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DIGITAL
ELECTRONICSLECTURE 2:ANALOG DIGITAL
INTERFACE
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SIGNALS
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DIGITAL TO ANALOGCONVERTERS (DAC S ) DACs take an n-bit digital signal, and output a single
analog voltage. Calculate a weighted sum of the input bits Two common types of DAC:
R/2nR R/2R
Both based on voltage-summation circuits
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R/2nR DAC
Inverting summation circuit, but with weighted inputresistances
Therefore Vout = -(V 1 + V 2 /2 + V 3 /4)
Image http://www.allaboutci
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R/2nR DAC
Connect digital input with MSB -> V1 Each bit has half the impact on Vout as the previous bit Can be expanded to any number of bits
Image http://www.allaboutci
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R/2R DAC Disadvantage of R/2nR DAC: Resistor values must all be
exact, even one wrong resistance will impact the output R/2R uses less resistors, only two different resistances.
Image http://www.allaboutci
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ANALOG TO DIGITAL CONVERSION(ADC) More complicated than DAC. More varied architectures:
Flash Ramp Dual Slope Approximation
Basic principle: Sampling Similar to Decimal-to-Binary conversion
Series of divisions to find quotient and remainder Use a reference voltage V REF
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ADC GENERAL PRINCIPLES What fraction of V REF is V in ? Resolution: Voltage gap between output code Range: Highest recordable input (V REF )
Image National Sem
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ADC GENERAL PRINCIPLES Sample rate: The rate at which the the input voltage is
converted to a single digital output. Nyquist Theorum:
If the highest frequency in an analogue input is B Hz, then a
sample rate of 2B samples/sec will complete determine it Sample size: The number of bits in the output. Determines
the maximum resolution/range.
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FLASH ADC
Series of 2n 1 comparators compare the input voltage toreference voltages
Priority encoder converts the comparator outputs to n-bitoutput
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FLASH ADC
Image http://www.allaboutci
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FLASH ADC
Advantages Fast Flexible (by changing the resistors and V REF we can easily
change resolution, range, even linearity)
Disadvantages Expensive, hardware intensive Does not scale well (8-bits require 255 comparators, 32-
bits require over 4,000,000,000)
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RAMP ADC
More complicated than Flash A digital counter counts up from zero A Digital-to-Analog converter converts the counter output
to analog voltage Comparator compares DAC output to input voltage. When the two are equal, the output of the counter is read
as the digital output.
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RAMP ADC
Image http://www.allaboutcircuits.c
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RAMP ADC Advantages
Less hardware than Flash Disadvantages
Low sample rate, not very responsive Inconsistent sample rate Inaccurate if input is varying
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SLOPE ADC Similar to ramp, but instead of using a digital counter and
DAC, simply use an analogue ramping circuit Integrating circuit provides a sawtooth waveform
Advantages:
Less hardware than ramp Disadvantages
Slow Even less accurate than ramp
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SLOPE ADC
Image http://www.allaboutcircuits.c
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DUAL SLOPE ADC Similar to Slope, but with two sawtooth waveform
generators, one increasing, one decreasing Composite output is triangular
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DUAL SLOPE ADC
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DUAL SLOPE ADC Advantages:
Less hardware than Flash Faster and more accurate than Ramp/Slope
Disadvantages
Slower than Flash
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SUCCESSIVE APPROXIMATIONADC Similar to Ramp/Slope ADC, but instead of counting
linearly, the counter circuit changes its output startingfrom the MSB
The comparator compares this output with the analogsignal, and feeds back to the counter circuit to change thenext least significant bit accordingly.
Similar to trial-and-fit decimal to binary conversion
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SUCCESSIVE APPROXIMATIONADC
Image http://www.allaboutcircuits.c
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Advantages: Much faster than Ramp/Slope, since the counter output
converges to the correct value quickly
SUCCESSIVE APPROXIMATIONADC