Physics 327 – Lab 8
2/16/16
Physics 327, Spring 2009
Lab 8: DAC and ADC
Goal: Convert digital signals to analog and analog to digital.
A. Introduction
This week we will set up and examine an Analog to Digital Converter (ADC) and a
Digital to Analog Converter (DAC). If time permits, you can connect the ADC and DAC
together to get back to original analog input. Arrange your board neatly to facilitate this.
Also, you will need +/- 12V and 5 V supply voltage levels, though +12V is only for an
op-amp.
B. The MC1408 (DAC0808) Digital to Analog Converter (DAC)
A drawing of an MC1408 DAC circuit is attached. Remember that the pins on the chip
are numbered counterclockwise; the order here is mangled for ease of drawing the
circuit. Set this circuit up.
Connect all input pins 5 -- 12 to ground, and measure Vout of the opamp. If it is not zero
adjust a 10 K pot to give zero out. The most significant bit (MSB) is pin #5 and the least
significant bit (LSB) is pin #12. The reference voltage will be +5 V. The output is
should be given by
R  PB7 PB6
PB0 
V out  Vref  o 

 
(1)
 R1  2
4
256 
where PB# is 1 if true (+5V) or 0 if false (0V,ie, grounded). Begin by grounding all digital
inputs and check the output. Then connect PB7 to +5V and measure V out. Next
ground PB7 and connect PB6 to +5V and measure V out. Repeat for each pin. When
only PB0 is connected can you see the difference between zero and V ref/256? Try a
few combinations (e.g. #5 and #6, #5,6,7) and check that the output agrees with the
formula.
Ground all inputs except #5 (PB7). Use the function generator with square pulse output
at about 1 kHz to measure the threshold for pin 5 to cause the output to change. It
should be about 1.35 V. Is the output shape a good square pulse? Describe the rising
part of the output in your report.
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Physics 327 – Lab 8
2/16/16
C. The ADC0809 Analog to Digital Converter (ADC)
A drawing of a 0809 ADC circuit is shown on the next page. Set this circuit up. Initially,
use a 500 kHz square pulse from the function generator as input to the clock. Clock
input voltage levels should extend at least from 0 to 5 V. The inputs must be
POSITIVE. Negative voltages can damage the ADC. Before applying the clock signal
to the ADC, display the clock signal on the scope(dc couple) and check with instructor.
Apply a DC voltage of about 0.5 V to IN0, pin 26. If the chip does not convert when you
turn power on, momentarily apply +5 V to the start pin, then leave it open. Connecting
EOC (end of conversion) to start should cause the chip to continuously convert once it
has been started. Does the digital output make sense? Remember the output is binary,
ranging from about 0 V to Vref, and from 0 to 11111111. Repeat this for at least four
other voltages to check linearity. Also adjust one voltage slightly to see if you can
determine what voltage change corresponds to a change of one unit in the LSB and
explain how you did this in your lab report.
Measure the conversion time of the ADC. Disconnect EOC from START. Use a 555
timer to put a ~1 kHz square pulse into START, and monitor this on the oscilloscope.
Also monitor EOC on the oscilloscope. The time difference between HILO of the start
and LOHI of EOC is the conversion time. Redo this exercise at a clock frequency of
200 kHz. Do you find any simple relationship between clock frequency and conversion
time? Can you explain it?
D. Analog to Digital to Analog (optional)
Reset the ADC for continuous conversion. Use the 555 at an output frequency to a few
hundred kHz as the clock input. Be careful: it does not give out a good square pulse at
too high a frequency. Reset the function generator to a few hundred Hz triangular pulse,
with a DC offset such that the signal is always positive, and connect to IN0. Hook up the
ADC outputs to the appropriate DAC inputs. Compare on the scope the input triangular
wave with the DAC / opamp output triangular wave. Show and explain the differences in
your lab report. What happens as you increase the input triangle frequency?
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Physics 327 – Lab 8
2/16/16
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