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Analog to Digital Converters (ADCs) & Digital to Analog Converters (DACs)

Analog to Digital Converters (ADCs) & Digital to Analog Converters (DACs)

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## Analog to Digital Converters (ADCs) & Digital to Analog Converters (DACs)

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**Analog to Digital Converters(ADCs)& Digital to Analog**Converters (DACs)**Analog to Digital Conversion (ADC)**• Analog to Digital Conversion is the process by which analog signals are converted to their digitized forms. • ADC occurs via three steps: - Sampling. - Quantization. - Encoding.**The idea of Sampling**• The analog signal is sampled at regular intervals of time. • The sampling rate must be at least twice the highest frequency of the signal ( Nyquist Criterion).**Quantization**• Quantization is defined as the process of converting the continuous sample amplitude into a discrete amplitude. Thus by then, the signal will be discrete in both; time and amplitude.**Quantization Error**• The difference between the actual analog value and quantized digital value.**Encoding**• Encoding is the process which assigns ones and zeros (stream of bits) for every quantization level. • The number of bits assigned for each level (n) depends on the levels’ number (L) ; such that L= 2^n.**Ideal ADC**Q= Δ=quantization step = full scale / levels’ number. In an ideal analog-to-digital converter, the quantization error is uniformly distributed between –Δ/2 and Δ/2. The resolution of the ADC is the smallest detectable change in voltage.**Types Of ADCs**• There are many types of ADCs such as: • Direct Conversion ADC. • Successive Approximation Register (SAR) ADC. • Integrating ADCs: Single slope, dual slope, and ramp ADC. • Sigma-Delta ADC (over sampled ADC). • ADC0804 Analog to Digital Converter. In our experiment ADC0804 , and the SAR ADC are explained.**ADC0804 Analog to Digital Converter**• ADC0804 is a 20-pin IC with an 8-bit resolution. • The analog input voltage range is from 0 V to 5 V, with 15 mW power consumption and 100 us conversion time .**SAR ADC**• It is one of the popular ADCs for 8-16 bits. • It has moderate conversion speeds ; the conversion time is around 1 Us. • It doesn’t consume a lot of power and its cost is low in comparison with the other types. • On the other hand, it requires a sample and hold circuit and it can have missing output codes.**Example:**Vref=5V, Vs=3.127V, 4-bits.(b1b2b3b4) Set b1=1 all b2b3b4=000 >> 1000 Vdac=8*5/(2^4)=2.5V (Vdac=2.5)>(Vs=3.127) No!! b1=1 2. Set b2=1 all b3b4=00 >> 1100 Vdac=12*5/(2^4)=3.75V (Vdac=3.75)>(Vs=3.127) Yes b2=0 3. Set b3=1 b4=0 >> 1010 Vdac=10*5/(2^4)=3.125V (Vdac=3.125)>(Vs=3.127) No!! b3=1 4. Set b4=1 >>1011 Vdac=11*5/(2^4)=3.4375V (Vdac=3.4375)>(Vs=3.127) Yes b4=0 Final Result=1010**Digital To Analog Conversion ( DAC)**• Digital to analog converter (DAC) is the device which converts digital signals to analog ones. • Most of the DACs consist of a network of resistors and analog switches. • The switches control the currents or voltages that are derived from a particular reference voltage and provide analog output current.**Types Of DACs**• There are many types of DACs such as: • Weighted resistor DAC. • R-2R DAC. • General purpose DAC ( DAC 0800). • Frequency to voltage converter. • Pulse width modulation. In our experiment, R-2R DAC and the general purpose DAC are explained.**R-2R DAC**• The Block diagram of R-2R DAC is as shown below: • The analog output is given by the following equation: • The highest resistance value is only 2R and thus requires an area inside the IC less 8 times than that of the weighted resistor DAC which highest resistance is 16R.**The DAC analog output is represented by current. This**current is the ratio of the input code to the full scale voltage (Vref) as given in the following equation: • So, an operational amplifier is needed to convert the current to voltage level.**The General Purpose ( DAC 0800)**• The block diagram of DAC 0800 is as shown below: • It’s clear that it’s a 8-bit DAC with 16 pins IC. • The conversion time is around 100ns.**Bipolar/ Unipolar DACs**• For unipolar DACs, the output voltage is given by: • For bipolar DACs, the output voltage is given by: