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Data Converter

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Title: Data Converter


1
Data Converter
2
Design Of A Wireless Sensing
3
Analog to Digital (A/D) Converter
  • Input signal
  • Sampling rate
  • Throughput
  • Resolution
  • Range
  • Gain

4
Fundamentals of Sampled Data Systems
Analog-to-Digital converters (ADCs) translate
analog quantities, wich are characteristic of
most phenomen in the real world to digital
language, used in information processing,
computing, data transmission, and control
systems Digital-to-Analog converters (DACs) are
used in transforming transmitted or stored data,
or the results of digital processing, back to
real world variables for control, information
display, or further analog processing
5
Digital Number
Digital number used are all basically binary
that is, each bit or unit of information has
one of two possible states. These state are
off, false, or 1 on,
true , or 0 It is also possible to
represent the two logic state by two different
levels of current however, this is much less
popular than using voltages . Word are
groups of levels representing digital numbers
the levels may appear simultaneously in paralel
, on a bus or groups of gate inputs or
outputs, serially (or in time sequence) on a
single line, as a sequence of parallel bytes
(i.e. byte serial) or nibbles (small
bytes) A unique parallel or serial grouping of
digital levels, or a number, or code, is assigned
to each analog level which is quantized (i.e.,
represents a unique portion of the analog range).
6
Typical Digital Code
A typical digital code would be this array
The meaning of the code, as either a number, a
character, or a representation of an analog
variable is unknow until the code and the
conversion relationship have been defined
7
Unipolar Code
8
BipolarCodes
9
Quantization The Size of a Least Significant Bit
(LSB)
Resolution N 2N VOLTAGE (10V FS) ppm FS FS dB FS
2-bit 4 2.5V 250.000 25 -12
4-bit 16 625mV 62.500 6.25 -24
6-bit 64 156mV 15.625 1.56 -36
8-bit 256 39.1mV 3.906 0.39 -48
10-bit 1.024 9.77mV (10mV) 977 0.098 -60
12-bit 4.096 2.44mV 244 0.024 -72
14-bit 16.384 610mV 61 0.0061 -84
16-bit 65.536 153mV 15 0.0015 -96
18-bit 262.144 38mV 4 0.0004 -108
20-bit 1.048.576 9.54mV (10mV) 1 0.001 -120
22-bit 4.194.304 2.38mV 0.24 0.000024 -132
24-bit 16.777.216 596nV 0.06 0.000006 -144
The resolution of data converters
10
The Ideal Transfer Function (ADC)
The theoretical ideal transfer function for an
ADC is a straight line, however, the practical
ideal transfer function is a uniform staircase
characteristic shown in Figure .
11
The Ideal Transfer Function (DAC)
The DAC theoretical ideal transfer function would
also be a straight line with an infinite number
of steps but practically it is a series of points
that fall on the ideal straight line as shown in
Figure
12
Sources of Static Error
Static errors, that is those errors that affect
the accuracy of the converter when it is
converting static (dc) signals, can be completely
described by just four terms. These are
Each can be expressed in LSB units
or sometimes as a percentage of the FSR
  • offset error,
  • gain error,
  • integral nonlinearity and
  • differential nonlinearity.

13
Offset Error - ADC
The offset error is efined as the difference
between the nominal and actual offset points.
14
Offset Error - DAC
For a DAC it is the step value when the digital
input is zero. This error affects all codes by
the same amount and can usually be compensated
for by a trimming process. If trimming is not
possible, this error is referred to as the
zero-scale error.
15
Gain Error - ADC
The gain error is defined as the difference
between the nominal and actual gain points on the
transfer function after the offset error has been
corrected to zero. For an ADC, the gain point is
the midstep value when the digital output is full
scale,
16
Gain Error - DAC
For a DAC it is the step value when the digital
input is full scale. This error represents a
difference in the slope of the actual and ideal
transfer functions This error can also usually be
adjusted to zero by trimming.
17
Differential Nonlinearity (DNL) Error - ADC
DNL is the difference between an actual step
width (for an ADC) and the ideal value of 1 LSB.
Therefore if the step width is exactly 1 LSB,
then the differential nonlinearity error is zero.
If the DNL exceeds 1 LSB ? nonmonotonic (this
means that the magnitude of the output gets
smaller for an increase in the magnitude of the
input) If the DNL error of 1 LSB there is also
a possibility that there can be missing codes
i.e., one or more of the possible 2n binary codes
are never output.
18
Differential Nonlinearity (DNL) Error - DAC
The differential nonlinearity error shown in
Figure is the difference between an actual step
height (for a DAC) and the ideal value of 1 LSB.
Therefore if the step height is exactly 1 LSB,
then the differential nonlinearity error is zero
19
Integral Nonlinerity (INL) Error - ADC
The integral nonlinearity error shown in Figure
is the deviation of the values on the actual
transfer function from a straight line. This
straight line can be either a best straight line
which is drawn so as to minimize these deviations
or it can be a line drawn between the end points
of the transfer function once the gain and offset
errors have been nullified (end-point linearity )
20
Integral Nonlinerity (INL) Error - DAC -
The name integral nonlinearity derives from the
fact that the summation of the differential
nonlinearities from the bottom up to a particular
step, determines the value of the integral
nonlinearity at that step.
21
Absolute Accuracy (Total) Error -ADC-
The absolute accuracy or total error of an ADC as
shown in Figure is the maximum value of the
difference between an analog value and the ideal
midstep value. It includes offset, gain, and
integral linearity errors and also the
quantization error in the case of an ADC
22
Absolute Accuracy (Total) Error -DAC-
23
Sampling Theory
Prior to the actual analog-to-digital conversion,
the analog signal usually passes through some
sort of signal conditioning circuitry which
performs such functions as amplification,
attenuation, and filtering. The
lowpass/bandpass filter is required to remove
unwanted signals outside the bandwidth of
interest and prevent aliasing. There are two
key concepts involved in the actual
analog-to-digital and digital-to-analog
conversion process An understanding of
these concepts is vital to data converter
applications.
  • discrete time sampling and
  • finite amplitude resolution due to quantization.

24
Sampling Theory
The system shown in Figure is real-time system
i.e., the signal to the ADC is continuously
sampled at a rate equal to fS, and the ADC
presents a new sample to the DSP at this rate. In
order to maintain real-time operation, the DSP
must perform all its required computation within
the sampling interval, 1/fS, and present an
output sample to the DAC before arrival of the
next sample from the ADC.
25
The Need for a Sample-and-Hold Amplifier (SHA)
Function
Most ADCs today have a built-in-sample-and-hold
function, thereby allowing them to process ac
signals. This type of ADC is referred to as a
sampling ADC If the input signal to a SAR ADC
(assuming no SHA function) changes by more than
1LSB during the conversion time (8ms is the
example), the output data can have large errors,
depending on the location of the code Most ADC
architectures are subject to this type of error
some more, some less with the possible
exception of flash converters having well-matched
comparators
26
Input Frequency Limitations of Nonsampling ADC
(Encoder)
This implies any input frequency greater than 9.7
Hz is subject to conversion errors, even though a
sampling frequency of 100 kSPS is possible with
the 8ms ADC (this allows an extra 2ms interval
for an external SHA to reacquire the signal after
coming out of hold mode).
27
Sample-and-Hold Function Required for Digitizing
AC Signals
Sample-and-hold amplifier (SHA) Track-and-hold
amplifier (THA).
28
The Nyquist Criteria
A continuous analog signal is sampled at discrete
intervals, fS,which must be carefully chosen to
ensure an accurate representation of the original
analog signal The Nyquist criteria requiries that
the sampling frequency be at least twice the
highest frequency contained in the signal, or
information about the signal will be lost If the
sampling frequency is less than twice the maximum
analog signal frequency, a phenomen know as
aliasing will occur
  • A signal with a maximum frequency .. must be
    sampled at a rate .... or information about the
    signal will be lost because of aliasing
  • Aliasing occurs whenever ...
  • A signal which has frequency components between
    .. and.... must be sampled at a rate ...... in
    order to prevent alias components from
    overlapping the signal frequencies

29
Aliasing in Time Domain
In order to understand the implications of
aliasing in both the time and frequency domain,
first consider the case of a time domain
representation of a single tone sinewave sampled
as shown in Figure
30
Matlab Example - 1
31
Matlab Example - 2
32
Matlab Example - 3
33
Matlab Example - 4
34
Matlab Example - 5
35
Aliasing in Frequency Domain
Consider the case of a single frequency sinewave
of frequency fa sampled at a frequency fs by an
ideal impulse sampler. Also assume that fs gt 2fa
as shown. The frequency-domain output of the
sampler shows aliases or images of the original
signal around every multiple of fs, i.e. at
frequencies equal to Kfs fa, K 1, 2, 3,
4, .....
36
Baseband Antialiasing Filter
Baseband sampling implies that the signal to be
sampled lies in the first Nyquist zone. It is
important to note that with no input filtering at
the input of the ideal sampler, any frequency
component (either signal or noise) that falls
outside the Nyquist bandwidth in any Nyquist zone
will be aliased back into the first Nyquist
zone. For this reason, an antialiasing filter is
used in almost all sampling ADC applications to
remove these unwanted signals. The antialiasing
filter transition band is therefore determined by
the corner frequency fa, the stopband frequency
fs fa, and the desired stopband attenuation,
DR. The required system dynamic range is chosen
based on the requirement for signal
fidelity. For instance, a Butterworth filter
gives 6-dB attenuation per octave for each filter
pole (as do all filters). Achieving 60 dB
attenuation in a transition region between 1 MHz
and 2 MHz (1 octave) requires a minimum of 10
polesnot a trivial filter, and definitely a
design challenge.
37
Oversampling Relaxes Requirementson Baseband
Antialiasing Filter
The effects of increasing the sampling frequency
by a factor of K, while maintaining the same
analog corner frequency, fa, and the same dynamic
range, DR, requirement. The wider transition band
(fa to Kfs fa) makes this filter easier to
design
38
Comparing a Nyquist rate (a) and Oversampling
strategies (b)
39
Data Converter AC Error
The only errors (dc or ac) associated with an
ideal N-bit data converter are those related to
the sampling and quantization processes. The
maximum error an ideal converter makes when
digitizing a signal is ½ LSB. The transfer
function of an ideal N-bit ADC is shown in Figure
40
Quantization Noise as a Function of Time
41
FFT diagram of a multi-bit ADC with a sampling
frequency FS
This noise is approximately Gaussian and spread
more or less uniformly over the Nyquist bandwidth
dc to fs/2.
42
Theoretical Signal-to-Quantization Noise Ratioof
an Ideal N-Bit Converter
43
Procces Gain
In many applications, the actual signal of
interest occupies a smaller bandwidth, BW. If
digital filtering is used to filter out noise
components outside the bandwidth BW, then a
correction factor (called process gain) must be
included in the quation to account for the
resulting increase in SNR.
44
(No Transcript)
45
SINAD, ENOB, SNR
46
Dynamic Range
47
Spurious Free Dynamic Range (SFDR)
Probably the most significant specification for
an ADC used in a communications application is
its spurious free dynamic range (SFDR). SFDR of
an ADC is defined as the ratio of the rms signal
amplitude to the rms value of the peak spurious
spectral content measured over the bandwidth of
interest. SFDR is generally plotted as a
function of signal amplitude and may be
expressed relative to the signal amplitude (dBc)
or the ADC full-scale (dBFS) as shown in Figure
48
Aperture Time, Aperture Delay Time, and Aperture
Jitter
49
Design a Low-Jitter Clock for High-Speed Data
Converter
Many modern, high speed, high performance ICs
ADCs require a low-phase-noise (low-jitter)
clock that operates in the GHz range Conventional
crystal oscillators may provide a low jitter
clock signal, but are not generally available in
oscilating frequencies above 120 MHz
Typical high-speed data converter system
50
Jitter in clock signal degrades the ADC
signal-to-noise ratio.
Jitter is generally defined as short-term,
non-cumulative variation of the significant
instant of a digital signal from its ideal
position in time. Figure illustrates a sampling
clock signal that contains jitter. Jitter
generated by the clock is caused by various
internal noise sources, such as thermal noise,
phase noise, and spurious noise. A clock signal
that has cycle-to-cycle variation in its duty
cycle is said to exhibit jitter. Clock jitter
causes an uncertainty in the precise sampling
time, resulting in a reduction of dynamic
performance.
51
How Clock Jitter Degrades ADC's Signal-to-Noise
Ratio (SNR)
52
The functional diagraman integer-N PLL system
Consists of a phase detector (or comparator), an
output charge-pump, a dual modulus prescalar, an
N counter, and an R counter. The N counter
consists of a main (M) counter and a swallow or
auxiliary (A) counter. The N counter then works
in conjunction with the dual modulus pre-scalar
(P)
53
Basic DAC Structures
1-Bit DAC Changeover Switch (Single-Pole, Double
Throw, SPDT)
switching an output between a reference and
ground or between equal positive and negative
reference voltages, as a 1-bit DAC
Such a simple device is a component of many more
complex DAC structures, and is used, with
oversampling, as the basic component in many of
the sigma-delta DACs
54
The Comparator A 1-Bit ADC
As a changeover switch is a 1-bit DAC, so a
comparator is a 1-bit ADC. If the input is above
a threshold, the output has one logic value,
below it has another. Comparators used as
building blocks in ADCs need good resolution
which implies high gain. This can lead to
uncontrolled oscillation when the differential
input approaches zero. In order to prevent this,
hysteresis is often added to comparators using a
small amount of positive feedback
55
The Comparator A 1-Bit ADC cont.
Most modern comparators used in ADCs include a
built-in latch which makes them sampling devices
suitable for data converters. A typical
structure is shown in Figure
The latch thus performs a track-and-hold
function, allowing short input signals to be
detected and held for further processing.
56
ADC Architectures
Flash Converters Successive Aproximation
ADCs Pipelined ADCs Integrating ADC Sigma-Delta
ADC
57
Classification ADC
Most ADC applications today can be classified
into four broad market segments (a) data
acquisition, (b) precision industrial
measurement, (c) voiceband and audio, and
(d) high speed (implying sampling rates
greater than about 5 MSPS). A very large
percentage of these applications can be filled by
A basic understanding of these, the
three most popular ADC architecturesand their
relationship to the market segmentsis a useful
supplement to the selection guides and search
engines.
  • successive-approximation (SAR),
  • sigma-delta (?-?), and
  • pipelined ADCs

58
ADC Architectures, applications, resolution and
sampling rates - 1
59
ADC Architectures, applications, resolution and
sampling rates - 2
60
Flash Converters
Flash analog-to-digital converters, also known as
parallel ADCS, are the fastest way to convert an
analog signal to a digital signal. An N-bit
flash ADC consists of 2N resistors and 2N1
comparators arranged as in Figure. Since 2N1
data outputs are not really practical, they are
processed by a decoder to generate an N-bit
binary output.
  • very large bandwidths.
  • consume a lot of power,
  • have relatively low resolution,
  • can be quite expensive

61
Architecture Detail
The reference voltage for each comparator is one
least significant bit (LSB) greater than the
reference voltage for the comparator immediately
below it.
62
Sparkle Codes and Metastability
Normally, the comparator outputs will be a
thermometer code, such as 00011111. Errors may
cause an output like 00010111 (i.e., there is a
spurious zero in the result). This out of
sequence "0" is called a sparkle. This may be
caused by imperfect input settling or comparator
timing mismatch. The magnitude of the error can
be quite large. Modern converters employ an
input track-and-hold in front of the ADC along
with an encoding technique that suppresses
sparkle codes. When a digital output of a
comparator is ambiguous (neither a one nor a
zero), the output is defined as metastable.
Metastability can be reduced by allowing more
time for regeneration. Gray-code encoding can
also greatly improve metastability.
63
Successive-Approximation ADCs
The successive-approximation ADC is by far the
most popular architecture for data-acquisition
applications, especially when multiple channels
require input multiplexing. Modern IC SAR ADCs
are available in resolutions from 8 bits to 18
bits, with sampling rates up to several MHz.
Output data is generally provided via a
standard serial interface (I2C or SPI), but some
devices are available with parallel outputs
64
Operation Algorithm
In order to process rapidly changing signals, SAR
ADCs have an input sample-and-hold (SHA) to keep
the signal constant during the conversion
cycle. The conversion starts with the internal
D/A converter (DAC) set to midscale. The
comparator determines whether the SHA output is
greater or less than the DAC output, and the
result (the most-significant bit (MSB) of the
conversion) is stored in the successive-approximat
ion register (SAR) as a 1 or a 0. The DAC is
then set either to 1/4 scale or 3/4 scale
(depending on the value of the MSB), and the
comparator makes the decision for the second bit
of the conversion The result (1 or 0) is stored
in the register, and the process continues until
all of the bit values have been determined. At
the end of the conversion process, a logic signal
(EOC, DRDY, BUSY, etc.) is asserted. The
acronym, SAR, which actually stands for
successive-approximation registerthe logic block
that controls the conversion processis
universally understood as an abbreviated name for
the entire architecture.
65
Basic Successive-Approximation ADC
The overall accuracy and linearity of the SAR ADC
are determined primarily by the internal DACs
characteristics
66
Functional block Diagram of a modern 1-MSPS SAR
The sequencer allows automatic conversion of the
selected channels, or channels can be addressed
individually if desired. Data is transferred via
the serial port. SAR ADCs are popular in
multichannel data-acquisition applications
67
Pipelined ADCs for High-Speed Applications(Sampli
ng Rates Greater than 5 MSPS)
The low-power CMOS pipelined converter is the ADC
of choice, not only for the video market but for
many others as well Today, markets that require
high speed ADCs include many types
of The pipelined ADC has its origins
in the subranging architecture A block diagram
of a simple 6-bit, two-stage subranging ADC is
shown in Figure
  • instrumentation applications (digital
    oscilloscopes, spectrum analyzers, and medical
    imaging).
  • video, radar, communications (IF sampling,
    software radio, base stations, set-top boxes,
    etc.),
  • consumer electronics (digital cameras, display
    electronics, DVD, enhanced-definition TV, and
    high-definition TV)

68
6-bit, two-stage subranging ADC
The output of the SHA is digitized by the
first-stage 3-bit sub-ADC (SADC)usually a flash
converter. The coarse 3-bit MSB conversion is
converted back to an analog signal using a 3-bit
sub-DAC (SDAC). Then the SDAC output is
subtracted from the SHA output, the difference is
amplified, and this residue signal is digitized
by a second-stage 3-bit SADC to generate the
three LSBs of the total 6-bit output word
69
Residue waveform at input of second-stage SADC
This waveform is typical for a low-frequency ramp
signal applied to the analog input of the ADC.
In order for there to be no missing codes, the
residue waveform must not exceed the input range
of the second-stage ADC, (Figure A). The
situation shown in Figure B will result in
missing codes when the residue waveform goes
outside the range of the N2 SADC, R, and falls
within the X or Y regionswhich might be
caused by a nonlinear N1 SADC or a mismatch of
interstage gain and/or offset.
70
The error-corrected subranging ADC architecture
A basic 6-bit subranging ADC with error
correction is shown in Figure, with the
second-stage resolution increased to 4 bits,
rather than the original 3 bits. Additional
logic, required to modify the results of the N1
SADC when the residue waveform falls in the X
or Y overrange regions, is implemented with a
simple adder in conjunction with a dc offset
voltage added to the residue waveform. In this
arrangement, the MSB of the second-stage SADC
controls whether the MSBs are incremented by 001
or passed through unmodified.
71
Pipelined architecture
In order to increase the speed of the basic
subranging ADC, the pipelined architecture has
become very popular. This pipelined ADC has a
digitally corrected subranging architecture in
which each of the two stages operates on the data
for one-half of the conversion cycle, and then
passes its residue output to the next stage in
the pipeline prior to the next phase of the
sampling clock. The interstage track-and-hold
(T/H) serves as an analog delay line it is
timed to enter the hold mode when the first-stage
conversion is complete. This allows more settling
time for the internal SADCs, SDACs, and
amplifiers, and allows the pipelined converter to
operate at a much higher overall sampling rate
than a nonpipelined version.
72
Generalized pipeline stages and timing
73
Clock Issues in Pipelined ADCs
Notice that the phases of the clocks to the T/H
amplifiers are alternated from stage to stage
such that when a particular T/H in the ADC enters
the hold mode it holds the sample from the
preceding T/H, and the preceding T/H returns to
the track mode. The held analog signal is passed
along from stage to stage until it reaches the
final stage in the pipelined ADC
74
Dual Slope ADCs
The dual-slope ADC architecture was truly a
breakthrough in ADCs for high resolution
applications such as digital voltmeters,
etc. The input signal is applied to an
integrator at the same time a counter is
started, counting clock pulses. After a
pre-determined amount of time (T), a reference
voltage having opposite polarity is applied to
the integrator. At that instant, the accumulated
charge on the integrating capacitor is
proportional to the average value of the input
over the interval T.
75
Dual Slope ADCs cont.
The integral of the reference is an
opposite-going ramp having a slope of VREF/RC. At
the same time, the counter is again counting from
zero. When the integrator output reaches zero,
the count is stopped, and the analog circuitry is
reset. Since the charge gained is proportional to
VIN T, and the equal amount of charge lost is
proportional to VREF tx, then the number of
counts relative to the full scale count is
proportional to tx/T, or VIN/VREF. If the output
of the counter is a binary number, it will
therefore be a binary representation of the input
voltage.
76
?-? ADC architecture
Modern ?-? ADCs for applications requiring high
resolution (16 bits to 24 bits) and effective
sampling rates up to a few hundred hertz. High
resolution, together with on-chip
programmable-gain amplifiers (PGAs), allows the
small output voltages of sensors such as weigh
scales and thermocouples to be digitized
directly. Proper selection of sampling rate and
digital filter bandwidth also yields excellent
rejection of 50-Hz and 60-Hz power-line
frequencies. ?-? ADCs offer an attractive
alternative to traditional approaches using an
instrumentation amplifier (in-amp) and a SAR ADC.
77
The basic concepts ?-? ADC architecture - 1
Figure A shows a noise spectrum for traditional
Nyquist operation, where the ADC input signal
falls between dc and fS/2, and the quantization
noise is uniformly spread over the same bandwidth
78
The basic concepts ?-? ADC architecture - 2
In Figure B, the sampling frequency has been
increased by a factor, K, (the oversampling
ratio), but the input signal bandwidth is
unchanged. The quantization noise falling
outside the signal bandwidth is then removed with
a digital filter. The output data rate can now
be reduced (decimated) back to the original
sampling rate, fS. This process of oversampling,
followed by digital filtering and decimation,
increases the SNR within the Nyquist bandwidth
(dc to fS/2). For each doubling of K, the SNR
within the dc-to-fS/2 bandwidth increases by 3
dB.
79
The basic concepts ?-? ADC architecture - 3
Figure C shows the basic ?-? architecture, where
the traditional ADC is replaced by a ?-?
modulator. The effect of the modulator is to
shape the quantization noise so that most of it
occurs outside the bandwidth of interest, thereby
greatly increasing the SNR in the dc-to-fS/2
region.
80
First-order sigma-delta ADC
The heart of this basic modulator is a 1-bit ADC
(comparator) and a 1-bit DAC (switch). The
output of the modulator is a 1-bit stream of
data. The noise-shaping function by acting as a
low-pass filter for the signal and a high-pass
filter for the quantization noise.
81
Sigma-Delta Modulator Waveforms
Because of negative feedback around the
integrator, the average value of the signal at B
must equal VIN. If VIN is zero (i.e., midscale),
there are an equal number of 1s and 0s in the
output data stream. As the input signal goes more
positive, the number of 1s increases, and the
number of 0s decreases. Likewise, as the input
signal goes more negative, the number of 1s
decreases, and the number of 0s increases. The
ratio of the 1s in the output stream to the total
number of samples in the same intervalthe ones
densitymust therefore be proportional to the dc
value of the input
82
Example
Analog input 3/8
83
Second-order ?-? modulator
84
24-bit ?-? Converter
85
Some General Trends in Data Converters
The general trends in data converters are
summarized in Figure
86
Low Power, Sleep, and Standby Modes
In order to conserve power, especially in
battery-powered applications, most modern data
converters have some type of low-power, sleep, or
standby mode, where the major portion of the
internal circuitry is powered downusually
initiated by the application of a signal to
one of the pins, software control via internal
control registers. additional power savings
can be achieved by disabling some or all of the
external clocks. Sleep-mode power supply current
? from a few µA to tens of mA depending upon the
normal-mode power dissipation. Recovery time
from the sleep mode, or power-up time ? but
generally is in the order of a few µs to 100 µs.
87
ADC Serial Output Interfaces
Serial outputs on SAR-based and S-? ADCs since
their conversion architecture is essentially
serial. If an ADC is operating continuously, the
period of the sampling clock must be long enough
to transfer all the serial data across the
interface at the interface data rate, with some
appropriate amount of headroom.
Example
A 16-bit, 1-MSPS sampling ADC requires a serial
output data rate of at least 16 MHz, which would
not be a problem with most modern ?P, ?Cor DSPs.
88
ADC Parallel Output Interfaces
Parallel ADC output interfaces are popular,
straightforward, and must be used when the
product of sampling rate and resolution exceeds
the capacity available serial links.
Example
Using a maximum LVDS serial data link of 600
Mbits/s requires parallel data transmission for
resolutions/sampling rates greater than 8 bits at
75 MSPS, 10 bits at 60 MSPS, 12 bits at 50 MSPS,
14 bits at 43 MSPS, 16 bits at 38 MSPS, etc.
89
Data Converter VoltageReferences
The accuracy of a data converter is determined by
a voltage reference of some sort. An exception
to this, of course, is an ADC which operates in a
ratiometric mode, where both the input signal and
input range scale proportionally to the
reference.
Example
Voltage references have a major impact on the
performance and accuracy of analog systems. A
5-mV tolerance on a 5-V reference corresponds to
0.1 absolute accuracyonly 10 bits. For a
12-bit system, choosing a reference that has a
1-mV tolerance may be far more cost effective
than performing manual calibration, while both
high initial accuracy and calibration will be
necessary in a system making absolute 16-bit
measurements.
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Ratiometric
ADC can be driven from a single supply voltage
which is also used to excite the remote bridge.
Both the analog input and the reference input to
the ADC are high impedance and fully
differential. By using the and SENSE outputs
from the bridge as the differential reference to
the ADC, the reference voltage is proportional to
the excitation voltage which is also proportional
to the bridge output voltage.
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Some Popular ADC/DAC Reference Options
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  • converter which requires an external reference.
    It is generally recommended that a suitable
    decoupling capacitor be added close to the
    ADC/DAC REF IN pin
  • converter that has an internal reference, where
    the reference is also brought out to a pin on
    the device. This allows it to be used other
    places in the circuit,
  • provided the loading does not exceed the
    rated value.
  • converter which can use either the internal
    reference or an external
  • one, but an extra package pin is required.
    If the internal reference is used,
    REF OUT is simply externally
    connected to REF IN, and decoupled if required.
  • If an external reference is used as shown, REF
    OUT is left floating, and the
    external reference decoupled and applied to the
    REF IN pin.
  • shows an arrangement whereby an external
    reference can override the
  • internal reference using a single package
    pin. The value of the resistor, R, is
    typically a few kO, thereby allowing the low
    impedance external reference to override the
    internal one when connected to the REF OUT/IN
    pin.
  • shows how the external reference is connected to
    override the internal reference.

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Types of Voltage References
Basic Bandgap Reference
Simple Diode Reference Circuits
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Selecting an A/D Converter
The selection checklist can be broken up into two
areas
  • primary facts which cannot be compromised, and
  • secondary factors which may allow the designer
    some flexibility

Primary What is the required level of system
accuracy? How many bits of resolution are
required? What is the nature of the analog
input signal? How fast must the converter
operate (conversion speed)? What are the
environmental conditions? Is a track-and-hold
circuit required?
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Selecting an A/D Converter
Secondary Does the system have multiple
channels? Should the reference be internal or
external? What are the drive amplifier
requirements? What are the digital interface
requirements? What type of digital output
format is required? What are the timing
conditions?
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Caracteristics ADC
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How to Save Power ?
The serial interface consists of the CS, SCLK,
and SDATA lines A normal conversion requires
sixteen serial clock pulses for completion.
shows how the power-down mode can be entered by
controlling the CS signal
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Texas Instruments - ADS7807
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Analog Devices - AD7466
The AD7466, a micropower, 12-bit SAR-type ADC
housed in a 6-lead SOT-23 package. It can be
operated from 1.6 V to 3.6 V and is capable of
throughput rates of up to 200 kSPS. The current
consumption in power-down mode is typically 8 nA.
The AD7466 consumes 0.9 mW max when operating at
3 V, and 0.3 mW max for 1.8 V operation at 100
kSPS.
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