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Analog to Digital Conversion

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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

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

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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 = 0.01953125 Volts

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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

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

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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

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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.

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How is it done Digital-Ramp ADC http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html

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How is it done Successive Approximation ADC http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html

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How is it done Flash ADC http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html

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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.

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ADC0820

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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

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ADC0820 – RD Mode

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

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

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ADC0820 – WD-RD Mode

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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.

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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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References http://en.wikipedia.org/wiki/Analog_to_digital_converter http://en.wikipedia.org/wiki/Analog_to_digital_converter http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html

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