Digital vs. Analog Meters: How Do They Compare?

Digital meters have revolutionized the field of electrical measurements, offering enhanced accuracy and convenience compared to their analog counterparts. 

 

Image used courtesy of Adobe Stock

 

With the advancement of technology, digital voltmeters, digital multimeters, and others have become widely used for accuracy, resolution, convenience, and versatility. These meters provide precise and accurate readings, thanks to digital readout displays.

With an analog meter, the accuracy of the readout depends on the user's ability to interpret the pointer's position on the meter scale. With larger meter scales, accuracy can be higher than with smaller scales. A mirror scale is provided on more expensive analog meter movements to help align the eye vertically with the scale, reducing the possibility of parallax error.

A digital meter offers enhanced resolution eliminates any uncertainty regarding the correctness of reading, and provides excellent accuracy. An analog meter will respond more quickly to a change in values.

Digital meters can be read accurately by untrained people, avoiding confusion with the analog multimeter's multiple scales.

A digital multimeter is similar to an analog multimeter. The major difference is that the meter movement is replaced with a digital converter. This electronic circuit replaces the meter function and drives the components which, when illuminated, make up the digits.

 

Digital Voltmeter

The basic digital meter always responds to a voltage. A typical range is 0-200 mV. To use it as a multimeter, the appropriate multipliers and shunts are connected around the converter and the digital readout.

Figure 1 shows the block diagram from the circuit for an elementary digital voltmeter. The probes are placed across a voltage, and multiplying resistors reduce the voltage according to the values selected by the range switch.

 

Figure 1. Digital voltmeter circuit principle. Image used courtesy of Amna Ahmad

 

The output is connected to a converter, which processes the voltage value and sends output signals to the appropriate bars of the display.

 

Readout Display

Each numeral in a display group comprises a maximum of seven bars. The full scale is usually specified as 199 or 1999 and increases in multiples of 10 (decades) as the number of digits increases. A 199 display can also be described as a 2½ digit display. Similarly, the 1999 display is sometimes called a 3½ digit display.

Originally, the required range had to be selected, and the decimal point shifted along the display to match. Figure 2 illustrates the circuit in a meter of this type. Modern digital displays are auto-ranging, and the decimal point shifts along to suit the input. The electronic circuits are, accordingly, more complex.

 

Figure 2. Digital multimeter circuit principle. Image used courtesy of Amna Ahmad

 

The only adjustment is to select the resistance, current or voltage function, and the rest is done automatically. Even the polarity of DC voltages and currents is catered for with suitable indicators.

Digit sizes can range from 5 mm to 12 mm or larger, depending on the manufacturer and the targeted use of the meter. The display size can be important, for example, if a reading has to be made from a distance.

Display types can range from light-emitting diodes to liquid crystal displays. Vacuum and gaseous displays are also popular. Gaseous displays are excellent for reading under very weak illumination, while a liquid crystal display (LCD) is almost useless without a supplementary light source.

A gaseous discharge display is hard to read in bright sunlight, while a liquid crystal display can be easily read. The LCD is, however, more temperature dependent, and if the display's temperature gets too high, the whole screen goes either blank or black and cannot be read. Permanent damage can occur under these conditions.

 

Digital Multimeter

Digital multimeter circuits of the modern type are far more complicated than analog meter circuits, so only a basic circuit has been shown in Figure 2. With this circuit, the user must select the range and type of measurement. However, it does illustrate the principles behind the operation of such a meter.

An input attenuator and a function selection switch make up digital multimeter inputs. The attenuator is sometimes automated and combined with the auto-range function. The converter and digital readout are not always sensitive enough for multimeter use, so an amplifier is often provided. It also serves the additional role of providing isolation between the converter and the attenuator, preventing possible loading of the circuit being tested.

 

Comparing Digital And Analog Meters

Here are the key comparisons of digital and analog meters.

• For normal operation, the digital instrument is more accurate. The more expensive the meter, the greater the accuracy.
• Both types need an internal battery source of power.
• Both types use a rectifier to convert AC to DC, but the analog meter has to use a separate scale for AC voltages. The digital meter has to have a correcting circuit to compensate for this.
• Ohmmeter functions in an analog meter use a non-linear scale. The digital meter has no scale, and its non-linear tendency has to be corrected with a special constant-current circuit.
• The input impedance of a digital meter is far higher than an analog-type circuit (analog meter). It may be 20 kΩ/V while, for the digital meter, it may be 20 MΩ/V, meaning less interference or effect on the tested circuit.
• A digital multimeter is subject to large errors in the presence of a radio-frequency field. Normally this has minimal effect on the analog meter.
• The digital meter is far more sensitive to circuit conditions than the analog meter. In some circumstances, this can lead to misleading readings.
• Analog meters' on-resistance readings can have negative polarities (from the internal battery) on the positive probe. This has to be checked before use because of possible directional errors in current-sensitive devices such as diodes.
• Digital meters have constant polarities on the resistance ranges, causing no confusion.
• An analog meter is more responsive to changing values than a digital instrument. Because of its circuit configuration, the digital device takes an appreciably longer time to respond to the new value and settle again.

 

Digital vs. Analog Meter Summary

Digital reading meters, including digital voltmeters and digital multimeters, have revolutionized the field of electrical measurements. Firstly, these meters provide precise and accurate readings, ensuring reliable measurement results. With their digital readout display, users can easily read and interpret the measured values, enhancing convenience and reducing the chances of errors. Digital meters offer a wider range of functions and measurement capabilities than analog meters, making them versatile tools for various applications. Additionally, digital meters often incorporate advanced features like data logging, peak hold, and auto-ranging, providing enhanced functionality and flexibility.