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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 numbers

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Analog to Digital Conversion

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Analog to digital conversion l.jpg

Analog to Digital Conversion

Introduction l.jpg


  • An analog-to-digital converter(ADC, A/D, or A to D) is a device that converts continuous signals to discrete digital numbers

  • 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 conversion

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Important terminologies in ADC

  • Resolution

  • Response type

    • Linear ADCs

    • Non-linear ADCs

  • Accuracy

  • Sampling rate

  • Aliasing

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  • 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:

    • 5/28 = 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 signals

      • m-law

      • A-law

      • Dolby

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  • 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 Ts, the sampling time.

  • A bandlimited analog signal must be sampled at a frequency fs = 1/Ts that is twice the maximum frequency (fa) of the bandlimited signal

  • fs = 2fa is known as the Nyquist Sampling frequency

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  • 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 doneDigital-Ramp ADC

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How is it doneSuccessive Approximation ADC

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How is it doneFlash ADC

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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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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 (fs = 2*fa)

  • Outputs signal when data is valid

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ADC0820 – RD Mode

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ADC0820 – WD-RD Mode

t1 = tINTL= 800 ns

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ADC0820 – WD-RD Mode

t1 = tINTL= 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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