Page 230 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 12: A/D and D/A Converters

The steady-state accuracy test involves setting a known digital number in the input register, Notes
measuring the analog output with an accurate meter, and comparing with the theoretical value.
Checking for monotonicity means checking that the output voltage increases regularly as the
input digital signal increases: This can be accomplished by using a counter as the digital input
signal and observing the analog output on an oscilloscope. For proper monotonicity, the output
waveform should be a perfect staircase waveform, as shown in Figure 12.16. The steps on the
staircase waveform must be equally spaced and of the exact same amplitude. Missing steps, steps
of different amplitude, or steps in a downward fashion indicate malfunctions.
Figure 12.16: Correct Output Voltage Waveform for Monotonicity Test
+V +V

DAC R
+ +
V V
o o
– –

c
() () ()
a
b
The monotonicity test does not check the system for accuracy, but if the system passes the test, it
is relatively certain that the converter error is less than 1 LSB. Converter accuracy and resolution
are the subjects of the next section.
A D/A converter can be regarded as a logic block having numerous digital inputs and a single
analog output as seen in Figure 12.16b. It is interesting to compare this logic block with the
potentiometer shown in Figure 12.16c. The analog output voltage of the D/A converter is controlled
by the digital input signals, while the analog output voltage of the potentiometer is controlled by
mechanical rotation of the potentiometer shaft. Considered in this fashion, it is easy to see how a
D/A converter could be used to generate a voltage waveform (saw tooth, triangular, sinusoidal,
etc.). It is, in effect, a digitally controlled voltage generator!
12.8: Suppose that in the course of a monotonicity check on the 4-bit converter in
Figure 12.13, the waveform shown in Figure 12.17 is observed. What is the probable malfunction
in the converter?
Figure 12.17: Irregular Output Voltage for Example 12.8

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