DA/AD CONVERTER

What is a DAC? Weighted Resistor DAC R-2R DAC Capacitive DAC DAC associated error Delta-Sigma DAC Conclusion

Digital-to-Analog Converter: An electronic device, often an integrated circuit, that converts a digital number into a corresponding analog voltage or current.

A/D Converter• In order to change an analog signal to

digital, the input analog signal is sampled at a high rate of speed.

• The amplitude at each of those sampled moments is converted into a number equivalent – this is called quantization.

• These numbers are simply the combinations of the 0s and 1s used in computer language – this called encoding.

Digital to Analog (D/A, DAC, or D-to-A) Conversion

• Converting discrete signals into discrete analog values that represent the magnitude of the input signal compared to a standard or reference voltage – The output of the DAC is discrete analog steps.– By increasing the resolution (number of bits), the

step size is reduced, and the output approximates a continuous analog signal.

DAC In an electronic circuit, a combination of high voltage (+5V) and low voltage (0V) is usually used to represent a binary number. For example, a binary number 1010 is represented by

Weighting 23 22 21 20

Binary Digit 1 0 1 0

State +5V 0V +5V 0V

DACs are electronic circuits that convert digital, (usually binary) signals (for example, 1000100) to analog electrical quantities (usually voltage) directly related to the digitally encoded input number.

DACs are used in many other applications, such as voice synthesizers, automatic test system, and process control actuator. In addition, they allow computers to communicate with the real (analog) world.

Reg

iste

r

VoltageSwitch

ResistiveSummingNetwork

Amplifier

Input BinaryNumber

Analog VoltageOutput

The two most popular types of resistive summing networks are:

Resistive divider circuit/Weighted binary resistance type

-Utilizies summing amplifier to form the weighted sum of all non-zero bits in the input

b)Ladder resistance (R-2R) type -Maintains constant current through all

branches (No voltage transients)

Uses a summing amplifier circuit to generate output.Analysis uses KCL and Op-Amp properties to determine Vout.

Rf = R

8R4R2RR Vo

-VREF

iI

Least Significant Bit

Most Significant Bit

Disadvantages Large number of resistors required for high

bit DACs High resistor values required for high bit

DACs (32,768R for common 16-bit DACs) Larger resistors cause more error in analog

output

Only Input 4 is HIGH

Only Input 3 is HIGH

Used for Digital-to-analog converter!

Binary Weighted R-2R

Pros Easily understood

Only 2 resistor values

Easier implementation

Easier to manufacture

Faster response time

Cons

Limited to ~ 8 bits

Large # of resistors

Susceptible to noise

Expensive

Greater Error

More confusing analysis

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Digital Oscilloscopes◦ Digital Input◦ Analog Ouput

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Signal Generators◦ Sine wave generation◦ Square wave generation◦ Triangle wave generation◦ Random noise generation

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For the selection of an IC DAC, there are several parameters that can determine the suitability of a particular device.

ResolutionThe number of bits making up the input data word that will ultimately determine the output step voltage as a percentage of full-scale output voltage. Example: Calculate the resolution of an 8-bit DAC. Solution: Resolution = 8 bits

Percentage resolution = %391.0%100

256

1%100

2

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What is ADC(Analog to Digital Converter) Why ADC is needed Application of ADC A/D conversion process

What is ADC An electronic integrated circuit whichtransforms a signal from analog (continuous)to digital (discrete) form. Analog signals are directly measurablequantities. Digital signals only have two states. Fordigital computer, we refer to binary states, 0and 1.

• A simple hypothetical A/D converter circuit with one analog input signal and three digital output lines with eight possible binary combinations: 000 to 111– Shows the graph of digital output for FS V analog input

• The following points can be summarized in the above process:– Maximum value this quantization process reaches is 7/8 V for a 1 V analog signal;

includes 1/8 V an inherent error– 1/8 V (an inherent error) is also equal to the value of the Least Significant Bit

(LSB) = 001.– Resolution of a converter is defined in terms of the number of discrete values it

can produce; also expressed in the number of bits used for conversion or as 1/2n where n =number of bits

– The value of the most significant bit (MSB) -100- is equal to ½ the voltage of the full-scale value of 1 V.

– The value of the largest digital number 111 is equal to full-scale value minus the value of the LSB.

– The quantization error can be reduced or the resolution can be improved by increasing the number of bits used for the conversion

Why ADC is needed Microprocessors can only perform complexprocessing on digitized signals. When signals are in digital form they are lesssusceptible to the deleterious effects ofadditive noise. ADC Provides a link between the analogworld of transducers and the digital world ofsignal processing and data handling.

Application of ADC ADC are used virtually everywhere where ananalog signal has to be processed, stored, ortransported in digital form. Some examples of ADC usage are digital voltmeters, cell phone, thermocouples, anddigital oscilloscope. Microcontrollers commonly use 8, 10, 12, or16 bit ADCs, our micro controller uses an 8 or10 bit ADC.

Numerous methods are used for converting analog signals to digital form. Five most commonly used methods are listed below:

• Staircase ramp• Successive approximation• Dual slope• Voltage to frequency• Parallel (or flash)

Conversion from analog to digital form inherently involves comparator action where the value of the analog voltage at some point in time is compared with some standard.A common way to do that is to apply the analog voltage to one terminal of a comparator and trigger a binary counter which drives a DAC.

The clear pulse resets the counter to the zero. The counter then records in the binary form the number of pulses from clock line. The clock is a source of pulses equally spaced in time. Since the number of pulses counted increases as the input linearly with time, the binary word representing this count is used as the input of a DAC (Digital to Analog Converter) whose output is the stair case waveform

As long as the analog input VA is greater than VD, the comparator has an output which is high and the AND gate is open for transmission of the clock pulse to the counter. When VD exceeds VA the comparator output changes to low value and the AND gate is disabled. This stops the counting at the time when VA ≈ VD and the counter can be read out as the digital word representing the analog input voltage.

Advantages of Counter ADC   Very simple design  Cheap due to its simple design

Disadvantage of Counter ADC Variable conversion time.  Slow operation

Illustration of 4-bit SAC with 1 volt step size Illustration of 4-bit SAC with 1 volt step size

• Much faster than the digital ramp ADC because it uses digital logic to converge on the value closest to the input voltage.

• A comparator and a DAC are used in the process.