April 14, 2004

Measuring Ohms

To get to know the 6430, I've decided to measure Ohms. According to 2-4 of the instruction manual, for the triax to alligator clip, red=HI, black=GUARD, and green=LO. Although I could have left LO floating, I connected it to the chassis ground. (I'll try the floating... later.)

For the two-wire configuration, per the diagram in the manual, SENSE is left open, and the resistor (the device under test or DUT) connected between HI and LO. GUARD is the same voltage as HI (2-6) - I left this disconnected.

2-wire.PNG

There are four items to configure in resistance measurement:

  • SOURCE - AUTO or MANUAL,
  • GUARD - OHMS or CABLE,
  • SRC RDBK - source readback mode (ENABLE or DISABLE), and
  • OFFSET COMPENSATION - ENABLE or DISABLE
For this first measurement, I set SOURCE to AUTO (CONFIG->Ω->SOURCE->AUTO) to make the device select its own test current. If you do not do this, it will give some wierd number - around 700 kΩ - because the source current is zero. I left everything at it's default (CABLE/ENABLE/DISABLE). Testing a resistor that Nick left on the table, I found it to be 1.00011-1.00013 MΩ with the value of the last digit fluctuating over time. Does it make sense? Unfortuantely the resistance value was encoded using 5 bands. Thanks to Sam Engström's graphically-nifty translator, I was able to decode it in a jiffy, 1 M&Ohm; with 1% tolerance. Good. The source current, Isrc, was 1.00000 μA and the compliance voltage limit, Cmpl, 2.10000 V.

Now, 1 MΩ was still kind of small, so I scrounged around and found a brown-brown-blue-gold resistor - 11 MΩ±5%. (Incidentally, in case you ever need them, html symbols found here.) Measurement? 11.2178 MΩ. Nice. I'll use this resistor for the remainder of my tests though it would be good if I could find something in the 100 MΩ range... I scrounged some more, and found a brown-gray-blue-gold resistor - 19 MΩ±5%. Measurement? 17.7025 MΩ. Ahh, I'll use this one.

Several quick tests. First of all, what does offset compensation do? It attempts to take out thermal EMF by measuring resistance (V/I) at a specific source level and then subtracting a resistance measurement made with the source set to zero. Result, 17.7084 MΩ. Interesting. Next step, 4-wire.

Posted by torque at April 14, 2004 5:02 PM
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