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I'm attempting to design an ohmmeter that can measure resistance from 0.01 to 10k Ohms. The circuit is based on the 4 wire Kelvin resistance measurement. So far the low end measurements are fine but I'm having difficulty adapting the circuit to measure from 10 to 10k Ohms.

How can I adjust this circuit to measure resistance from 0.01 to 10k Ohms? Thanks

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  • \$\begingroup\$ @Andyaka - I am the same OP. I took the advice of one of the respondents and built this new circuit but I've hit another wall. \$\endgroup\$ – Dan Oct 24 '15 at 16:08
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    \$\begingroup\$ When no answers have been posted on the other question, please proceed to modify the old question with an edit section explaining what has happened - no need to create a new question. Probably best if you made alterations and deleted this one. \$\endgroup\$ – Andy aka Oct 24 '15 at 16:11
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    \$\begingroup\$ With one LM324 you are far away, anyway it is complicated even for professional instruments to acheive such span, you would need a stable current source, low noise amplifier, PGA, filters and ratiometric ADC. \$\endgroup\$ – Marko Buršič Oct 24 '15 at 16:12
  • \$\begingroup\$ In what way does the circuit not work for high values? Which resistor in the diagram is meant to be the DUT? What is the value of I2? \$\endgroup\$ – The Photon Oct 24 '15 at 16:13
  • \$\begingroup\$ I agree with Andy. It is your choice which one you want to delete, but it is kind of irritating to see basically the same question reposted by the same person in a short period of time. \$\endgroup\$ – mkeith Oct 24 '15 at 17:05
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You are attempting to measure a resistor value range that spans 6 decades or a range of 1 million to 1. That is difficult to do using strictly linear analog techniques.

You have a couple of options.

1) Split the range into smaller segments. Most Ohmmeters have multiple ranges, each range is a factor of 10 larger or smaller than the adjacent range.

2) Use a log amplifier to compress your 6 decade range into a more reasonable span. Although the actual accuracy is reduced, you are able to easily see the approximate resistance value.

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