Study of Digital to Analog Converter

• Basic 4-bit weighted resistance

• 4-bit R-2R network module

• 10-bit IC Type AD7533 with mechanical switches

• 10-bit IC Type AD7533 with mechanical switches

Introduction  

Digital systems have become extremely common in almost all facets of modern life, be it consumer electronics, entertainment electronics, photography, medical electronics, industries, communication or any number of simple and complex systems. One of the major reasons for this development is the ease with which digital signals can be processed, stored, transmitted and converted to the form suitable for our use. This situation has been helped by the advancement in technology which has resulted in a vast reduction in cost of the devices and an enormous increase in their capabilities. An application which was not technically or economically feasible say five to ten years back is now a household item. An example is a digital camera. There are now more and more complex and sophisticated systems which are becoming commonplace everyday. It is therefore imperative for all students of science and engineering to familiarize themselves with at least the basic processes involved in a digital system. The present unit is designed with the aim to study the techniques of conversion of digital signals into analog form which is an integral part of any digital system.

The digital-to-analog converter (D/A converter), as the name suggests, is a block placed towards the end of a digital system which typically converts the digital signals obtained after all processing into an analog form suitable for our comprehension. As a simple example, the digital music recorded on a CD must be decoded by some processor and converted to analog form before it is amplified and fed to a speaker to generate the sound signals that we can hear.

There are two basic techniques of D/A conversion, viz., weighted resistance method and R-2R network method. While the first is of theoretical interest only, it is the second method which finds widespread application. In the present experimental unit both these methods are explored using discrete component based 4-bit circuits and also an IC based 10-bit system. These enable the user to study the basic performance of the converter circuits. In addition, an 8085 microprocessor and interfacing circuit is provided for the student to perform some advanced experiments to gain a better insight into realistic application of these circuits. These include generation of arbitrary waveforms of different frequencies, producing a voltage-time profile and simple signal processing experiments. A knowledge of assembly language programming is a pre-requisite for this section.

The total unit is supplied with a DVM on the panel for all dc measurements, the necessary power supplies and reference voltage sources for the complete unit, microprocessor unit with sample programmes and an exhaustive user manual. No additional equipment, except a general purpose CRO is required for conducting the experiments. The CRO is needed to view the waveforms generated by the microprocessor program.

Experiments  

• Study of the circuit diagram and performance of a 4-bit weighted resistance type D/A converter. The digital inputs are to be given by operating four mechanical switches and the output read on the digital panel meter provided.

• The above experiment is conducted on a 4-bit discrete component R-2R network based unit.

• Manual operation of a 10-bit IC D/A converter type AD7533 through 10 mechanical switches. Due to a much better resistance matching in the integrated circuit, the performance is seen to be far superior compared to the discrete 4-bit circuits.

• Operation of the IC based circuit through the microprocessor kit provided. Some typical waveform generation exercises are suggested and solutions are provided. More problems can be attempted by the student with help from his supervisor.

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