07-11-2012, 05:59 PM
DIGITAL TO ANALOG CONVERTER
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Theory
For processing, transmission and storage process it is often convenient to express analog variable in digital forms. The operation of digital communication system requires conversion of information from analog to digital and digital to analog forms.
DAC uses binary weighted resistors or binary ladder as a basis for translation of a digital signal to an equivalent analog signal.
Design
Resolution required = 6.25% = 6.25/100 = 1/16
(Resolution defines the smallest increment in output voltage obtained due to change in input. Here it is the change in output voltage due to LSB)
As LSB has a weight of 1/16 in a 4-bit system, the required system is 4-bit size. The minimum resistance is seen by the digital source with bit 3. Hence 2R=20K(R=10K)
Binary ladder network
Here only two values of resistors are used as shown in figure.
Left end of the ladder network (close to lowest order bit) is terminated in a resistance 2R. Each digital source is also connected through a resistance of 2R. Each of other resistors is R. Using Thevenin’s theorem and Superposition theorem it can be shown that the network with the inputs b0,b1,b2,b3 is equivalent to four parallel voltage sources, each with internal resistance R and having voltages b0(Vref/16), b1(Vref/8), b2(Vref/4), b3(Vref/2) respectively. Hence the output voltage of the circuit is given by
Vo=Rf/R (Vref/16) (b0+2b1+4b2+8b3)
The load seen by each digital source will be 3R if the output of ladder is loaded with 2R. This will result in a lower output voltage, but output voltage will still be a properly weighted sum of the binary input bits.
Procedure
1. Connections are made as shown in the figure.
2. Power supply toe the IC is switched on.
3. The bits b0,b1,b2,b3 are set to one (Applying 5V from the kit).
4. The potentiometer is adjusted to get a full scale output of 5V.
5. Bits b0,b1,b2,b3 are set at all possible values and corresponding output voltages are noted.