EE 308 Mar 6 2002 Analog Digital Converters An Analog to Digital A D converter converts an analog voltage into a digital number There are a wide variety of methods used for A D converters Examples are Flash Parallel Successive Approximation Sigma Delta Dual Slope Converter A D converters are classified according to several characteristics Resolution number of bits typically 8 bits to 24 bits Speed number of samples per second several samples sec to several billion samples sec Accuracy how much error there is in the conversion High resolution converters are usually slower than low resolution converters The HC12 has a 10 bit successive approximation A D converter which can be used in 8 bit mode The HC12 uses an analog multiplexer to allow eight input pins to connect to the A D converter 1 EE 308 Mar 6 2002 Comparator A comparator is used in many types of A D converters A comparator is the simplest interface from an analog signal to a digital signal A comparator compares two voltage values on its two inputs If the voltage on the input is greater than the voltage on the input the output will be a logic high If the voltage on the input is less than the voltage on the input the output will be a logic low V V V REF CC V IN If Vin Vref then Vout Vcc If Vin Vref then Vout 0 2 OUT EE 308 Mar 6 2002 Flash Parallel A D Converter A flash A D converter is the simplest to understand A flash A D converter compares an input voltage to a large number of reference voltages An n bit flash converter uses 2 1 comparators The output of the A D converter is determined by which of the two reference voltages the input signal is between Here is a 3 bit A D converter 5V Vin 4 375 V 3 750 V 3 125 V 2 500 V 1 875 V 1 250 V 0 625 V Dout 0 000 V 3 EE 308 A Mar 6 2002 Flash A D Converter bit Flash A D converter requires 2 1 comparators An 8 bit Flash A D requires 255 comparators A 12 bit Flash A D converter would require 4 095 comparators Cannot integrate 4 095 comparators onto an IC The largest flash A D converter is 8 bits Flash A D converters can sample at several billion samples sec 4 EE 308 Mar 6 2002 A D Converter Resolution and Quantization If the voltage input voltage is 3 2516 V the lowest 5 comparators will be turned on and the highest 2 comparators will be turned off The output of the 3 bit flash A D converter will be 5 101 For a 3 bit A D converter which has a range from 0 to 5 V an output of 5 indicates that the input voltage is between 3 125 V and 3 750 V A 3 bit A D converter with a 5 V input range has a quantization value of 0 625 V The quantization value of an A D converter can be found by where is the highest voltage the A D converter can handle is the lowest voltage the A D converter can handle and is the number of bits of the A D converter The HC12 has a 10 bit A D converter The typical voltage range used for the HC12 A D is 5 V and 0 V so the HC12 has a quantization value of The dynamic range of an A D converter is given in decibels dB 0 2143 5 7 6 8 9 A 10 bit A D converter has a dynamic range of BA C D 5 EE 308 Mar 6 2002 A D Sampling Rate The rate at which you sample a signal depends on how rapidly the signal is changing If you sample a signal too slowly the information about the signal may be inaccurate A 1050 Hz signal sampled at 500 Hz 1 0 5 0 0 5 1 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 1 0 5 0 0 5 1 1 0 5 0 0 5 1 t ms A 1 050 Hz signal sampled at 500 Hz looks like a 50 Hz signal To get full information about a signal you must sample more than twice the highest frequency in the signal Practical systems typically use a sampling rate of at least four times the highest frequency in the signal 6 EE 308 Mar 6 2002 Digital to Analog D A Converters Many A D converters use a D A converter internally A D A converter converts a digital signal to an analog voltage or current To understand how most A D converters work it is necessary to understand D A converters The heart of a D A converter is an inverting op amp circuit The output voltage of an inverting op amp circuit is proportional to the input voltage RF R V 0 R R0 Vout 7 R F 0 V R0 8 V V V V R3 R2 R1 R0 E R R R R 3 2 1 0 RF Vout R R 0 F V R0 R R 1 F V R1 R R 2 F V R2 R R 3 F V R3 An inverting op amp can produce an output voltage which is a linear combination of several input voltages Digital to Analog D A Converters EE 308 Mar 6 2002 9 V V V V Ref Ref Ref Ref E R R R R 0 0 0 0 8 4 2 RF Vout R R R R R R 0 F 0 F 0 F Ref Ref V Ref V V F Ref 4 V V 0 F 8V R 4 R Ref 1 2 4 8 Ref 0 2V R 2 R Ref Ref V R 8 R 0 F V Ref By using input resistors which scale by factors of 2 a summing op amp can produce an output which follows a binary pattern Digital to Analog D A Converters EE 308 Mar 6 2002 V Ref E 10 B B B B 3 2 1 0 R R R R 0 0 0 0 8 4 2 RF 4 Bit Digital to Analog Converter B R 0 F 0 F 3 R R R B Vout B 2 V V B 1 Ref Ref B 0 B B 0 2B 1 4 B 2 8B 3 By using switches on the input resistors a summing op amp can produce an output which is a binary number representing which switches are closed times a reference voltage Digital to Analog D A Converters EE 308 Mar 6 2002 EE 308 Mar 6 2002 Slope A D Converter A simple A D converter can be constructed with a counter and a D A converter The counter counts from 0 to 2 1 The counter drives the input of the D …
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