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# Analog to Digital Conversion - PowerPoint PPT Presentation

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

• 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

• Resolution

• Response type

• Accuracy

• Sampling rate

• Aliasing

• 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

• Output binary value changes approximately with the analog value within the resolution (or ½ the resolution)

• Uses techniques known as companding to ‘magnify” the low amplitude analog signals

• m-law

• A-law

• Dolby

• Accuracy depends on

• Quantization error

• Non-linear error caused by the physical imperfections of ADC

• 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

• 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 doneSuccessive Approximation ADC

• 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.

• 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

t1 = tINTL= 800 ns

t1 = tINTL= 800 ns

• 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.

• Use LM741 Operational Amplifier

• Eqs: Vo =V1 + v2 (for all resistors equal)

Vo = (R1+R2)/R2 (V1 R4 + V2R3)/ (R3+R4)

• http://en.wikipedia.org/wiki/Analog_to_digital_converter