Transcript Lab 5 Digital Oscilloscope and Remote Signal Processing

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