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Measure degradation of the electrical resistance

Question asked by microar on Oct 16, 2015

Degradation of the electrical resistance

I need to measure degradation of an electric resistance, with respect to another reference.

Both are in the same medium, a protected by a resin and the other exposed. They are very low, between 1milli ohms and 5 ohms maximum.

Degradacion de resistencia AD623.png

As want to measure varies relative to the other in order to quantify the effect of degradation, an alternating signal applied by the AF points, measure the differential voltages between BC, obtaining voltage reference system and between CD, for the external voltage.

The two VRef and Vext outputs connect to the instrumentation amplifier and get its

(VRef - VEXT) = VDif, with an additional gain.

Then I use a filter Buterbord of 3 active stages, for the sign.

Rline is the resistance presented by the cable between the sensor and the measurement system, about 10 meters 6-conductor shielded cable of 0.75mm diameter each.

The output signal is very unstable.

As I can improve the design?.
Is there any other method of measurement that gives me better, I thought to use a Wheatstone bridge or Kevin, but being so low value of resistence, not know how to create it.