Breadboarding the Ohmmeter

*Update 4 June 2019 : Last few days I was busy with a digital thermometer based on ADS1220 and there I found better way to make resistance measurements. Actually this approach is used in bench-top multimeters. Instead using long-term stable voltage reference and resistor to generate exact current value, it is better to use low noise current source which pass through both a resistor under test and a reference resistor. In this case the reference resistor is connected to the voltage reference inputs of the ADC. In result the ADC will show ratio between the resistor under test and the reference resistor. 

*End of the update

I tried to make an ohmmeter, based on the current source schematics found in the Linear technology LTC6081/LTC6082 datasheet, page 14. The other option was to use the Howland current source, but I will try this later.

The KiCad schematic is uploaded into the git repository, together with the LTSpice simulation:

I played with the simulator and found that measured resistor can have higher value then the reference, but the power supply voltage should be increased and eventually additional voltage divider can be put between the voltage reference and the first op-amp as it is shown in the LT schematics. This will allow measuring of resistance from hundreds mOhms to several MOhms only with 2 reference resistors : 1K and 100K, thus decreasing the final cost. An attention should be paid for the PCB layout for guarding the leakage current.

Using the LTC2052 op-amps, the current source was implemented on the breadboard with 10KOhm reference resistor (Vishay S102 series) and nominal 10Ohm resistor for measurement (9.92 Ohm measured with Keithley 2002), 2.5V reference voltage and 100KOhm LTC5400 quad matched resistor network.

The noise was ~19 uV peak to peak for duration of ~7h:40m.
The data in the Y axis are the ADC measurements in microvolts, data in the X axis are the sample counts. The ADC was set in low noise mode with ~6.9 samples per seconds.

I expect to have much lower noise when the first PCB prototype will be tested. After voltmeter calibration, I have to compare the results with the reference DMM.