22-02-2013, 12:02 PM
OPAMP AS A DIGITAL TO ANALOG CONVERTOR (R-2R METHOD)
OPAMP AS A DIGITAL.docx (Size: 89.49 KB / Downloads: 23)
OBJECTIVE: -
Study to design Digital to Analog Converter (R-2R method) using op-amp.
EQUIPMENT:-
1. OPT-01 Trainer kit.
2. Power Supply.
3. Patch cords.
4. DSO.
5. Multi-meter.
THEORY:-
D/A converter with R and 2 R resistors:-
Figure shows a D/A converter with Rand 2 R resistors. As before, the binary inputs are
simulated by switches b0 through b3, and output is proportional to the binary inputs. Binary
inputs can either be in the high (+5V) or low (0V) state. Assume that the most significant bit
(MSB) switch b3 is connected to +5V and other switches are connected to ground as in
Figure. Thevenizing the circuit to the left of switch b3, Thevenin’s equivalent resistance Rth
is
Rth = [{[(2R|2R +R)|2R]+R} |2R]+R
=2R =20k
In this figure the (-) input is at virtual ground (V2= 0V); therefore, the current through Rth
(=2R) is zero. However, the current through 2R connected to +5 V is 5V/20 k = 0.25 mA.
The same current flows through Rf and in turn produces the output voltage
V0 = -(20 k)(0.25 mA = - 5V
Using the same analysis the output voltage corresponding to all possible combinations of
binary inputs can be calculated as shown in Figure. The maximum or full-scale of -9.375 is
obtained when all the inputs are high. The output voltage equation can be written as
V0 = -RF (b3/2R + b2/4R + b1/8R + b0/16R)
Where each of the inputs b3, b2, b1 and b0 may be either high (5V) or low (0V)
The great advantage of the D/A converter is that it requires only two sets of precision
resistance values; nevertheless, it requires more resistors and is also more difficult to
analyze than the binary weighted resistor types. As the number of binary inputs is increased
beyond four both D/A converter circuits get complex and their accuracy degenerates.
Therefore, in critical applications an integrated circuit specially designed as a D/A converter
should be used.
CONCLUSION: -
We studied the design of Digital to Analog Converter op-amps and calculated and observed
values are verified.