I built a digital multimeter in 1992 to be used as a measurement instrument in my home electronic lab.

The multimeter incudes as main component, a low cost, dual slope A/D converter, type CA3162 [2].

Specification:
-instruments: AC/DC voltmeter, ohmmeter, AC/DC amperemeter
-instruments resolution: 0.1% of full scale
-A/D converter linearity: ± 1 count (1/999)
-voltmeter AC/DC scales: 1V, 10V, 100V, 1000V
-voltmeter AC/DC input impedance: 8 to 10 MOhm
-voltmeter DC accuracy: ± 0.5% of full scale
-voltmeter AC accuracy: ± 0.8% of full scale
-ohmmeter scales: 1 kOhm, 10 kOhm, 100 kOhm, 1000 kOhm
-ohmmeter accuracy: ± 0.5% of full scale
-amperemeter AC/DC scales: 1 mA, 10 mA, 100 mA, 1000 mA
-amperemeter DC accuracy: ± 0.5% of full scale
-amperemeter AC accuracy: ± 0.8% of full scale
-powered from mains AC: 230 VAC / 50 Hz
-power consumption: 5 W
-calibration signal output: 5.00 V / 5.00 mA for quick instrument test
-protection by proper schematic design (using special input circuits, voltage suppressors, fuses and so on) for accidental connection of measurement leads to power line voltages up to 250V AC/DC on all instruments, all scales.

The multimeter is composed of several modules, each one having its own PCB. Below are the schematic diagrams (designed by me):
-A/D converter (based on CA3162) - figure 7
-display (LED digits with common cathode) - figure 8
-power supply (based on LM723) - figure 9
-ohmmeter + sign circuit (SC) - figure 10

More schematic diagrams:
-precision rectifier (for AC) + amplifier x10 (amperemeter) - figure 11
-block diagram - figure 12
-voltage divider - figure 13
-panel switches layout - figure 14

This project includes some precision operational amplifiers LM108 with good thermal stability, low offset and low bias currents. These components are ideals for instrumentation applications.

The fourth digit module from LED display has been modified to be able to display "+", "-" and "~" signs. The modifications have been made by breaking apart and changing the plastic mask windows. Same module also shows "u" in voltmeter mode, "i" in amperemeter mode and "r" in ohmmeter mode (figure 8). Plus (+) and minus (-) LEDs from fourth module are driven by "sign circuit". (figure 10)

There is a voltage follower at A/D converter input, made with M108 operational amplifier (figure 7). It is used as impedance adapter between voltage divider and A/D converter. CA3162 has a multiplexed BCD output. For driving the LED display is provided a 4511 - BCD to 7 segment decoder.

"Sign circuit" is built with general-purpose operational amplifier A741, used here as a voltage comparator. It has resistors of different values at the inputs (pin 2 and 3) to compensate its voltage/current input offset. There is another A741 in the module used to create -1 V reference voltage for ohmmeter.

Ohmmeter is built with an inverting amplifier (figure 10) starting from a reference voltage at input. Output voltage is in proportional relationship with the measured resistor. A non-inverting amplifier with a fixed gain of 10 is used as amperemeter (figure 11). It has the role to allow the use of shunts with low values when measuring currents.

The following appropriate actions have been taken to improve the accuracy of the instruments (as stated in specification):
-selection of operational amplifiers with lowest input voltage/current offset to be used in project.
-use of metal film resistors with low tolerance on all modules from in the measurement chain
-power supply for all modules and reference voltage for ohmmeter made with precision voltage regulator LM723
-resistors from divider and other critical places have been calibrated to precise values by selection or by using serial/parallel combinations.

Disclaimer: The information on this web site is provided "AS IS", without warranty of any kind. The author has made the best efforts to ensure the design and the information provided are reliable. Under no circumstances shall the author be liable for any direct, indirect, incidental, special or consequential loss, damage, expense or injury incurred or suffered which is claimed to resulted from use of this site, even if expressly advised of the possibility of such loss, damage, expense or injury, including, without limitation, any fault, error, omission, interruption or delay with respect thereto.

Links:
[1] - Wikipedia - Multimeter: http://en.wikipedia.org/wiki/Multimeter
[2] - A/D Converter, 3-Digit Display: CA3162
[3] - Precision operational amplifiers: LM108