adc/dac/analog interface - asian institute of technologyesl.ait.ac.th/courses/at74.08/class10 -...
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ADC/DAC/Analog interface
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Analog I/O
• Analog inputs – convert to digital using an Analog
to Digital converter (A/D or ADC)
• Analog output – convert digital output to analog
using a Digital to Analog converter (D/A or DAC)
• A/D outputs and D/A inputs can be attached to
digital I/O ports
• Design issues to consider – number of bits of
accuracy, conversion time delay, and sample rate
needed by application
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The MCP3008 10-bit 200Ksps A/D chip used in Phidget modules has an SPI interface.
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Off-the-shelf ADC
• Resolution – smallest distinguishable change in input
• Precision – number of distinguishable inputs
• Accuracy – the absolute error of the entire system
• Monotonic – no missing codes
• Linear – constant resolution
• Speed – time to convert
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Analog-digital interface
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Processing analog signal
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Analog input signal
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Analog input signal
• For periodic waveforms, the duration of
the waveform before it repeats is called
the period of the waveform
• The rate at which a regular vibration
pattern repeats itself (frequency =
1/period)
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Frequency of a Waveform
• The unit for frequency is cycles/second,
also called Hertz (Hz).
• The frequency of a waveform is equal to
the reciprocal of the period.
frequency = 1/period
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Frequency of a Waveform
• Examples:
frequency = 10 Hz
period = .1 (1/10) seconds
frequency = 100 Hz
period = .01 (1/100) seconds
frequency = 261.6 Hz (middle C)
period = .0038226 (1/ 261.6) seconds
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Waveform Sampling (Quantization)
• To represent waveforms on digital computers,
we need to digitize or sample the waveform.
• side effects of digitization:
– introduces some noise
– limits the maximum upper frequency range
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Sampling rate
• The sampling rate (SR) is the rate at which
amplitude values are digitized from the
original waveform.
– CD sampling rate (high-quality):
SR = 44,100 samples/second
– medium-quality sampling rate:
SR = 22,050 samples/second
– phone sampling rate (low-quality):
SR = 8,192 samples/second
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Sampling rate
• Higher sampling rates
allow the waveform to
be more accurately
represented
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Nyquist Theorem
• Nyquist Theorem:
We can digitally represent only frequencies up to half the sampling rate.
– Example:
CD: SR=44,100 Hz
Nyquist Frequency = SR/2 = 22,050 Hz
– Example:
SR=22,050 Hz
Nyquist Frequency = SR/2 = 11,025 Hz
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Nyquist Theorem
Sampling rate (SR) > 2 fmax
fmax is the largest signal frequency of interest
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Nyquist Theorem and Aliasing
• Graphical Example 1a:
– SR = 20,000 Hz
– Nyquist Frequency = 10,000 Hz
– f = 2,500 Hz (no aliasing)
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Nyquist Theorem and Aliasing
• Graphical Example 1b:
– SR = 20,000 Hz
– Nyquist Frequency = 10,000 Hz
– f = 5,000 Hz (no aliasing)
(left and right figures have same frequency, but have
different sampling points)
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Nyquist Theorem and Aliasing
• Graphical Example 2:
– SR = 20,000 Hz
– Nyquist Frequency = 10,000 Hz
– f = 10,000 Hz (no aliasing)
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Nyquist Theorem and Aliasing
• Graphical Example 2:
– BUT, if sample points fall on zero-crossings
the sound is completely cancelled out
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Nyquist Theorem and Aliasing
• Graphical Example 3:
– SR = 20,000 Hz
– Nyquist Frequency = 10,000 Hz
– f = 12,500 Hz, f' = 7,500
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Nyquist Theorem and Aliasing
• Graphical Example 3:
– Fitting the simplest sine wave to the sampled
points gives an aliased waveform (dotted line
below):
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Sample of sine wave at different
freq.
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Processing analog signal
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Low pass filter
• Allow only low
frequency value to
pass
• Prevent aliasing
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Layout of ADC
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ADC precision
• Number of ADC bit output (n):
n = input range (r) / input resolution y
E.g., input range of 1, and resolution of 0.0001
n = 10000 alternatives or 15 bits value
Assume linear ADC
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Sample and hold circuit
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Sample and hold circuit
• Using op-amp to hold signal strength
unity gain buffer
Vin
Vout
control
+
-
+
-
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Sample and hold signal
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Hold circuit output
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Quantized output
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Convert analog value to digital
value
Bipolar codes Offset binary 2s binary
+5.00 1111 0111
+2.50 1100 0100
+0.04 1000 0000
+0.00 1000 0000
-2.50 0100 1100
-5.00 0000 1000
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ADC types
• Flash ADC
- fast
• Successive Approximation ADC
- most popular
• Sigma Delta ADC
- highest output precision
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Flash ADC
• Use reference voltage and differential op-
amp to generate digital output
• Fast conversion
• To increase the number of bits, it requires
larger hardware support
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Two-bit flash ADC
Vin X3 X2 X1 Z1 Z0
V<2.5 0 0 0 0 0
2.5V<5.0 0 0 1 0 1
5.0V<7.5 0 1 1 1 0
V 7.5 1 1 1 1 1
+
-
+
-
+
-
REF
20 k
10V
20 k
20 k
20 k Vin
20 k
5V
20 k
5V
20 k
5V
Z1
Z0
2.5V
5.0V
7.5VX3
X2
X1
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Flash converter
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256R ladder network
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Successive Approximation ADC
• Similar to Flash ADC, but use only one set
of hardware
• Require longer time
• Fixed size of hardware
• Widely used
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Successive Approximation ADC
Vin
+
-
N bit
Successive
Approximation
Clock
Done
Go
N bit
ADC
n
5V
Digital
output
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The MCP3008 10-bit 200Ksps A/D chip used in Phidget modules has an SPI interface.
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Successive approximation counting
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Block diagram of internal AD
converter
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AD converter
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3-bit AD converter
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Delta-Sigma ADC
• Quantization noise is uniformly distributed
among frequency spectrum
• Using over-sampling technique to reduce the
impact of quantization noise
• Results in higher bit-precision
• Slow
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Frequency domain
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Oversampling by K times
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Signal pin of AD converter
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Digital to Analog Converter
(DAC)
50
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Layout of DAC
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Analog of digital conversion
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Analog voltage of zero order
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Reconstruction
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Summing Amplifier
• Scale factors for the 2
inputs can be
independently adjusted by
proper choice of R2 and R1.
• Any number of inputs can
be connected to summing
junction through extra
resistors.
• This is an example of a
simple digital-to-analog
converter.
1
1v
1i
R
2
2v
2i
R
3
ov3
iR
Since negative amplifier input is
at virtual ground,
Since i-=0, i3= i1 + i2,
2v
2
31
v
1
3ov
R
R
R
R
Chap 11-55
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Binary weight register method
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DA converter
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DA pin signal
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Connection of DAC
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Analog interfacing network
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Analog-digital-analog path
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Applications
• Body Sensor Network
• Personal Health Monitor
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Body Sensor Network
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Personal Health Monitor
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Questions?
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