Good resource for "simple" OHMMETER design

[geomit]

I am trying to look for good resources where I can find designs of a simple auto-ranging ohmmeter (200 to 20Mohms).

I am attempting to embedd on in some of my equipment so it would be nice to have a some reference designs.

Thanks.

 

[nbucska]

Why don't you build it as a PC plug-in card ?

<nbucska@pcperipherals DOT com> subj: eng-tips

 

[Madcow]

This might get you started.

http://ww1.microchip.com/downloads/en/AppNotes/00512e.pdf

For auto ranging use some of the GPIO pins to switch in and
out different values.
I have used four GPIO pins and four resistors. each resistor
in series with one pin. The other ends get tied together and
then tied to one of the analog input pins. The test resistor is the connected from that point to return. By
setting one of the outputs, five volts is applied to the
resistor this forms a divider with the test resistor. I then preformed an A to D conversion and read the midpoint voltage and calculated the resistor's value. I started with the largest value resistor first and if the midpoint voltage was below 200mv (just a value I picked) I would
change the resistor by switching that output pin to an input and enable another resistor. This worked fairly well.

Good luck

 

[Madcow]

The 20meg range will probably be difficult do to leakage
currents. This app. note might help if you need the higher ranges or more accuracy.

http://www.intersil.com/data/AN/AN046.pdf