Comparators, DAC, and ADC by Dr. Amin Danial Asham

References Introduction to Instrumentation, Sensors, and Process Control-William C. Dunn Data Sheet of TI-DAC0808 Data Sheet of TI ADC0808

Comparator A comparator is basically a very high gain differential amplifier that compares two analog signals 𝑉 𝑎 and 𝑉 𝑏 and based on is greater 𝑉 𝑜𝑢𝑡 is either 1 or 0. If 𝑉 𝑏 > 𝑉 𝑎 then 𝑉 𝑜𝑢𝑡 = V cc (supply) Logic 1 If 𝑉 𝑎 > 𝑉 𝑏 then 𝑉 𝑜𝑢𝑡 =0 Logic 0

Digital to Analog Converter (DAC) DAC is a device that gets a binary coded value and generate a corresponding analog voltage. The n-bits DAC can generate 2 𝑛 different levels of analog signal 𝑉 0 from 0 volt up to 𝑉𝑚𝑎𝑥 (full scale volt of DAC). That means DAC can generate 2 𝑛 −1 nonzero levels. 𝐷 0 𝐷 1 𝐷 2 𝐷 𝑛 𝑉 𝑜 DAC 𝑉 𝑟𝑒𝑓

Digital to Analog Converter (DAC) (continue) The output of voltage of DAC is 𝑉 𝑜 = 𝑉 𝑟𝑒𝑓 ( 2 𝑛−1 + 2 𝑛−2 +…+ 2 0 )/ 2 𝑛 Therefore for the maximum output voltage 𝑉 𝑚𝑎𝑥 is 𝑉 𝑚𝑎𝑥 = 𝑉 𝑟𝑒𝑓 2 𝑛 −1 2 𝑛 Resolution: resolution of DAC usually refers to the number of bits. The voltage corresponding to the LSB is called 1 LSB 1 𝐿𝑆𝐵= 𝑉𝑟𝑒𝑓 2 𝑛 volt

Digital to Analog Converter (DAC) (continue) 𝐷 0 𝐷 1 𝐷 2 𝑉 𝑟𝑒𝑓 3-bits DAC

Digital to Analog Converter (DAC) (continue) Example: for a 8 bit DAC with a full scale 4.9805 volts. Find 𝑉 𝑟𝑒𝑓 and 1LSB Solution: S𝑖𝑛𝑐𝑒 𝑉 𝑚𝑎𝑥 = 𝑉 𝑟𝑒𝑓 2 8 −1 2 8 ∴ 𝑉 𝑟𝑒𝑓 = 𝑉 𝑚𝑎𝑥 2 8 2 8 −1 =5 volts 1𝐿𝑆𝐵= 𝑉 𝑟𝑒𝑓 2 𝑛 = 𝑉 𝑚𝑎𝑥 2 𝑛 −1 =0.01953

Analog to Digital Converter (ADC) Analog to digital converter converts an analog signal to a binary value. 𝑉 𝑖𝑛 is the input analog signal 𝐷 𝑛 𝐷 𝑛−1 … 𝐷 0 is the output binary value corresponding to the input analog signal 𝑉 𝑖𝑛 , where 𝐷 𝑛 is MSB and 𝐷 𝑜 is the LSB. Resolution refers to the number of bits of the output binary value of the ADC. 𝐷 0 𝐷 1 𝐷 2 𝐷 𝑛 𝑉 𝑖𝑛 ADC 𝑉 𝑟𝑒𝑓

Analog to Digital Converter (ADC) (continue) ADC with a resolution of n bits divides the full scale of the input analog signal into 2 𝑛 levels. 1 LSB or Quantization level (Q) is the voltage change of the in signal the causes the output of the converter change from one level to another. 1𝐿𝑆𝐵= 𝑉 𝑟𝑒𝑓 2 𝑛 The output code represents an integer equals to: 𝑉 𝑖𝑛 𝑉 𝑟𝑒𝑓 × 2 𝑛

Analog to Digital Converter (ADC) (continue) Flash converter: As shown the analog input signal is compare with different levels of voltages obtained by dividing 𝑉 𝑟𝑒𝑓 . When the input voltage exceeds certain level the corresponding comparator gives an output 1. The output of the comparators converted to binary value using an encoder. Flash converters are the fastest technique for converting analog voltages into digital signals, but expensive. 𝑉 𝑟𝑒𝑓

Analog to Digital Converter (ADC) (continue) Successive approximation is a parallel feedback ADC that feeds back a voltage from a DAC, as shown. A comparator compares the analog input sign the analog signal generated from the current binary value via DAC . If the generated signal is not matching the input a new value generated by the counter and then compared again and so no. The comparator is controlling the counter operation. Conversion is started by start signal and gives a feedback after completion

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