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

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

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:

5/28 = 0.01953125 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 signals -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 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

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 doneDigital-Ramp ADC

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

How is it doneSuccessive Approximation ADC

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

How is it doneFlash ADC

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

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 (fs = 2*fa)

Outputs signal when data is valid

ADC0820 – RD Mode

ADC0820 – WD-RD Mode

t1 = tINTL= 800 ns

ADC0820 – WD-RD Mode

t1 = tINTL= 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)

References

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