854 Analogue To Digital Conversion

In previous chapters there have been many references to interfaces and analogue to digital conversion. A/D conversion is necessary because many sensor signals are of analogue (varying voltage) form. In order for the control computer to function these analogue signals must be converted to binary codes (digital signals). It is therefore appropriate to consider the basics of the design of an A/D converter that could be used at an ECM interface. Conversion from an analogue voltage to a digital code (word) can be done in a number of ways. Figure 8.13 shows one type of A/D converter that is known as a 'flash' converter.

V,n input voltage analogue Fig. 8.13 Flash-type analogue to digital converter

The flash converter consists of four comparators and an encoder circuit which takes the comparator outputs and converts them into a binary code. An electronic comparator is a circuit which continuously compares two signals. One of the inputs, at each comparator, is a reference voltage. When the input voltage matches the reference voltage the comparator outputs a logic 1. The reference voltages shown here are 1 V up to 4 V. Table 8.1 shows the input/output performance of the converter.

The encoder contains logic devices (gates etc.) and this enables it to output the binary codes for 1 to 4. These binary codes (0s and 1s) are used by the ECM

Table 8.1 Performance of the flash converter

A/D converter Comparator Encoder input voltage outputs output range A B C D

Table 8.1 Performance of the flash converter

A/D converter Comparator Encoder input voltage outputs output range A B C D

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

Get My Free Ebook


Post a comment