Voltage to frequency converters

Abstract

One definition of a digital instrument is 'One in which the indicated or recorded value is expressed directly in terms of the decimal number system’. Until very recently there would have been little objection to such a definition; however instruments are now appearing on the market which present their outputs in analogue form although they rely on the use of digital techniques to perform their function. Indeed within the next few years it will become increasingly difficult to divide instruments into rigid analogue or digital classes, as designers are now beginning to combine analogue and digital techniques within one instrument to great effect.

The first laboratory instruments to be recognised as digital appeared in the early 1950's. Event counting came first, followed by frequency measurement and digital voltage measurement appeared soon afterwards. These instruments, for the first time provided an automatic indication by means of a digital display of the analogue quantity being measured.

This analogue quantity could have been voltage, current, frequency, phase shift, pulse width or any other form of signal. The voltage etc. could stand for temperature, pressure, velocity, distance, shaft position, shock, vibration or any of the other physical phenomena which can be changed to a voltage form by a suitable transducer. Indeed it is modern measurement practice to develop an electrical signal analogous to the phenomena of interest at the earliest opportunity so that subsequent processing can be carried out electronically.

The principal advantages of analogue systems is initial cost, flexibility, simplicity and familiarity. The disadvantage of these systems is limited accuracy. It is difficult to read a dial type indicator closer than + 1% which does not include the inaccuracy of the mechanism itself.

Digital readouts are inherently more accurate because the human reading error can be eliminated. The readouts can be made with virtually any number of significant digits. The limitations of digital techniques are chiefly lack of flexibility and cost. Analogue devices can be rescaled by simply adjusting a potentiometer; digital units require changing a module. The cost element is constantly changing and with further development of low cost integrated circuits and increased mass production may eventually become less than that of analogue systems.

Because of these advantages, digital systems are becoming more popular with designers and are being used in practically all branches of electrical engineering, from measurements to communications and computers. However an analogue form is often needed somewhere in the system and therefore the ability to change from one system to another is of great importance. To utilise the best features of analogue or digital systems, it is frequently necessary to convert from one system to the other. The resultant hybrid exhibits some advantages in price, accuracy, or size that is unattainable with a conventional approach. The two possibilities are Analogue-to-digital and Digital-to-analogue conversions. In the following chapter, the main types of Analogue-to- Digital converters are discussed.