22-02-2013, 12:01 PM
To design a DAC using Weighted Resistor method.
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OBJECTIVE: - To design a DAC using Weighted Resistor method.
(I) PRACTICAL SIGNIFICANCE: - DAC is used in digital modulator.
(II) COMPETENCY/SKILL: - Design and testing.
(III) EXPERIMENT OBJECTIVE:-To design a DAC using Weighted Resistor method.
(IV) THEORETICAL BACKGROUND: -
An output voltage value depending on the converter full-scale range corresponds to every one the 8 input stages, normally indicated with FSR (Full Scale Range).If we define "quantum" Q the smallest variation of the output voltage, it results
Q = FSR / 2n
Where, n is the bit number composing the input code: it is therefore evident that the resolution of the digital number conversion depends on the n value.The maximum FS (Full Scale) value of the output voltage is:
FS = (1 - 2n) FSR = (2n – 1) Q
Suppose FSR 10v, then
Q = 10 / 23 = 1.25v
FS = (23 – 1) ×1.25 = 8.75v
Fig. Shows a D/A converter using an op-amp and binary – weighted resistors. Although in this figure the op-amp is connected in the inverting, it can also be connected in the non-inverting mode. Since the number of binary input is four, the converter is called a 4-bit (binary digit) converter. When switch b0 is closed (connected to +5V), the voltage across R is 5v/10 K=0.5mA. However the input bias current IB is negligible; hence the current through feedback resistor RF is also 0.5mA, which, in turn produces an output voltage of –(1K)(0.5mA) = -0.5V.
Note that the op-amp is working as current – to- voltage converter. Now suppose that switch b1 is closed and b0 is opened. This action connects R/2 to the positive supply of +5V, causing twice as much current (1mA) to flow through RF, which in turn doubles the output voltage. Thus the output voltage Vo is –1V when switch b1 is closed. Similarly, if both switches b0 and b1 are closed, the current through RF will be 1.5mA, which will be converted to an output voltage of –(1K)(1.5mA) = -1.5 V.
PROCEDURE: -
1) Connect the circuit as per the diagram.
2) Connect the D0-D3 of the logic switches to the corresponding jacks B0-B3 of the converter.
3) Set the switches S0-S3 to logic level 0.
4) Connect the VREF socket to +5v.
5) Connect a multimeter as voltmeter for DC, to the output Vo of the converter.
6) Switch the logic switches in binary progression & measure & record the output voltage in correspondence of every combination of the input code.