HAL-42b, what accuracy are you after? If it is just 0.1% then Zad's suggestions are the way to go. No switching needed.

If you are after as much accuracy as you can achieve, my suggestion may be the way to go.

Now as far as the equations for my circuit, just assume the current trough the two resistors is I .

You are measuring:

V_Rref = I * Rref

V_Rut = I * Rut

Now what you are measuring is the ratio

V_Rut/V_Rref = I*Rut/(I*Rref) = Rut/Rref

So

Rut = Rref * V_Rut/V_Rref

Calibration inaccuracies in the voltage applied to the resistors and in the voltmeter don't affect the result at all.

In the light of all of the above, let's consider that we are using a resistive ladder as a reference resistor and range switch at the same time. The resistive ladder consists of 5 steps, 1k 10k 100k 1M 10M. ADC operates between 0V - 5V however it is a good idea to switch to the upper range when we measure 4 V across the Resistor under test. In this case we have:

Vref | Rref | | Range |

5 V | 1k | | 4k |

5 V | 10k | | 40k |

5 V | 100k | | 400k |

5 V | 1M | | 4M |

5 V | 10M | | 40M |

So far so good, but I'd like to have 400R and 40R ranges as well. Also what about 4 wire measurements and 4R and 0.4R ranges?

If you are going after accuracy, you may want to minimize the number of precision resistors - really stable resistors are very expensive, particularly if you are going to use Vishay foil resistors.

A trick HP has used is to add a switchable precision x1/x10 amplifier in front of the voltmeter. A single switch can turn the x10 on and off, and it is pretty easy to make a x10 divider that you can calibrate to at least 0.001% without special equipment and you can get up to 0.00001% if your divider resistors were stable enough (I didn't miscount the zeros there - that is 1 part in 10-7).

This way, you get 3 ranges out of one precision resistor.

1K ohms can do the 100ohm, 1K and 10K ranges.

1M can do the 100K, 1M and 10M ranges

With these ranges, then if you did a 6 1/2 meter, you can read down to 0.0001 ohms.

For 4 wire ohms, you need to be able to switch the fixed voltmeter lead separately to each resistor.

Anyway, it all comes down to what you are after here.

Richard