ECE 03 – Electronics 3: Electronics System & Design Final Exam Dec 2024 PDF

ECE 03 – ELECTRONICS 3: ELECTRONICS SYSTEM & DESIGN BSECE III ENGR. JOEL ANTHONY L. SEVILLA DECEMBER 2024 INTERFACING TECHNIQUES An interface circuit is a signal conditioning circuit used to bring signal from the sensor up to the format that is compatible with th...

ECE 03 – ELECTRONICS 3: ELECTRONICS SYSTEM & DESIGN BSECE III ENGR. JOEL ANTHONY L. SEVILLA DECEMBER 2024 INTERFACING TECHNIQUES An interface circuit is a signal conditioning circuit used to bring signal from the sensor up to the format that is compatible with the load device. ADC and DAC interface Transducer A device that converts the physical variable to an electrical variable. The electrical output of the transducer is an analog current or voltage that is proportional to the physical variable it is monitoring. Some common transducers: thermistors, photocells, photo diodes, flow meters, pressure transducers, and tachometers. Analog to Digital Converter (ADC) The transducer’s electrical analog output serves as the analog input to the ADC. The ADC converts this analog input to a digital output. This digital output consists of a number of bits that represent the value of the analog input. Digital System The digital representation of the process variable is transmitted from the ADC to the digital computer, which stores the digital value and processes it according to a program of instructions that it is executing. The program might perform calculations or other operations on this digital representation. Digital to Analog Converter (DAC) This digital output from the computer is connected to a DAC, which converts it to a proportional analog voltage or current. Actuator The analog signal from the DAC is often connected to some device or circuit that serves as an actuator to control the physical variable. A device that causes a machine or other device to operate. ACTUATORS ADC and DAC interface MICROCONTROLLER INTERFACES Microcontrollers are useful to the extent that they communicate with other devices, such as sensors, motors, switches, keypads, displays, memory and even other microcontrollers. Many microcontroller designs typically mix multiple interfacing methods. In a very simplistic form, a microcontroller system can be viewed as a system that reads from (monitors) inputs, performs processing and writes to (controls) outputs. INTERFACE CIRCUIT Amplifiers: Most passive sensors produce weak output signals with magnitudes on the order of microvolts (μV) or picoamperes (pA). Therefore, an amplification of the sensor output signals has to be made with a voltage gain up to 10,000 and a current gain up to 1 million. The amplifiers are composed of standard building blocks, such as operational amplifiers and various discrete components. Operational Amplifiers: By using Op Amps and discrete components (resistors, capacitors, inductors, etc.), you may create an infinite number of useful circuits, such as; amplifiers, summers, integrators, differentiators. Operational amplifiers usually have limited frequency bandwidths. There are programmable operational amplifiers, which allow the user to control the bias current and, therefore, the first stage frequency response. The higher the current, the faster would be the response. Analog Inputs/Outputs Voltage-based control and monitoring. Advantages Simple interface Low cost for low-resolutions High speed Low programming overhead Disadvantages High cost for higher resolutions Not all microcontrollers have analog inputs/outputs built-in Complicates the circuit design when external ADC or DAC are needed. Short distance, few feet maximum. Digital to Analog Conversion Ladder Network Conversion A ladder network accepts inputs of binary values at, typically, 0 V or Vref and provides an output voltage proportional to the binary input value. Ladder Network Conversion A ladder network with four input voltages, representing 4 bits of digital data and a dc voltage output. The output voltage is proportional to the digital input value as given by The function of the ladder network is to convert the 16 possible binary values from 0000 to 1111 into one of 16 voltage levels in steps of Vref/16. Using more sections of ladder allows having more binary inputs and greater quantization for each step. For example, a 10- stage ladder network could extend the number of voltage steps or the voltage resolution to Vref/2¹⁰ or Vref/1024. A reference voltage of Vref = 10 V would then provide output voltage steps of 10 V/1024 or approximately 10 mV. More ladder stages provide greater voltage resolution. In general, the voltage resolution for n ladder stages is The digital inputs D0, D1, D2, and D3 are usually derived from the output register of a digital system. The 2⁴ = 16 different binary numbers represented by these 4 bits for each input number, the D/A converter output voltage is a unique value. In this case, the analog output voltage Vo is equal in volts to the binary number. In general, Analog output = K × digital input where K is the proportionality factor and it is constant value for a given DAC. The analog output can be a voltage or current. When it is a voltage, K will be in voltage units, and when the output is current, K will be in current units. For the DAC of K = 1 V, so that Vo = (1 V) × digital input We can use this to calculate Vo for any value of digital input. Example, with a digital input of 11002 = 1210, we obtain Vo = 1V × 12 = 12V Example 1. For a 5-bit DAC digital input of 10100, an output current of 10mA is produced. What will Io be for a digital input of 11101? Solution 1. Convert 101002 to 2010 & 111012 to 2910 find K and Io Analog output = K × digital input 10mA = K x 20 K = 0.5 mA Io = 0.5 mA × 29 Io = 14.5 mA Example 2. What is the largest value of output voltage from an 8-bit DAC that produces 1.0V for a digital input of 00110010? Solution 2. Convert 001100102 to 5010 find K and Vo Analog output = K × digital input 1 V = K x 50 K = 20 mV The largest output will be 111111112 = 25510 Vo (max) = 20 mV x 255 Vo (max) = 5.1 V DAC CIRCUITRY Basic circuit for four-bit DAC employing an op amp as a summing amplifier. Analog to Digital Conversion An analog to digital converter takes an analog input voltage and, after a certain amount of time, produces a digital output code that represents the analog input. The A/D conversion process is generally more complex and time consuming than the D/A process, and many different methods have been developed and used. Analog to Digital Conversion Several important types of ADCs utilize a DAC as part of their circuitry. The timing for the operation is provided by the input clock signal. The control unit contains the logic circuitry for generating the proper sequence of operations in response to the start command, which initiates the conversion process. Analog to Digital Conversion The op-amp comparator has two analog inputs and a digital output that switches states, depending on which analog input is greater. The basic operation of ADCs of this type consists of the following steps: 1. The start command pulse initiates the operation. 2. At a rate determined by the clock, the control unit continually modifies the binary number that is stored in the register. 3. The binary number in the register is converted to an analog voltage, VAX, by the DAC. 4. The comparator compares VAX with the analog input VA. As long as the comparator output stays HIGH. When VAX exceeds VA by at least an amount equal to VT (threshold voltage), the comparator output goes LOW and stops the process of modifying the register number. At this point, VAX is a close approximation to VA. The digital number in the register, which is the digital equivalent of  VAX, is also the approximate digital equivalent of VA, within the resolution and accuracy of the system. 5. The control logic activates the end of conversion signal, EOC, when the conversion is complete. Digital-Ramp ADC It is one of the simplest versions of the general ADC using a binary counter as the register and allows the clock to increment the counter one step at a time until VAX ≥ VA. It is called a digital- ramp ADC because the wave form at VAX is a step-by-step ramp or a staircase. It is also referred to as a counter-type ADC. Digital ramp ADC Digital-Ramp ADC It contains a counter, a DAC, an analog comparator, and a control AND gate. The comparator output serves as the active-LOW end-of-conversion signal, 𝐸𝑂𝐶. If we assume that VA, the analog voltage to be converted, is positive, the operation proceeds as follows: 1. A START pulse is applied to reset the counter to zero. The HIGH at START also inhibits clock pulse form passing through the AND gate into the counter. 2. With all 0’s at its input, the DAC’s output will be VAX = 0V. 3. Since VA > VAX, the comparator output, 𝐸𝑂𝐶, will be HIGH. 4. When START returns LOW, the AND gate is enabled and clock pulses get through to the counter. 5. As the counter advances, the DAC output, VAX, increases one step at a time. 6. This continues unit VAX reaches a step that exceeds VA by an amount equal to or greater than VT (typically 10 to 100 μV). At this point,  𝐸𝑂𝐶 will go LOW and inhibit the flow of pulses into the counter and the counter will stop counting. 7. The conversion process is now complete as signaled by the HIGH-to-LOW transition at 𝐸𝑂𝐶, and the contents of the counter are the digital representation of VA. 8. The counter will hold the digital value until the next START pulse initiates a new conversion. Assume the following values for the digital ramp ADC of : clock frequency = 1 MHz; VT = 0.1 mV; DAC has full scale output = 10.23 V and a 10-bit input. Determine the following values: a) The digital equivalent obtained for VA=3.728 V b) The conversion time c) The resolution of this converter a) The DAC has a 10-bit input and a 10.23-V F.S. output. Thus, the number of total possible 𝑛 steps is 2 − 1 and so the step size is 10 # of steps = 2 − 1 = 1023 10.23 𝑉 step size = = 10 mV 1023 VAX increases in steps of 10 mV as the counter counts up from 0. a) Since VA is 3.728 V and VT is 0.1 mV, VAX must reach 3.7281 V or more before the comparator switches LOW. This will require 3.7281 𝑉 # of steps = 10 𝑚𝑉 = 372.81 = 373 𝑠𝑡𝑒𝑝𝑠 At the end of the conversion, the counter will hold the binary equivalent of 373, which is 0101110101, the desired digital equivalent of VA = 3.728 V, as produced by this ADC. b) Conversion time = # of steps x time Conversion time = (373)(1/1MHz) Conversion time = 373 μs c) Resolution = step size of the DAC in % 1 Resolution = x 100% 1023 Resolution = 0.1% Analog Digital Conversion Voltage to Frequency Flash ADC Successive Approximation Dual-Slope Integration Delta-Sigma Successive approximation ADC Successive Approximation ADC’s are popular for use with microcontrollers due to low-cost and ease of interfacing. A successive approximation ADC consists of: Successive Approximation Register Result Register DAC Comparator Successive approximation ADC Successive-approximation register counts by trying all values of bits starting with the most significant bit and finishing at the least- significant bit. Throughout the count process, the register monitors the comparator's output to see if the binary count is less than or greater than the analog signal input, adjusting the bit values accordingly. This way, the DAC output Successive approximation ADC eventually converges on the analog input signal and the result is presented in the Result register.