Download A/D successive approximation converter.

5. AD converter with successive approximation
Questions, preparation at home
a)
b)
c)
d)
e)
Do you have USB flash-disc with you?
What are the basic blocks of the AD converter with successive approximation?
What is the purpose of the voltage comparator?
Compare Fig.3 and Fig.6 in [1]. What are the differences?
Which bit is being sent first in the serial communication protocol of the WSH570 and
ADS8512 ADCs? MSB or LSB?
f) At which edge of the clock signal are the data valid? Rise or fall?
g) What are the differences between the two ADCs? (serial protocol, power supply, input
ranges, resolution etc.)
h) The logic analyzer has two basic modes: synchronous and asynchronous analyses. The
acquisition of the input signals can be done in two different ways: state analyses, time
analyses. Can you describe the difference?
Task
a) Describe the functionality of the AD converter with successive approximation and make
drawing of the diagram to your notebook. Write up the differences between the ADCs
WSH 570 a ADS8512.
b) Explore the functionality and possibilities of the digital oscilloscope MSO4000 with
integrated logic analyzer. Be ready to explain how to set-up a mode of single acquisition,
continual acquisition, time base parameters, analog channel gain, selection of active
digital channels and the storage of acquired data on USB drive.
c) Check the connections / wiring of the tools with the AD converters, power supply
connection. Switch on the power supplies. Switch on the power supplies for the AD
converters FIRST (basically, circuits should be powered before there is some voltage on
their inputs).
d) In the time analyses mode show on the oscilloscope screen following signals: (do it for
two different voltage levels at the ADC input: 4.950 and 5.050V)
1) Parallel output B1 to B8, STATUS (status output), S OUT (serial output), CLK
(clock signal), START A/C – connected to the digital inputs of the logic analyzer.
2) Output before the voltage comparator – output at the application board of the
WSH570 ADC (see Fig.1 and Fig.3), marked as COMP [1]. Signal must be
connected to the analog input channel of the oscilloscope.
Store the waveforms to your USB flash-disc and after necessary modifications insert into
your notebook – to the protocol about the measurement.
e) Explain the connection between the analog value of the signal COMP (output of the DA
converter summed with the input) and the digital output word of the converter.
f) Measure the transfer function of the AD converters in few points (e.g. 0,000 V; 0,039 V;
1,000 V; 2,500 V; 5,000 V; 7,500 V; 9,000 V; 9,961 V). You will need to apply some
type of a mathematical correction to one of the outputs (e.g. to cut the output of the
ADS8512 ADC to 8-bits, invert the bit values).
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Optional measurements
g) Measure the time of one conversion of the WSH570 ADC.
h) Measure the time of one conversion of the ADS8512 ADC; measure also the time needed
to send the output data through the serial interface.
i) Measure the clock signals for both ADCs, describe the differences (frequency, duty cycle).
j) Measure the amplitude of an overshoot on DATA CLK signal of the ADS8512 ADC
(overshoot is generated at rise and fall edge of the SPI clock signal).
k) Observe the changes in the signals if the frequency of triggering pulses is increased
(increased in such a way, that the triggering period is shorter than the time of a single
conversion)
l) Compare the representation (how they are displayed) of the triggering pulses acquired by
the analog input channel and the digital input channel, explain the differences.
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Settings of the instruments
Impulse generator settings:
output A
frequency 100 kHz
pulse length 1 µs
maximum amplitude
Block diagram of the measurement setup
The signals shown on Fig.2 correspond to circuit-diagram shown on Fig.1. More detailed
information about the internal circuit of the WSH570 ADC can be found in [1] (as well as the
detailed description of the functionality of all inputs).
Fig.1 Block diagram of the measurement
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Fig.2 Acquired analog and digital signals
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Description of an AD converter with successive approximation
The 8-bit ADC Tesla WSH 570 [1] made by “hybrid” technology contains: reference voltage
supply, 8-bit DAC, voltage comparator, approximation register and clock generator. See Fig.3 for
simplified circuit diagram.
The ADC converts the input analog voltage by successive approximation by 8 internally
generated, binary scaled voltage weights (5V, 2.5V, 1.25V...). The ADC sends 9 clock pulses
during the conversion (STATUS signal is at high level) and eight data bits which are synchronous
with the clock. The parallel output is not valid during the conversion. The conversion is finished
and the parallel output valid when the STATUS output goes back to low level.
Fig.3 Simplified internal structure of the WS 570 ADC
Compare the block diagram shown on Fig.3 with the implementation of the ADS8512 ADC,
which is shown on first page of [2]. Application circuit of the ADS8512 ADC is shown on Fig.4,
it requires minimal amount of external components for its operation.
Fig.4 Application circuit of the ADS8512 ADC [2]
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The application board of the ADS8512 ADC allows to connect: input voltage to (R1IN),
conversion start input (/CONV), output indicating that the conversion is in progress (/BUSY) and
two output signals of the serial interface SPI (DATA a DATACLK). The converter can convert an
input voltage within a range –10V...+10V, thanks to built-in input resistor divider. On the
contrary the WSH570 ADC can convert only positive input voltage within a range 0...10V (-1
LSB). One LSB corresponds to 39,06 mV for WSH570 and to 4,88 mV for ADS8512 ADC. The
output coding of both ADC is shown in Tab.1.
Tab.1 Output codes of the two used ADCs
Output code CSB
UX [V]
ADC WSH 570
Input range: 0...+10 V
MSB
+10
+ 9,961
+ 9,922
...
+ 7,500
...
+ 5,039
+ 5,000
+ 4,961
...
+ 2,500
...
+ 0,039
0
LSB
0000 0000
0000 0001
...
0011 1111
...
0111 1110
0111 1111
1000 0000
...
1011 1111
...
1111 1110
1111 1111
Output code
ADC ADS8512
Input range: –10...+10 V
MSB
0111 1111
0111 1111
0111 1111
...
0110 0000
...
0100 0000
0100 0000
0111 1111
...
0000 0010
...
0000 0000
0000 0000
LSB
1111
1000
0000
0000
0111
0000
1000
0000
0111
0000
Literature
[1] Documentation to WSH570 ADC
[2] ADS8512 ADC datasheet, available online:
http://focus.ti.com/docs/prod/folders/print/ads8512.html
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