Download ADC in Analog Electronics and more Study notes Analog Electronics in PDF only on Docsity! What is ADC An electronic integrated circuit which transforms a signal from analog (continuous) to digital (discrete) form. Analog signals are directly measurable quantities. Digital signals only have two states. For digital computer, we refer to binary states, 0 and 1. ADC process uI(t) C Quantizing & Encoding Dn-1 D1D0 … uI′(t)S S/H circuit Input analog signal Output digital signal 2 steps Sampling and Holding (S/H) Quantizing and Encoding (Q/E) Accuracy of A/D Conversion There are two ways to best improve the accuracy of A/D conversion: increasing the resolution which improves the accuracy in measuring the amplitude of the analog signal. increasing the sampling rate which increases the maximum frequency that can be measured. Accuracy of A/D Conversion Low Accuracy Time R es ol ut io n Time R es ol ut io n Improved Types of A/D Converters By: Todd Sifleet Dual Slope A/D Converter Successive Approximation A/D Converter Flash A/D Converter Delta-Sigma A/D Converter Other Voltage-to-frequency, staircase ramp or single slope, charge balancing or redistribution, switched capacitor, tracking, and synchro or resolver How Does it Work Cont. At t<0, S1 is set to ground, S2 is closed, and counter=0. At t=0 a conversion begins and S2 is open, and S1 is set so the input to the integrator is Vin. S1 is held for TINT which is a constant predetermined time interval. When S1 is set the counter begins to count clock pulses, the counter resets to zero after TINT Vout of integrator at t=TINT is VINTINT/RC is linearly proportional to VIN At t=TINT S1 is set so -Vref is the input to the integrator which has the voltage VINTINT/RC stored in it. The integrator voltage then drops linearly with a slop -Vref/RC. A compartor is used to determine when the output voltage of the integrator crosses zero When it is zero the digitized output value is the state of the counter. Dual Slope A/D Converter Pros and Cons PROS Conversion result is insensitive to errors in the component values. Fewer adverse affects from “noise” High Accuracy CONS Slow Accuracy is dependent on the use of precision external components Cost Flash A/D Converter Fundamental Components (For N bit Flash A/D) 2N-1 Comparators 2N Resistors Control Logic ADC Resolution Comparison 0 5 10 15 20 25 Sigma-Delta Successive Approx Flash Dual Slope Resolution (Bits) Type Speed (relative) Cost (relative) Dual Slope Slow Med Flash Very Fast High Successive Appox Medium – Fast Low Sigma-Delta Slow Low ADC Types Comparison Successive Approximation ADC Circuit •Uses a n-bit DAC to compare DAC and original analog results. •Uses Successive Approximation Register (SAR) supplies an approximate digital code to DAC of Vin. •Comparison changes digital output to bring it closer to the input value. •Uses Closed-Loop Feedback Conversion Successive Approximation ADC Process 1. MSB initialized as 1 2. Convert digital value to analog using DAC 3. Compares guess to analog input 4. Is Vin>VDAC • Set bit 1 • If no, bit is 0 and test next bit Closed-Loop SAR DAC Output VIN - + 1000 0000 Is Vin > ½ ADC range? 0 0 If no, then test next bit Vref Successive Approximation Example Example 10 bit ADC Vin= 0.6 volts (from analog device) Vref=1 volts Find the digital value of Vin N=2n (N of possible states) N=1024 Vmax-Vmin/N = 1 Volt/1024 = 0.0009765625V of Vref (resolution) Successive Approximation MSB (bit 9) Divided Vref by 2 Compare Vref /2 with Vin If Vin is greater than Vref /2 , turn MSB on (1) If Vin is less than Vref /2 , turn MSB off (0) Vin =0.6V and V=0.5 Since Vin>V, MSB = 1 (on) Successive Approximation Next Calculate MSB-1 (bit 8) Compare Vin=0.6 V to V=Vref/2 + Vref/4= 0.5+0.25 =0.75V Since 0.6<0.75, MSB is turned off Calculate MSB-2 (bit 7) Go back to the last voltage that caused it to be turned on (Bit 9) and add it to Vref/8, and compare with Vin Compare Vin with (0.5+Vref/8)=0.625 Since 0.6<0.625, MSB is turned off
