eet260: a/d and d/a converters. a/d conversion we will consider process and circuits required to...
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EET260: A/D and D/A converters
A/D conversion We will consider process and circuits required to
convert an analog waveform into a digital output.
011001111000101011001110111111111111111011001010011101010011
Analog-to-digital conversion
0 1 2 3 4 5 6
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
Time (ms)
Vol
tage
(V
)
A/D conversion A modern A/D converter is a single-chip IC
which performs the following processes.
[ADC IC picture]
Anti-aliasingfilter
Sample-and-hold(S/H) circuit
A/D converter
011001111000101011001110111111111111111011001010011101010011
Sampling and aliasing Sampling is basically a multiplication process,
and it gives rise to sidebands just like AM modulation does.
analog input
sampling pulses
sampled output
The frequency spectrum resulting from sampling looks like:
Decreasing the sampling frequency ( fs ), we get:
If we continue to decrease fs to a value less than the Nyquist rate, aliasing (overlapping of the spectra) occurs:
Sampling and aliasing (frequency)
We get a carrier and upper and lower sidebands at multiples of the sampling frequency: fs, 2fs, 3fs , etc.
Sampling and aliasing To eliminate the problem, a low-pass filter called
an anti-aliasing filter is included at the input of the A/D converter to block any frequencies higher than one-half the sampling frequency.
Anti-aliasingfilter
Sample-and-hold(S/H) circuit
A/D converter
Sample-and-hold circuit A/D conversion begins with sampling, which is
carried out by a sample-and-hold (S/H) circuit. The S/H circuit takes a precise measurement of
the analog voltage at specified intervals.
Anti-aliasingfilter
Sample-and-hold(S/H) circuit
A/D converter
Sample-and-hold circuit A sample-and-hold (S/H) circuit accepts the
analog input signal and passes it through, unchanged, during its sampling mode.
In the hold mode, the amplifier remembers or memorizes a particular voltage level at the instant of sampling.
The output is a fixed DC level whose amplitude is the value at the sampling time.
Conversion The constant S/H output during the sampling
interval permits accurate quantization. The last step is the conversion from an analog
voltage into a binary number. Two common converters are:
Successive-approximation converter Flash converter
Anti-aliasingfilter
Sample-and-hold(S/H) circuit
A/D converter
Comparator Comparators are found in both types of A/D
converters. A comparator compares two analog inputs and
produces a binary output.
+A
B
+Vcc= +5 V
-Vcc= 0 V
C
inputs outputs
if A > B C is true (+5 V)
if A < B C is false (0 V)
Successive-approximation converter
This converter contains an 8-bit successive-approximations register (SAR).
Successive-approximation converter
Special logic in the register causes each bit to be turned on one at a time from MSB to LSB until the closest binary value is stored in the register.
At each clock cycle, a comparison is made. If the D/A output is greater than the analog input, that
bit is turned off (set to 0) If the D/A output is less than the analog input, that bit
is left on (set to 1). Process repeats until 8 bits are checked.
Successive-approximation converter
If the clock frequency is 200-kHz, how long does it take to complete the conversion for an 8-bit D/A converter?
Successive-approximation converter
Successive-approximation converters are fast and consistent.
Conversion times range from 0.25 to 200 ms and 8-, 10-, 12-, and 16-bit versions are available.
Flash converter A flash converter uses a large resistive voltage
divider and multiple analog comparators.
Flash converter The encoder logic circuit converts the 7-bit input
from the comparators into a 3-bit binary output. The flash converter produces an output as fast
as the comparators can switch and the signals can be translated to binary levels by the logic circuits.
Flash converters are the fastest type of A/D converter.
Flash converter The number of comparators is equal to 2N – 1,
where N is the number of desired output bits.
0 1 2 3 4 5 6 7 8 9 10 110
200
400
600
800
1000
1200
n-bits
Num
ber
of c
ompa
rtor
s (2
n )
Number of Comparators Required for an n-bit Flash ADC
Flash converter Flash A/D converters are complicated and
expensive but are the best choice for high-speed conversions. Conversion speeds < 100 ns are typical. Speed less than 0.5 ns are possible.
6-, 8-, 10-bit flash converters are available.
Digital-to-analog (D/A) conversion We will consider a circuit which convert a digital
signal into and analog output.
Analog-to-digital conversion
0 1 1 1 0 0 1 1 0 1 1 1 1 1 1 1 1 0 0 1 1 0 0 1
0 1 2 3 4 5 6
-1.00
-0.75
-0.50
-0.25
0.00
0.25
0.50
0.75
1.00
Time (ms)
Vol
tage
(V
)
D/Aconversion
D/Aconversion
Digital-to-analog (D/A) conversion One of the most popular D/A circuits is the R-2R
ladder shown below
R-2R ladder DAC The output voltage is given .
f 1 1Ref 1 2
12 2 2
n no n
R b b bV V
R
b0
b1
b2
b3Vref
Vo
Assume the R-2R ladder DAC circuit below as the following values: R = 50 k and Rf = 100 k. Assume Vref = 1 V. Determine the D/A converter output for the following binary inputs.
a. 00002
b. 00012
c. 01012
d. 11112
e. What is the resolution of this D/A converter. b0
b1
b2
b3Vref
Vo
Example Problem 1