Analog to Digital Conversion

Introduction  An analog-to-digital converter (ADC, A/D, or A to D) is a device that converts continuous signals to discrete digital numbersanalog-to-digital converter  In electronics, a digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analog signal (current, voltage or charges). Digital-to-Analog Converters are the interface between the abstract digital world and the analog real life. Simple switches, a network of resistors, current sources or capacitors may implement this conversiondigital-to-analog converter

Important terminologies in ADC  Resolution  Response type Linear ADCs Non-linear ADCs  Accuracy  Sampling rate  Aliasing

Resolution  The resolution of the converter indicates the smallest analog value that it can convert to a digital number  If the ADC has 8 bits and the Full scale is 0-5 Volts, then the ADC voltage resolution is: 5/2 8 = Volts

Response type  Linear ADCs Output binary value changes approximately with the analog value within the resolution (or ½ the resolution)  Non-linear ADCs Uses techniques known as companding to ‘magnify” the low amplitude analog signalscompanding   -law  A-law  Dolby

Accuracy  Accuracy depends on Quantization error Non-linear error caused by the physical imperfections of ADC

Sampling rate  For ADC, a signal values are measured and stored at intervals of time T s, the sampling time.  A bandlimited analog signal must be sampled at a frequency f s = 1/T s that is twice the maximum frequency (f a ) of the bandlimited signal  f s = 2f a is known as the Nyquist Sampling frequency

Aliasing  If a signal values are measured and stored at frequencies greater than the Nyquist sampling rate, the signal can be reproduced exactly (within quantization and other non-linear error accuracy).  However, If a function is sampled at less than Nyquist rate, the resulting function may have different frequency content. This is known as aliasing. For example: If a 3 KHz sine wave is sampled at 4 KHz, the resulting signal will appear as a 1 KHz signal.

How is it done Digital-Ramp ADC

How is it done Successive Approximation ADC

How is it done Flash ADC

Analog to Digital chip: ADC0820  8-Bit High Speed µP Compatible A/D Converter with Track/Hold Function  Uses ½ flash conversion technique consists of 32 comparators a most significant 4-bit ADC a least significant 4-bit ADC  1.5 µs conversion time  Does not need external sample-and-hold for signals moving at less than 100 mV/µs.

ADC0820

Analog to Digital chip: ADC0820  Has many input modes, RD, WR- RD, WR-RD Standalone  Input pulse required to read analog data (Sample) Must sample at more than Nyquist rate (f s = 2*f a )  Outputs signal when data is valid

ADC0820 – RD Mode

ADC0820 – WD-RD Mode t 1 = t INTL = 800 ns

ADC0820 – WD-RD Mode t 1 = t INTL = 800 ns

ADC0820 – WD-RD Mode

Acquiring an Analog Signal  Input is a sinusoidal signal with peak to peak of 5 V  Voltage input in the range -2.5 to 2.5 V  Use Analog to Digital Converter ADC0820 Input’s analog voltage 0 to 5 V Requires adding 2.5 Volts to input signal before converted.

Op-Amp - Non-Inverting Adder  Use LM741 Operational Amplifier  Eqs: Vo =V1 + v2 (for all resistors equal) Vo = (R1+R2)/R2 (V1 R4 + V2R3)/ (R3+R4)

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