Building blocks of multimeter

A project log for DIY 6-digit multimeter

Open source multimeter that measures more for less

In theory, there is nothing complicated about multimeter, as usually up to the point of meeting the reality.

The main part of multimeter is ADC, converting voltage to digital value, to be processed and displayed. Well, ADC directly measures voltage, that goes for the voltmeter part. For measurement outside the ADC input range, voltage divider (allowing higher voltage to be measured) or amplifier (for lower voltages) is required.

To measure current, usual method is to convert current to voltage. Fortunately, there is very common part converting current to voltage - a resistor. In order to have the option of changing measurement ranges, resistor switching is usually applied.

To measure resistance, one needs current source to push current through the unknown resistor, while the voltmeter part of multimeter takes care of measurement of voltage drop on the resistor, being proportional to the resistor value.

That is the simple part and really, majority of multimeters do operate in this way. I sketched a quick sketch of such as multimeter

As you can see, it's really that simple, but this picture already uncovers a few usual devils hidden in details.

First of them is voltage reference above the central, ADC part. This voltage reference should be as stable as possible, with regard to temperature and ageing. It doesn't need to be very accurate (non-accurate voltage can be "calibrated out") but drift with temperature and age is hard to predict and compensate for.

Another part that makes the design a bit more tricky is isolator. Since the measured voltage could be on any potential, perhaps floating on higher voltage or you may have two multimeters measuring at different part of the circuit, it is usually needed to have the COMMON (black wire of your typical multimeter) disconnected from power ground or each other. That's why the reference/amplifier/ADC part is usually "floating" or "guarded" (often referred as inguard part) and the MCU/interface/user IO is usually "grounded", via power outlet or USB or other connection to PC (usually referred as outguard part). This separation is usually not needed for handheld multimeters, operated from battery, being floating per design.