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Session 1559

Analog to Digital (ADC) and Digital to Analog (DAC) Converters

M. Rabiee Eastern Kentucky University

Introduction: Electric voltage and current signals are often referred to as analog signals. Analog signals must be converted to digital signals prior to input into computers. Analog to Digital Converters (ADCs) are used to convert analog signals to digital signals. Inversely, the computer output that is in digital form must sometimes be converted to an analog signal prior to input to an electronic or electrical device. Digital to Analog Converters (DACs) are used to perform this operation. In this paper we will examine important characteristics associated with ADC/ DAC converters. In addition, we will explain how to specify a converter to meet the requirements of a specific system. We will also state and describe different types of ADC / DAC converters. Binary-weighted, and R- 2R Ladder type DAC circuits will be displayed and analyzed. Parallel Comparator or Flash, Dual Slope, and Successive Approximation type ADC circuits will be displayed and analyzed. We will explain extensively the advantage / disadvantage criteria associated with each type of ADC or DAC converters. Finally, two computer interfacing laboratory assignments will demonstrate the constructions of the R-2R Ladder type DAC, and Successive Approximation type ADC converters.

Digital to Analog Converter (DAC) Characteristic: When we select a Digital to Analog Converter (DAC) chip for an application, we must consider three characteristic associated with a DAC. First, resolution or step size defines the smallest voltage or current change possible for the DAC output signal. For example, an 8-bit DAC that generates a maximum output voltage of 5 volts has a step size or resolution of (5V / 2 8 ) = 19.5 mV. Sometimes, the resolution is stated in the percentage value. For the 8-bit DAC, the percent resolution is (1 / 2 8 ) x 100 = 0.39%. Settling time is the time required for the DAC output to settle to 99.5% of its new value. Typical settling time for a commercial DAC is 300 nanoseconds. Accuracy is used to state the percentage of actual output of a DAC to expected output. Usually the accuracy is 50% of the step size. For the 5 V, 8-bit DAC the accuracy is 0.975 mV. This implies that the maximum output is guaranteed by the DAC manufacturer to be within 5.00975V and 4.99025V.

Digital to Analog Converter (DAC) types: There are two methods of constructing a DAC; Binary Weighted type, and R-2R Ladder type. Figure 1 displays an 8-bit binary weighted type DAC circuit. The output voltage for a binary weighted DAC is found using the following equation.

V OUT = ( V REF ) * ( R F / R ) * ( D7 / 2 0 + D6 / 2 1 + ................. + D1 / 2 6 + D0 / 2 7 )

Note that the switches represent transistors that can be opened ( i.e., operates in turned off, or open collector to emitter state), or closed (i.e., operates in turned on, or saturated state).

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Rabiee, M. (1998, June), Analog To Digital (Adc) And Digital To Analog (Dac) Converters Paper presented at 1998 Annual Conference, Seattle, Washington. 10.18260/1-2--6916