DAC.3.V.3

DAC.3.V.3 - L303.5.v3 Drexel University Electrical and...

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L303.5.v3 5- 1 Drexel University Electrical and Computer Engr. Dept. Electrical Engineering Laboratory III, ECE L303 E. L. Gerber DIGITAL-TO-ANALOG CONVERSION Object The object of this experiment is to learn about digital to analog conversion. Basic operation of a D/A system will be covered and a simple IC device will be investigated. Introduction Mathematical operations on analog signals are more easily performed with the use of the digital computer. To enable digital processing, the analog signal is normally changed into a digital representation (analog-to-digital conversion), usually consisting of eight-bit digital words. After processing, it may be necessary to convert these words back into an analog signal for a particular application. A device that can perform this function is the digital-to-analog converter (D/A). Theory The digital-to-analog converter takes a digital input word and translates it into an analog output voltage. Thus, when the bits change in a digital word, a change occurs in the output voltage. For example, assume that the digital word consists of 4 bits, giving us 16 (=2 4 ) possible 4-bit combinations. Let’s further assume that the 16 combinations represent 0 to 15 V (in 1-V steps) with 0000 = 0 V and 1111 = 15 V (binary-to-decimal equivalence). Therefore, with an input word of 0100, the output of the D/A is 4 V. The word 1010 gives an output of 10 V and so on. Integrated-circuit D/A chips typically operate by having the digital input word control the amount of current flowing through a fixed output load resistor. Following the previous example, the current is increased in 15 equal increments as the input word varies from 0000 to 1111. If the current steps are 1 mA and the load resistor is 1 k , then the output voltage ranges from 0 to 15 V in 1-V steps. A Typical Digital-to-Analog Converter One type of ADC integrated circuit is the DAC0808. This device converts digital inputs to analog outputs with a resolution of 8 bits or 256 (=2 8 ) steps. Digital inputs to the device can be from TTL, CMOS, PMOS, etc. The full-scale output current ( I FS ) for this device is typically 2 mA. An equivalent circuit of the DAC0808 chip is shown in detail in Fig. 1. A simplified block diagram is shown on page 5-9. Referring to Fig. 2 and Fig. 3, the reference current I REF is controlled by the +V REF source and it flows around the inverting input of the input amplifier into the collector of Q 0 (the first BJT). This same current flows through the emitter resistor R of Q 0 to
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L303.5.v3 5- 2 Fig. 1 DAC 0808 Equivalent Circuit.
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L303.5.v3 5- 3 the negative supply voltage - V CC and sets up a voltage drop V R from the emitter to - V CC , that is equal to I REF × R . This same voltage drop appears from the emitter to - V CC for each transistor, Q 1 through Q 8 (the last BJT). Since the emitter resistor of
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This note was uploaded on 09/25/2011 for the course ECEL 303 taught by Professor Gerber during the Fall '07 term at Drexel.

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DAC.3.V.3 - L303.5.v3 Drexel University Electrical and...

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