Instructor: Paul Gordy

1
Lecture 6 EGR 262 Fundamental Circuits Lab
EGR 262 Fundamental Circuits Lab Presentation
for Lab 6 Analog-to-Digital Conversion - Hardware
Instructor Paul Gordy Office H-115 Phone
822-7175 Email PGordy_at_tcc.edu
2
Lecture 6 EGR 262 Fundamental Circuits Lab
Digital-to-Analog Conversion In order to control
analog outputs, digital outputs must first be
converted to analog form using a
digital-to-analog converter (also referred to as
a DAC or D/A converter).
Note Labs 5 9 deal with digital-to-analog
conversion.
Analog-to-Digital Conversion In order to read
analog inputs, the analog inputs must first be
converted to digital form using a an
analog-to-digital converter (also referred to as
a ADC or A/D converter).
Note Labs 6 7 deal with analog-to-digital
conversion.
3
Lecture 6 EGR 262 Fundamental Circuits Lab

• Analog-to-Digital Conversion
• There are several methods of performing
  analog-to-digital conversion, including
• Simultaneous A/D converter this method uses 2N
  comparators and an N-bit priority encoder to
  produce an N-bit output.
• Stairstep-ramp A/D converter this method uses a
  D/A converter and a counter. As the binary count
  advances, it is converted to an analog signal and
  compared to the analog input.
• Tracking A/D converter similar to the
  stairstep-ramp A/D converter, but uses an UP/DOWN
  counter so that each successive conversion starts
  with the last digital value and counts up or down
  until the new analog input value is detected.
• Single-slope A/D converter instead of using D/A
  converter like the previous two methods, this
  method uses a linear ramp generator to produce a
  constant-slope reference voltage. A counter is
  synchronized with the slope of the ramp.
• Dual-slope A/D converter similar to the
  single-slope A/D converter, but the input charges
  a capacitor linearly, producing a negative,
  variable-slope ramp. The capacitor then
  discharges linearly with a positive slope. A
  counter runs as the capacitor discharges,
  yielding a count proportional to the voltage.
  This method is commonly used with voltmeters and
  other test equipment.
• Successive-approximation A/D converter this is
  perhaps the most widely used method and is used
  in Labs 6-7. It has a much shorter conversion
  time than most other methods and the conversion
  time is the same for any analog input.

4
Lecture 6 EGR 262 Fundamental Circuits Lab

• Successive-approximation A/D converter
• The successive-approximation A/D converter
  consists of
• D/A converter
• Comparator
• Success-approximation register (or processing
  using the MicroStamp11)
• Operation of the successive-approximation A/D
  converter
• The bits of the D/A converter are enabled one at
  a time, starting with the MSB.
• As each bit is enabled, the comparator produces
  an output that indicates whether the analog input
  voltage is greater or less than the output of the
  D/A converter. If the D/A output is greater than
  the analog input, the comparator output is LOW
  and the bit is set LOW. If the D/A output is
  less than the analog input, the comparator output
  is HIGH and the bit is set HIGH.
• This process is repeated for each bit.
• See the example on the following page.

5
Lecture 6 EGR 262 Fundamental Circuits Lab
Example 4-bit successive approximation A/D
converter Note that this generic example is not
based on the MicroStamp11. In Labs 6-7, the SAR
(successive-approximation register) is
essentially replaced by the Microstamp11.
6
Lecture 6 EGR 262 Fundamental Circuits Lab
Generic 4-bit successive- approximation A/D
converter
3-bit successive-approximation A/D converter
using the MicroStamp11

• Notes
• The SAR (successive-approximation register) is
  replaced by the MicroStamp11.
• A buffering amplifier is added to control the
  input to the comparator.
• A clamp circuit is added to insure that
  appropriate digital inputs are generated for the
  MicroStamp11.

7
Lecture 6 EGR 262 Fundamental Circuits Lab

• Circuit Background Information
• In order to understand the A/D circuit to be
  built, a few topics will first be introduced,
  including
• Operational amplifiers (including buffering
  circuits)
• Comparators
• Clamping Circuits

Operational Amplifiers Operational amplifiers (or
op amps) were covered in some detail in EGR 260,
but a few points are reviewed here.
8
Lecture 6 EGR 262 Fundamental Circuits Lab
Operational Amplifiers Operational amplifiers (or
op amps) were covered in some detail in EGR 260,
but a few points are reviewed here. Refer to
Chapter 5 in Electric Circuits, 7th Edition, by
Nilsson for additional information. Operational
Amplifier - An operational amplifier (op amp) is
a high gain differential amplifier with nearly
ideal external characteristics. Internally the
op amp is constructed using many transistors.
Terminology V non-inverting input voltage V-
inverting input voltage Vo output voltage Io
output current I non-inverting input
current I- inverting input current ?VDC
positive and negative DC supply voltages used to
power the op amp (typically ?5V to ?30V) ?V V
- V- difference voltage
Note Sometimes the supply voltage connections
are not shown
9
Lecture 6 EGR 262 Fundamental Circuits Lab

• Closed-loop
• Most commonly used
• Some sort of feedback from output to input
  exists
• The input voltage, Vin, is defined according
  to the application

• An op amp circuit can be easily analyzed using
  the following ideal assumptions.
• Ideal op-amp assumptions
• Assume that ?V 0, so V V-
• Assume the input resistance is infinite, so I
  I- 0
• Realize the all voltages defined above are node
  voltages w.r.t. a common ground (as illustrated
  below)

10
Lecture 6 EGR 262 Fundamental Circuits Lab
Example Determine an expression for Vo in the
inverting amplifier shown below. Discuss
limitations to the output based on the supply
voltages. Discuss saturating the op amp.
11
Lecture 6 EGR 262 Fundamental Circuits Lab
Example Determine an expression for Vo in the
unity-gain buffer shown below. Discuss loading
problems that occur in circuits.
12
Lecture 6 EGR 262 Fundamental Circuits Lab
Discuss how the buffer might be used to eliminate
loading at the output from the R-2R ladder
network used in the D/A converter from Lab 5.
13
Lecture 6 EGR 262 Fundamental Circuits Lab
Comparators A comparator is a circuit that
compares an input voltage to a fixed reference
voltage and indicates whether the input is larger
or smaller than the reference voltage.
Although specific comparator ICs can be
purchases, an operational amplifier in the
open-loop configuration (no feedback connection)
can easily function as a comparator. If is a
circuit that compares an input voltage to a fixed
reference voltage and indicates whether the input
is larger or smaller than the reference voltage.
14
Lecture 6 EGR 262 Fundamental Circuits Lab
LMC660 In lab we will use the LMC660 quad op amp
(quad indicates 4 op amps per IC). If we use
supply voltages of 0V and 9V, the circuit will
function as follows
Pinout for the LMC660
15
Lecture 6 EGR 262 Fundamental Circuits Lab
Potentiometers Three styles of potentiometers are
shown below. The center lead in each style is
referred to as the wiper. Potentiometers are
also sometimes called pots or trim pots.
Potentiometer symbols
wiper
16
Lecture 6 EGR 262 Fundamental Circuits Lab
Uses of potentiometers Potentiometers have two
key uses 1) Adjustable resistors (or
rheostats) In this case, only two leads are
required. Use the center lead (wiper) and either
end lead.
Symbol
2) Voltage dividers (or potentiometers) In this
case, all three leads are used as the
potentiometer acts like a voltage divider. A 10k
potentiometer can be thought of as two series
resistors, where the sum of the two resistors is
always 10k. Adjusting the wiper changes the
value of R1 and R2 (R2 10k R1).
R2
R1
Symbol
wiper
R1 R2 10k (for a 10k potentiometer)
17
Lecture 6 EGR 262 Fundamental Circuits Lab
Connecting a potentiometer as a voltage divider
18
Lecture 6 EGR 262 Fundamental Circuits Lab
Using a potentiometer to provide a reference
voltage for a comparator
19
Lecture 6 EGR 262 Fundamental Circuits Lab

• Clamping Circuit In Lab 6 a diode is used in a
  clamping circuit. Before discussing clamping
  circuits, lets quickly review diodes
• Diode Characteristics
• Diodes act somewhat like voltage-controlled
  switches where
• The switch is closed when a positive voltage is
  placed across the diode
• The switch is open when a negative voltage is
  placed across the diode
• The characteristics of an ideal diode are shown
  below

20
Lecture 6 EGR 262 Fundamental Circuits Lab
Actual Diode Characteristics Actual diodes
typically require a small amount of voltage, Vo,
before they act essentially like closed switches
(short circuits). In most applications, the
breakdown region is simply something to be
avoided. This will not concern us as the diode
used in lab will have breakdown voltages of
around 1000 V.
21
Lecture 6 EGR 262 Fundamental Circuits Lab

• Diode Models
• Diodes models are often used to analyze circuits
  containing diodes. The characteristics of a
  common diode model are shown below.
• The diode acts like a 0.7V source when
  forward-biased.
• The diode acts like an open circuit when
  reverse-biased.

22
Lecture 6 EGR 262 Fundamental Circuits Lab
Clamping Circuits Since a diode acts like a 0.7V
source, it can be used to clamp any output of
0.7V or greater to 0.7V. Similarly, if a voltage
source of value Vx is added in series with the
diode, a voltage can be easily clamped to Vx
0.7V.
23
Lecture 6 EGR 262 Fundamental Circuits Lab
Clamping Circuit used in Lab 6 The output of the
comparator will produce either 0V or 9V. This
needs to be converted to 0V or 5V so that it is a
suitable input for the MicroStamp11 in Lab 7. We
can use a diode and our 5V supply to clamp the
comparator output to 5.7V. Then a final
adjustment potentiometer can be added to adjust
the 5.7V to 5.0V. The circuit is shown below.

• Discuss the operation of this circuit.
• The output current of an op amp is typically in
  the mA range and the op amp can be destroyed if
  the output current is too large. If we want the
  output current to be around 1mA, what value of
  Rlimit should be used? (Hint What are the node
  voltages on either side of the resistor?)

24
Lecture 6 EGR 262 Fundamental Circuits Lab
Discuss
Note that Lab 6 involves no programming! It
does, however, use the circuit and program from
Lab 5, so do not take the circuit apart. Lab 6
deals only with the hardware portion of the A/D
circuit. We will deal with the software issues
in Lab 7 and will then have a fully functioning
A/D converter.
25
Lecture 6 EGR 262 Fundamental Circuits Lab
Final Schematic
5V
O1
PD0
1
20
220 ?
O7
a

PD1
2
19
a
5V
O2
O6
b
3
18
O3
c
4
17
b
f
g
O4
O0
d
5
16
O5
e
6
15
c
e
I7
f
7
14

d
I6
g
8
13

220 ?
9
12

10 k?
10
11


Common-anode 7-segment display (see data sheet
for pinout)

MicroStamp11

5V
9V
9V
_
Vin
LMC660

Vbuffer
Vclamp
VDAC
PA3

LMC660
Vcomp
Vref
_
0V
Rlimit
2R
10 k?
0V
R
Buffer
5V
PD0
1N4007
Vo
10 k? pot
2R
10 k? pot
R
PD1
5V
2R
Comparator
Clamp Circuit
2R
R-2R Ladder Network (D/A Converter)
26
Lecture 6 EGR 262 Fundamental Circuits Lab
4.1. Pre-lab Tasks (1) Draw the schematic of a
unity-gain buffer. Explain how the circuit
works. Write an expression for the output
voltage. (2) Draw the schematic of a comparator
circuit that compares a reference voltage (0-9V)
generated by a 10 k? potentiometer to the input
voltage generated by your bu?er circuit. Your
circuit should generate a voltage that is either
0 or 9 volts. Explain how the circuit works.
Write an expression for the output voltage. (3)
Draw the schematic of a clamp circuit that
clamps the comparators voltage to either zero or
?ve volts. Explain how the circuit works. Write
an expression for the output voltage. (4) Show
the calculation for the resistance in the clamp
circuit so that the maximum current drawn from
the source is around 1 mA. (5) Show a
complete schematic including the MicroStamp11,
R2R ladder network, buffer, comparator, and clamp
circuit. (6) Draw the breadboard layout for the
schematic above.
27
Lecture 6 EGR 262 Fundamental Circuits Lab

• 4.2. In-lab Tasks
• (1) Note It is best to build and test one
  circuit at a time rather than building the entire
  circuit.
• Build the buffer circuit and connect it to the
  DAC you built earlier. Test your circuit by
  measuring the buffer output voltage versus the
  buffer input voltage from the DAC for all 8
  possible cases. See sample table on following
  slides. 
• Build the comparator circuit and connect it to
  the bu?er. Test your circuit by measuring the
  comparators output voltage as a function of at
  least 10 di?erent reference voltage levels
  between 0 and 9 volts. Repeat this test for each
  of the 8 possible voltages that can be generated
  by your DAC (80 total measured values). See
  sample table on following slides.
• Build the clamp circuit and connect it to the
  comparator. (The thumbwheel potentiometer seems
  to work best here.) Adjust the potentiometer on
  the output of the clamp circuit to exactly 5V
  when the output of the comparator circuit is 9V
  and then put a piece of tape on the potentiometer
  so that it will not be accidentally changed.
  Test your circuit by measuring the clamps output
  as a function of at least 10 di?erent reference
  voltage levels. Repeat this test for each of the
  8 possible voltages that can be generated by your
  DAC (80 total measured values). See sample table
  on following slides.
• (4) Measure the threshold voltage where the
  comparator/clamp circuit transitions from zero to
  5 volts as a function of the reference voltage
  for each of the 8 possible DAC voltages. See
  sample table on following slides.
• (5) Describe what happened in the lab.
• (6) Demonstrate your circuit to the instructor.
  The instructor will double check the correctness
  of your results and completeness of your lab
  book, sign o? on the book and let you move on to
  the next lab. You may be asked to redo some of
  the tasks if they are not correct or complete.

28
Lecture 6 EGR 262 Fundamental Circuits Lab
1) Buffer output versus buffer input (Buffer
Input DAC Output)
Buffer Input (V) Buffer Output (V)








29
Lecture 6 EGR 262 Fundamental Circuits Lab
2) Comparator output versus input from DAC for 8
reference voltages
Comparator Output Voltage Comparator Output Voltage Comparator Output Voltage Comparator Output Voltage Comparator Output Voltage Comparator Output Voltage Comparator Output Voltage Comparator Output Voltage Comparator Output Voltage Comparator Output Voltage
DAC Output (V) Vref 0.0 V Vref 0.5 V Vref 1.0 V Vref 1.5 V Vref 2.0 V Vref 2.5 V Vref 3.0 V Vref 3.5 V Vref 4.0 V Vref 4.5 V








Graphing this data In the Post-Lab you will need
to graph this data. This can be done with either
10 graphs (one for Vref 0.0, one for Vref
0.5, etc) or a single 3D column chart.
3) Clamping circuit output versus input from DAC
for 8 reference voltages
Clamping Circuit Output Voltage Clamping Circuit Output Voltage Clamping Circuit Output Voltage Clamping Circuit Output Voltage Clamping Circuit Output Voltage Clamping Circuit Output Voltage Clamping Circuit Output Voltage Clamping Circuit Output Voltage Clamping Circuit Output Voltage Clamping Circuit Output Voltage
DAC Output (V) Vref 0.0 V Vref 0.5 V Vref 1.0 V Vref 1.5 V Vref 2.0 V Vref 2.5 V Vref 3.0 V Vref 3.5 V Vref 4.0 V Vref 4.5 V








30
Lecture 6 EGR 262 Fundamental Circuits Lab
4) Buffer output versus buffer input (Buffer
Input DAC Output)
Count Threshold Voltage (Vref where clamp circuit output changes from 0V to 5V)
0
1
2
3
4
5
6
7
4.3. Post-Lab Tasks (1) Plot the buffer output
voltage and input buffer voltage for each of the
8 cases. Assess how well the buffer works. (2)
Plot the experimental data for the comparator
circuit. Note Use either 8 graphs of Vcomp
vs. Vref (1 for each DAC input) or else use one
3D graph (80 points total). Assess how well the
comparator circuit works. (3) Plot the
experimental data for the clamp circuit. Note
Use either 8 graphs of Vclamp vs. Vref (1 for
each DAC input) or else use one 3D graph (80
points total). Assess how well the clamp
circuit works.