Analog/Digital Interface

8/13/2019 Analog/Digital Interface

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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



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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.



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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



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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



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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



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Advantages: Much faster than Ramp/Slope, since the counter output

converges to the correct value quickly

SUCCESSIVE APPROXIMATIONADC

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Analog/Digital Interface