Analogue and Digital
Multimeters

Article : Andy Collinson
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Overview
In electronics, one of the most popular test instruments is the Multimeter. These come in two varieities, Analogue and Digital. This article will explain the basic differences.

Analogue Multimeter
This type of multimeter contains a moving coil meter, a selector switch and can be passive, (using resistive networks to switch ranges )or active, ( requiring a battery and electronics to perform certain functions or amplify weak voltages and currents). The analogue meter has a screw to adjust the zero position. This needs to be done regularly, in accordance with the user manual.

The analogue multimeter has printed scales and a fine pointer. When reading the meter your eye needs to be at right angles to the plane of the meter, otherwise you will get a "parallax error". A parallax error is a reading error caused when you are reading the scale from the wrong angle (not 90 degrees to plane of the meter). To minimize parallax, a mirror on the scale allows you to make an accurate reading. Simply move your head left or right until you cannot see the pointers reflection, then the reading will be accurate.

Various ranges are used With the help of a rotary selector switch. Most multi-meters will read AC and DC voltage and current, and resistance readings. Some meters can also read decibels, transistor hFE and capacitance like my Maplin M-2030 shown right. This meter is obsolete now and the MT-2017 is a similar replacement.

An analogue multimeter has a lower input impedance than a digital multimeter but can take continuous readings of changing voltages or currents. The needle mechanism is damped but any signal that changes up to 10 times a second can easily be displayed. They are a compact and portable instrument and widely used in electronics.


Digital Multimeter
The digital multimeter often abbreviated DMM, has an LCD display commonly 3 or 4 digits wide, however there are also bench models with 5 or 6 digits resolution. Some have a multifunction selector switch or push button to select the ranges, but auto-ranging types are also available. Some DMM's have a "hold" and "max" or "min" buttons that can capture the maximum and minimum value of a changing signal. There is no mistaking the reading from a DMM and they do not rely on calibration.

The DMM is capable of measuring AC and DC voltage, AC and DC current, resistance, conductance, and decibels. Often, a DMM has more features than an analogue meter, such as frequency, capacitance, inductance, temperature and transistor current gain.

The input impedance is also very high and has little affect on loading the circuit. Precise readings can be taken easily. A DMM uses a very fast ADC (analogue to digital converter) to take samples of the input signal and convert it to a readable format on the LCD display. Although the ADC can be very fast, your eyes can not keep up with the rate at which the LCD display would be updated. For reason there is a trade-off between sample rate and display rate, which is usually slowed down to a few readings per second. My Maplin M5010 shown left can take 3 readings per second, although this varies between meter manufacturer.




Comparison of Features

Accuracy
Analogue multimeters have a typical accuracy of 5%, but care has to be taken when reading to avoid parallax errors. As the meter ages, the magnetism in the coil degrades, which also affects the accuracy. A Digital multimeters accuracy is 3% but very easy to read and no adjustments are necessary. These figures are typical but vary between manufacturer and model, A modern top of the range DMM can have an accuracy as high as 0.5%.

Input Impedance
The input impedance of an analogue multimeter is measured in ohms per volt. This varies for different ranges and can have a shunting effect on the circuit being measured. A DMM input impedance is much higher and constant across the voltage ranges. For current measurements, both analogue and digital multimeters use a shunt resistor. This needs to be as low as possible so as not to affect the current measurement.

Calibration
An analogue meter has a screw adjustment that needs to be turned to make sure the pointer starts at zero. This has to be done in accordance with the user manual or if the meter has been knocked or subjected to shock.

Varying Signals
If you have a voltage or current or resistance that continually changes, then a DMM will vary its reading on the display. Although the ADC on a digital multimeter is capable of very fast readings, your eyes could not read the display at this speed so the readings are slowed to a few readings per second. An analogue meter is much faster and easily capable of reading signals at 50 times per second. However due to inertia of the meter needle the readings will not be of great accuracy. If signals change faster than this, than an oscilloscope is required.



Summary (Which is Best?)
It probably goes without saying that if you need an accurate versatile measuring instrument then a Digital Multi-Meter is the best choice. However don't overlook the analogue meter. Many currents and voltages change slowly but not so fast that an oscilloscope is required. These are ideal signals for the analogue meter.

Further Reading
Multimeter Tutorials External link to Electronics and Radio site.