An Introduction to ADC and DAC
The natural state of audio and video signals is analog. When digital technology was not yet around, they are recorded or played back in analog devices like vinyl discs and cassette tapes. The storage capacity of these devices is limited and doing multiple runs of re-recording and editing produced poor signal quality. Developments in digital technology like the CD, DVD, Blu-ray, flash devices and other memory devices addressed these problems. For these devices to be used, the analog signals are first converted to digital signals using analog to digital conversion (ADC). For the recorded audio and video signals to be heard and viewed again, the reverse process of digital to analog conversion (DAC) is used. ADC and DAC are also used in interfacing digital circuits to analog systems. Typical applications are control and monitoring of temperature, water level, pressure and other real-world data.
An ADC inputs an analog signal such as voltage or current and outputs a digital signal in the form of a binary number. A DAC, on the other hand, inputs the binary number and outputs the corresponding analog voltage or current signal.
Types of ADC
Direct-conversion ADC is also called flash conversion ADC. This process is extremely fast with a sampling rate of up to 1 GHz. The resolution is however, limited because of the large number of comparators and reference voltages required. The input signal is fed simultaneously to all comparators. A priority encoder then generates a digital output that corresponds with the highest activated comparator.
Successive-approximation ADC is a conversion technique based on a successive-approximation register (SAR). This is also called bit-weighing conversion that employs a comparator to weigh the applied input voltage against the output of an N-bit digital-to-analog converter (DAC). The final result is obtained as a sum of N weighting steps, in which each step is a single-bit conversion using the DAC output as a reference. SAR converters sample at rates up to 1Msps, requires a low supply current, and the cheapest in terms of production cost.
In an integrating ADC, a current, proportional to the input voltage, charges a capacitor for a fixed time interval T charge. At the end of this interval, the device resets its counter and applies an opposite-polarity negative reference voltage to the integrator input. Because of this, the capacitor is discharged by a constant current until the integrator output voltage zero again. The T discharge interval is proportional to the input voltage level and the resultant final count provides the digital output, corresponding to the input signal. This type of ADCs are extremely slow devices with low input bandwidths. Their advantage, however, is their ability to reject high-frequency noise and AC line noise such as 50Hz or 60Hz. This makes them useful in noisy industrial environments and typical application is in multi-meters.
Types of DAC
Pulse Width Modulator
A basic form of digital to analog conversion is Pulse Width Modulation (PWM). It is a process that involves transmission of analog information over a series of pulses. The transmitted data is encoded on the width of these pulses resulting to corresponding digital signals. The resultant variable width pulses represent the amplitude of an input analog signal. The reverse process is done in converting the PWM generated digital signal to analog. Typical application is in power and voltage regulation.
Cyclic Serial DAC
This type of DAC was developed during the early days of Pulse Code Modulation (PCM). It took advantage of the serial nature of generated PCM pulse streams. Implementation is however complex because the operation needs to observe proper sequence in reception of data - the LSB should be at the beginning of the sequence and the MSB should be the last.
Thermometer Coded DAC employs a number of equally weighted elements that contains an equal resistor segment for each possible value of DAC output. This is currently the fastest and highest precision DAC architecture available but the trade-off is high cost. This type of DAC requires very high sampling rates with achieved speeds of more than 1 billion samples per second.
Hybrid DACs use a combination of the above techniques plus either that of the binary-weighted or ladder type of DAC. The objective is to achieve the characteristics of an ideal DAC – high speed, high precision and low cost. A good example is the Segmented DAC, which utilizes the thermometer coded principle for the MSBs and the binary weighted principle for the LSBs. This makes the Segmented DAC a pratical and cost-effective solution.
Binary-Weighted Resistor DAC
The binary-weighted-resistor DAC employs the characteristics of the inverting summer Op Amp circuit. In this type of DAC, the output voltage is the inverted sum of all the input voltages. If the input resistor values are set to multiples of two: 1R, 2R and 4R, the output voltage would be equal to the sum of V1, V2/2 and V3/4. V1 corresponds to the most significant bit (MSB) while V3 corresponds to the least significant bit (LSB).
R-2R Ladder DAC
An enhancement of the binary-weighted resistor DAC is the R-2R ladder network. This type of DAC utilizes Thevenin’s theorem in arriving at the desired output voltages. The R-2R network consists of resistors with only two values - R and 2xR. If each input is supplied either 0 volts or reference voltage, the output voltage will be an analog equivalent of the binary value of the three bits. VS2 corresponds to the most significant bit (MSB) while VS0 corresponds to the least significant bit (LSB).
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