Đang chuẩn bị nút TẢI XUỐNG, xin hãy chờ
Tải xuống
Tham khảo tài liệu 'microengineering mems and interfacing - danny banks part 16', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Computer Interfacing 283 Column 3 2 1 0 16n 4096 256 16 1 Four thousand and ninety sixes Two hundred and fifty sixes Sixteens Units I e.g. I I 8 I F I 8F converted to decimal is 8 X 16 128 F X 1 15 Total in decimal 143 8F converted to binary 8 F 1 X 8 0 X 4 0 X 2 0 X 1 1 X 8 1 X 4 1 X 2 1 X 1 1 1 0 1 0 L 0 1 1 1 1 1 L_ 1 8F in binary is 10001111 FIGURE 12.3 Hexadecimal place values 8F converted to decimal and binary. Note that one hexadecimal digit converts exactly to four binary digits. 12.2.1 Pulse-Width Modulation PWM One of the easiest ways to implement digital-to-analog conversion is the PWM approach because this only requires that the digital system produce two voltage levels. As such it can be relatively easily implemented although its use is restricted without additional analog-signal-conditioning circuitry. Consider the signal displayed in Figure 12.4a. This is a square wave signal which could quite easily be produced from a digital circuit. The period for which a Tmark Tspace b FIGURE 12.4 PWM signal frequency f period T 1 f amplitude Vmax average signal level Vav mark-space ratio Tmark Tspace a 1 2 b 2 1. Copyright 2006 Taylor Francis Group LLC 284 Microengineering MEMS and Interfacing A Practical Guide the signal is high is referred to as the mark and the period for which it is low as the space. It is possible to calculate the average signal level from this av max mark T 12.1 It is normal to describe such signals in terms of mark-space ratio because the average signal level is independent of frequency i.e. T does not need to be known in order to find Vav . Equation 12.1 can be rewritten as Vv T __ V mark__ max T . T i mark i space 12.2 In Equation 12.2 Tmark and Tspace can be dimensionless. In Figure 12.4a the mark-space ratio T is 1 2 so the average signal level is 0.33Vmax. Compare this with Figure 12.p4b in which the mark-space ratio is reversed and Vav will be 0.66Vmax. By continuously varying the mark-space ratio the average signal .
