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CUSTOMER_CODE
SMUDE
DIVISION_CODE
SMUDE
EVENT_CODE
APR2016
ASSESSMENT_CODE BCA1040_APR2016
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
5709
QUESTION_TEXT
Explain in brief any four types of electronic DACs.
SCHEME OF
EVALUATION
Solution:
Explanation of each – 2.5 marks x 4 = 10 marks
Answer:
The Pulse Width Modulator, the simplest DAC type. A stable
current or voltage is switched into a low pass analog filter with a
duration determined by the digital input code. This technique is
often used for electric motor speed control, and is now becoming
common in high-fidelity audio.
Oversampling DACs or Interpolating DACs such as the Delta-Sigma
DAC, use a pulse density conversion technique. The oversampling
technique allows for the use of a lower resolution DAC internally. A
simple 1-bit DAC is often chosen because the oversampled result is
inherently linear. The DAC is driven with a pulse density modulated
signal, created with the use of a low-pass filter, step nonlinearity (the
actual 1-bit DAC), and negative feedback loop, in a technique called
delta- sigma modulation. This results in an effective high-pass filter
acting on the quantization (signal processing) noise, thus steering
this noise out of the low frequencies of interest into the high
frequencies of little interest, which is called noise shaping (very high
frequencies because of the oversampling). The quantization noise at
these high frequencies are removed or greatly attenuated by use of
an analog low-pass filter at the output (sometimes a simple RC lowpass circuit is sufficient). Most very high resolution DACs (greater
than 16 bits) are of this type due to its high linearity and low cost.
Higher oversampling rates can either relax the specifications of the
output low-pass filter and enable further suppression of
quantization noise. Speeds of greater than 100 thousand samples per
second (for example, 192kHz) and resolutions of 24 bits are
attainable with Delta-Sigma DACs.
The Binary Weighted DAC, which contains one resistor or current
source for each bit of the DAC connected to a summing point. These
precise voltages or currents sum to the correct output value. This is
one of the fastest conversion methods but suffers from poor
accuracy because of the high precision required for each individual
voltage or current. Such high-precision resistors and current-
sources are expensive, so this type of converter is usually limited to
8-bit resolution or less.
The R-2R Ladder DAC, which is a binary weighted DAC that uses a
repeating cascaded structure of resistor values R and 2R. This
improves the precision due to the relative ease of producing equal
valued matched resistors (or current sources). However, wide
converters perform slowly due to increasingly large RC-constants
for each added R-2R link.
The Thermometer coded DAC, which contains an equal resistor or
current source segment for each possible value of DAC output. An 8bit thermometer DAC would have 255 segments, and a 16-bit
thermometer DAC would have 65,535 segments. This is perhaps the
fastest and highest precision DAC architecture but at the expense of
high cost. Conversion speeds of >1 billion samples per second have
been reached with this type of DAC.
Hybrid DACs, which use a combination of the above techniques in a
single converter. Most DAC integrated circuits are of this type due
to the difficulty of getting low cost, high speed and high precision in
one device.
o The Segmented DAC, which combines the thermometer coded
principle for the most significant bits and the binary weighted
principle for the least significant bits. In this way, a compromise is
obtained between precision (by the use of the thermometer coded
principle) and number of resistors or current sources (by the use of
the binary weighted principle). The full binary weighted design
means 0% segmentation, the full thermometer coded design means.
100% segmentation.
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
5710
QUESTION_TEXT
Explain the important characteristics of DAC devices.
Characteristics of DAC devices.
Resolution (2 marks)
Maximum sampling frequency (2 marks)
SCHEME OF EVALUATION
Monotonicity (2 marks)
THP + N (2 marks)
Dynamic range(2 marks)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
72381
QUESTION_TEXT
Explain the rules for simplifying functions using K–map.
SCHEME OF
EVALUATION
Summary of rules for simplifying functions using Karnaugh maps
1. While implementing minterm function, cells in K-map should be
included with all 1’s but not 0’s. While implementing maxterm function,
cells in K-map should be included with all 0’s but not 1’s. 2. Group
only cells which are horizontally or vertically adjacent to each other.
3. In –map the group size should be in power of 2 .i.e., group size can
be 1, 2, 4, 8 and so on.
4. The largest size groups are used to obtain the simplest form. Use
the fewest groups possible.
5. In order to achieve the step 4, overlaps can be used.
6. A horizontal ‘wrap around’ can be done for 3-variable map,
horizontal and vertical ‘wrap around’ can be done for 4-variable
map.7. Include 'don't cares' within groups as needed to achieve the
goals of point 4 above. 'Don't cares' should not be included if by so
doing the groups are not made larger or fewer.
These are six rules. Any five should be there. Each carries 2 marks.
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
72383
QUESTION_TEXT
List the different quantities that can be measured using multi
meter.
SCHEME OF
EVALUATION
The quantities are:
● Volts is used as quantity for Voltage.
● amperes is used as quantity for current.
● ohms Volts is used as quantity for resistance.
● farads Volts is used as quantity for capacitors.
● hertz is used as quantity for frequency
● Duty cycle as a percentage.
● degrees Celsius or Fahrenheit is used as quantity for
temperature.
● Siemens is used as quantity for conductance.
● henrys is used as quantity for inductance.
● decibels is used as quantity for audio signal levels.
There are 10. Each carries 1 mark.
QUESTION_TYPE DESCRIPTIVE_QUESTION
QUESTION_ID
72385
QUESTION_TEXT Explain universal gates along with their truth tables.
The NAND gate is a digital logic gate that behaves in a manner that corresponds
to the truth table to the left. A LOW output results only if both the inputs to the
gate are HIGH. If one or both inputs are LOW, a HIGH output results. The NAND
gate is a universal gate in the sense that any Boolean function can be
implemented by NAND gates.
Digital systems employing certain logic circuits take advantage of
NAND's functional completeness. In complicated logical expressions,
normally written in terms of other logic functions such as AND, OR, and
NOT, writing these in terms of NAND saves on cost, because
implementing such circuits using NAND gate yields a more compact
result than the alternatives. NAND gates can also be made with more
than two inputs, yielding an output of LOW if all of the inputs are HIGH,
and an output of HIGH if any of the inputs is LOW. These kinds of gates
therefore operate as n-ary operators instead of a simple binary
operator.
(4 marks)
SCHEME OF
EVALUATION
(2 marks)
The NOR gate is a digital logic gate that implements logical NOR - it behaves
according to the truth table to the right. A HIGH output (1) results if both the
inputs to the gate are LOW (0). If one or both inputs are is HIGH (1), a LOW
output (0) results. NOR is the result of the negation of the OR operator. NOR is
a functionally complete operation - combinations of NOR gates can be
combined to generate any other logical function. By contrast, the OR operator
is monotonic as it can only change LOW to HIGH but not vice
versa.
(3 marks)
(1 mark)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
118226
QUESTION_TEXT
Write a note on Excess-3 code and its properties.
SCHEME OF
EVALUATION
Excess–3 code theory explanation
(4 marks)
Properties
(6 marks)
Excess–3 binary coded decimal (XS–3), also called biased representation
or Excess–N, is a numeral system used on some older computers that
uses a pre–specified number N as a biasing value. It is a way to represent
values with a balanced number of positive and negative numbers. The
smallest binary number represents smallest value. The greatest binary
number represents the largest value.
The primary advantage of XS–3 coding over BCD coding is that a
decimal number can be nines’ complemented(for subtraction) as easily
as a binary number can be ones’ complemented; just invert all bits.
Adding Excess–3 works on a different algorithm than BCD coding or
regular binary numbers. When you add two XS–3 numbers together, the
result is not an XS–3 number.