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lecture_note11 - CpE 390: Microprocessor Systems Lecture 11...

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Stevens Institute of Technology 1 CpE 390: Microprocessor Systems Lecture 11 68HC12 Analog to Digital Converter (1)
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Stevens Institute of Technology 2 A/D Converter Transducer temperature pressure light weight airflow humidity . . . Such as a sensor, load cell, photocall, or thermocouple . . signal conditioning circuit (optional) voltage A/D converter Computer Digital value Figure 10.1 The A/D conversion process - A transducer: convert a non-electrical quantity into an electrical voltage -A voltage shifter and scaler: transform the transducer output voltage into a range that handled by the A/D converter. - An analog to digital (A/D) converter can convert a electrical voltage to a digital value.
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Stevens Institute of Technology 3 68HC12 A/D conversion method: Successive Approximation Method V in (analog input) Digital-to-analog converter Output Latch Digital code Successive approximation register (SAR) Control Logic Clock Figure 10.2 Block diagram of a successive approximation A/D converter + - analog comparator V RH V RL
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Stevens Institute of Technology 4 Algorithm of Successive Approximation Method Start SAR[n-1, . .., 0] 0 i n - 1 SAR[i] 1 Convert the value in SAR to a voltage Is the Converted voltage greater than the input? 0 yes no i = 0? i - 1 Sop Figure 10.3 Successive approximation A/D conversion method - Initialize the SAR register to 0. - Starting from the most significant bit of SAR and work toward the least significant bit, for each bit 1. Guess the bit to be a 1. 2. Converts the value of the SAR to an analog voltage 3. Compares the D/A output with the analog input. 4.Clears the bit to 0 if the D/A output is larger.
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Stevens Institute of Technology 5 Signal Conditioning Circuits Optimal Voltage Range for the A/D Converter - A/D converter needs a low reference voltage VLREF (is often set to 0) and a high reference voltage VHREF (is often set to VCC ) to operate. - The A/D conversion result k corresponds to an analog voltage given by Vx = V LREF + (range × k) ÷ (2 n –1) where, n is the number of bits the A/D converter uses to represent a conversion result, n is also called the resolution of the A/D converter, range = V HREF –V HREF,
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Stevens Institute of Technology 6 Example 10.1 Suppose there is a 12-bit A/D converter with V LREF = 0.5V and V HREF = 3.5V. Find the corresponding voltage values for A/D conversion results of 20, 100, 800, 1200, 2400, and 3600. Solution: range = V HREF –V LREF = 3 V. V(20) = 0.5V + (20 × 3) ÷ (2 12 – 1) = 0.515 V V(100) = 0.5V + (100 × 3) ÷ (2 12 – 1) = 0.573 V V(800) = 0.5V + (800 × 3) ÷ (2 12 – 1) = 1.086 V V(1200) = 0.5V + (1200 × 3) ÷ (2 12 – 1) = 1.379 V V(2400) = 0.5V + (2400 × 3) ÷ (2 12 – 1) = 2.258 V V(3600) = 0.5V + (3600 × 3) ÷ (2 12 – 1) = 3.137 V
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Stevens Institute of Technology 7 Voltage Scaling and Shifting Circuit - Some transducers have output in the range of V 1 ~V 2 . -V 1 may be negative and V 2 may be different from the power supply.
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lecture_note11 - CpE 390: Microprocessor Systems Lecture 11...

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