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Analog to Digital Conversion

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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
- Linear ADCs
- Non-linear ADCs

- 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

- 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

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

- 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.

http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html

http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html

http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html

- 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
- http://hyperphysics.phy-astr.gsu.edu/HBASE/Electronic/adc.html