
> gauges etc.
> Anyway, that might be helpful!!!!
I might add that this solution is accurate as long as the wires connecting to your differential inputs are *equidistant* and equal *gauge*.

When you're measuring voltage differences in the millivolt range, a foot difference or gauge difference between two wires can throw off the measurement.

...I don't think that wires of different composition, such as those used in thermoelectric systems, would have that much impact on this method, thermoelectric offsets are in the microvolt range...

...but this is an efficient technique.

Opinions (and mistakes) expressed herein are my own and not those of my employer.

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