Secondary instruments work in two modes:

(i) Analog mode, and (ii) Digital mode.

Signals that vary in a continuous fashion and take on an infinite number of values in any given range are called analog signals. The devices which produce these signals are called analog devices.

In contrast, the signals which vary in discrete steps and thus take up only finite different values in a given range are called digital signals The devices that produce such signals are called digital devices.

Let us elaborate further on digital and analog instruments and systems. In an analog system the function varies continuously A typical example of variation is shown in Fig. On the other hand, the digital values are discrete and vary in equal steps. Each digital number is a fixed sum of equal steps which is defined by the number.

In order to convert an analog quantify into a digital number, the vertical displacement must be divided into equal parts. For example, in Fig, the vertical quantities are divided into 10 equal parts and each part has a length of 1 unit When dealing with digital numbers, a quantity between 0 to 0.5 is 0 while a quantity between 0.5 to 1.5 is 1 and a quantity between 1.5 to 2.5 is 2.

For example, a point A on the analogue curve is 5.5 from the origin but in digital system it would be read as 5 . From A to B is 6 and from B to C is 7 It apparently seems that if we adopt digital system, the errors involved will be considerable. But if we divide each of the 10 steps into 10 equal parts, we get 100 steps instead of 10.

And if these 100 steps are further divided into 10 parts each, we will have 1000 steps. This gives much better accuracy in converting analog quantities into digital numbers. We can go on subdividing further and further till the desired accuracy is achieved. But it should be kept in mind that a digital number is still a sum of equal units.

In a digital system, magnitude lying within one of these steps lose their identity and are all defined by the same number. For example, if we have ten steps, numbers lying between 2.5 to 3.5 i.e., 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, would all be read as 3.

From the above discussion we conclude that the difference between analog and digital information is that the analog output is a continuous function while the digital output is a discrete number of units. The last digit of any digital number is rounded to ±0.5 of the last digit. It should also be marked that the magnitude of the digital quantity is measured only at the instant the reading is taken. One reading persists till another reading is taken (unlike the analog quantity which is a continuous function).

The majority of present day instruments are analog type. The importance of digital instruments is increasing, mainly because of the increasing use of digital computers in both data processing and automatic control systems since digital computer works only with digital signals, any information supplied to it must be in digital form. The computer’s output is also in digital form Thus working with a digital computer at either the input or the output, we must use digital signals.

However, most of our present day measurement and control apparatus produce signals of analog nature, it is thus necessary to have both Analog to Digital (A/D) Converters at the in put to the computer and Digital to Analog (D/A) Converters at the output of the computer.

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