Digital Multimeter working principle; DMM working

Digital multimeters are most widely used in every industry as it used for measurement of Voltage, Current, Resistance, Capacitance and various other parameters. DMMs have been available for very several years. Yet it is still challenging to locate advice about how a digital multimeter works.

The operation of a digital multimeter is comparatively uncomplicated, although there are obviously differences in how digital mulimeters work if they're from different makers.Despite this, there are several similarities and some general principles of how digital multimeters work.

Fundamentals of how a DMM functions

The key process occurring within a digital multimeter for any measurement is that of voltage measurement. All other measurements are derived from this basic measurement.

So the best technique for understanding how a digital multimeter functions is in understanding this procedure.

There are many kinds to digital converter, ADC. Nonetheless the one that is most widely used DMMs in digital multimeters, is currently referred to as SAR or the successive approximation register. Those used in test equipment normally have 16 bits or perhaps more dependent upon the program, although some SAR ADCs may just have resolution amounts of 12 bits. Commonly with speeds per second resolution levels of 16 bits are generally used, for DMMs. These levels of speed are more than adequate for most DMM applications, where high degrees of speed aren't generally needed.

Consecutive approximation register ADC used in most DMMs

Most DMMs were used in by serial approximation register ADC

As the name implies, the successive approximation register ADC functions by successively homing in on the financial value of the voltage that is incoming.

The first stage of the procedure is for the sample and hold circuit to sample the voltage at the input of the DMM and to hold it steady.

With a steady input voltage the register begins at half its total scale value. This would typically need the most significant bit, MSB set to "1" and all the remaining ones set to "0". Assuming the input voltage could be anywhere in the range, the mid-range indicates that the ADC is set in the middle and this provides a more rapid settling time. As it only needs to transfer a maximum instead of possibly 100%.

The input voltage's output will begin at 1000. The comparator output will be low and that'll drive the register to a amount of 0100 if the voltage is less than half the maximum ability. If the voltage is above this, the register will go until it homes in on the nearest value.

It may be seen that SAR converters, need one approximating cycle for each output bit, i.e. an n-bit ADC will require n cycles.

DMM operation

To be able to completely understand how a digital multimeter operates although the analogue forms the essential component within the instrument, it is vital to look at a few of the other functions around the ADC.

Although the ADC will take very many samples the overall digital multimeter will not display or return every sample chosen. The samples are buffered and averaged to reach high precision and resolution. This will overcome the effects of modest variations for example sound, etc., sound created by the analogue first periods of the DMM being an important variable that must be overcome to achieve the greatest precision.

How a digital multimeter works

Operation flow diagram of a DMM for operation is shown as below

Measurement time

One of the key areas of understanding how a digital multimeter works is related to the measurement time. In addition to the fundamental measurement there are several other functions that are required and these require time. Consequently the measurement time of an electronic multimeter, DMM, may not always seem uncomplicated.

The overall measurement time is composed from several phases where different actions happen:

Switch time:

The switch time is the time needed for the instrument to settle after the input signal has been switched. Including the time to settle after a measurement type has been changed, e.g. from voltage to resistance, etc. In addition, it contains time to settle after the range has been shifted. If auto-ranging is included the meter will need if there is a range change required to settle.

Settling time:

A time that is particular will be demanded for the value to be measured to settle, once it has been applied to the input. This will overcome any input signal capacitance levels when high impedance evaluations are made, or ordinarily for instrument and the circuit to settle.

For AC measurements, the frequency of functioning must be taken into account because the minimum signal measurement time relies on the minimal frequency required of the measurement.

Auto-zero time: 

When autorange is chosen, or range changes are made, it is vital to zero the meter to ensure correctness. Once the correct range is selected, the auto-zero is functionality for that range.

ADC calibration time:

In some DMMs there is a calibration occasionally performed. This must be accounted for, especially where measurements are required under automatic or computer management.

It's always useful to understand how a digital multimeter functions in order to be able to make the greatest use and the most precise measurements. However it should be remembered that different multimeters from different manufacturers may work in different ways. Hence it is always helpful to consult with the manufacturer's instructions to comprehend how a digital multimeter works that are particular.