Transcript Lecture 16

Design Realization
lecture 16
John Canny
10/16/03
Last Time
 Basic electronics:
 Resistors, capacitors, inductors, amplifiers
This time
 Analog-Digital boundary
 Printed-circuit board design
Digital-analog boundary
 Because of the very high performance and
programmability of digital circuitry, the goal
today is to do as much as possible in the digital
realm, and only move to analog at the edges of
the system.
 The design problem then is to choose the right
AD and DA converters, and add any necessary
analog circuitry around them.
Frequency and wavelength
 Recall that the frequency of an sinusoidal AC
signal is the number of complete cycles in a
second.
 As an AC signal propagates as sound or radio,
it traces out a sinusoidal signal in space as
well. The length of a complete cycle is the
wavelength
v

f
where v is the speed of propagation.
Frequency and wavelength
 For sound, v is about 343 m/s, so
 in meters = 343/f with f in Hz.
 For light or radio, v is about 3 x 108 m/s. A
convenient form is
 in meters = 300/f with f in MHz.
Frequency spectrum
 20Hz-20,000 Hz, Audio frequency (for humans)
 20 kHz – few MHz (sound) ultrasonics
 30 kHz – 300 kHz (radio) long wave, over the
horizon radio and radar.
 300 kHz – 3MHz, medium-frequency radio, AM
 3 MHz – 30 MHz, high-frequency radio (CB and
short wave), long-range propagation common.
 30-300 MHz, very-high frequency (VHF) radio,
FM stations, some TV, Line-of-sight only. Slow
computer clock speeds.
Frequency spectrum (contd.)
 300 MHz - 3 GHz, ultra-high frequency (UHF),
some TV, cell phones, microwave ovens, Wi-Fi,
GPS, fast computer clock speeds.
 3GHz – 30GHz, microwave communications
including satellite TV
 30GHz – 300 GHz, exotic microwave systems
 300GHz – 3THz, microwave-IR DMZ
 3THz - 300THz (100 m - 1m) infrared, C02
and YAG lasers.
 700 nm – 400 nm visible light.
Digital-analog conversion
 Simplest method: PWM or Pulse-Width
Modulation. Adjust the off-time of a periodic
digital pulse signal.
PWM rate >> faster than output signal freq.
PWM and H-bridges
 PWM can be used to drive some (slow) devices
directly, like electric motors.
 A H-bridge configuration
allows the load (motor) to
be driven both ways.
 Switches 1-4 are closed
during the low “0” part of
the pulse signal, and 2-3
are closed during the “1”
part.
PWM
 To produce the desired signal, the PWM output
must be filtered with a low-pass filter.
 Here is a two-step low-pass filter using
capacitors and inductors: (capacitor values are
micro-farad/voltage)
PWM
 Low-pass filters can also be built with resistorcapacitor combinations:
 The corner frequency is given by f = 1/(2RC)
 E.g. R = 160 , C = 1 uF, f = 1000 Hz
Low-pass filtering
 An RC circuit reduces the signal in proportion to
frequency. At 10x the corner frequency, the
output is about 1/10 of the input.
 An LC circuit reduces the signal in proportion to
the square of the frequency. At 10x corner f, the
output is about 1/100 of the input.
 The corner frequency for an LC circuit is
f 
1
2 LC
PWM example
 For a D/A PWM circuit with max output
frequency of 1000 Hz, and 0.1% “ripple”
(appearance of the pulse signal):
 Set the pulse frequency to 10 k Hz (10x signal).
 The low-pass filter must attenuate by 1000x.
 Requires 3 stages of RC filtering, or two of LC
filtering, all with corner frequency of 1000Hz.
Other D/A methods
 Resistor network:
D/A ladder network
 A ladder resistor
network:
 High precision
resistors needed for
either network.
Break - Project proposals
 The project is ideally a survey paper + some
resources (books, web links, company names
etc.).
 Should be enough information for a peer of
yours to start a design in a new area.
Analog/Digital Conversion
 Ramp type:
Analog/Digital Conversion
 More efficient to use binary search, the
“successive approximation” or SAR method:
Analog/Digital Conversion
 Direct or “Flash” conversion,
one comparator per input
value.
 Fastest method, but complex,
limited precision.
Analog/Digital Conversion
 “Delta-Sigma” method converts analog input to
a 1-bit signal whose average value matches it.
 Highest precision method, but slow.
 Naive digital sample rate = analog sample rate x
2^precision
 In practice do much better than this with decimation
and digital filtering
ADC performance range:





Some off-the-shelf devices:
24-bit converters at audio frequencies (-)
18-bit converters up to 1 M conversions/sec
16-bit converters up to 100 M c/s (flash)
“Software radio” ADCs can directly handle
signals up to 10s of GHz, using undersampling.
 Manufacturers:
 Analog devices: www.analog.com
 National semiconductor: www.national.com
Dealing with Noise:
 Noise is present in all electronic systems. It
originates from:
 Thermal energy, present in all components
 Electromagnetic fields, either radio or power freq.
 Static electricity, atmospheric effects,…
 Simple “white” noise is spread evenly across
frequency.
 White, uncorrelated noise grows with the
square root of frequency and is measured in
Volts / Hz
Computing Noise:
 e.g., For an amplifier with a noise figure of 10
nV/Hz working at audio frequencies 1-10 kHz,
noise voltage (RMS) = 10-8 x 104 = 1 V
 RMS = Root-Mean-Square (square root of the
average squared signal) is a natural way of
measuring complex AC signals.
 In practice the 24-bit audio ADCs do not give 24
accurate bits because of noise.
Ex: Sound system
 ADCs do not have very good noise figures.
 For noise-sensitive apps (which many sensor
applications are) the ADC should be preceded
with an amplifier with good S/N figure.
 The amplifier usually has adjustable gain, which
allows the system to be adjusted for good noise
performance and full resolution without values
out-of-bounds.
 Ex: Analog devices AD1871, 24-bit stereo ADC
at 96 k c/s, with programmable input gain amps.
The art of electronics
 Practical electronics departs in several ways
from the ideal model:
 There is no perfect wire. Every connection has
finite resistance and finite inductance. If either
high current, or high frequency current passes
through a connection, it will cause a voltage
drop.
 This is particularly acute for power supply wires.
Stray capacitance
 There is no perfect connection point. Any two
conductors near each other form a capacitor.
Such stray capacitance can be strong between
nearby conductors on either side of a PC board,
or between pins on a chip.
 These effects are worst at high frequencies, and
with high voltages.
Feedback and isolation
 For both these reasons it’s a good idea to
physically separate large signals from small
ones, especially if the system does large
amplification (say 100-1000 times) – because
the large signals are controlled by the small
ones, which can lead to feedback and
uncontrolled oscillation.
 Don’t try for too much gain from a single stage
amplifier.
Power supply bypass
 Capacitors (and inductors or resistors) can be
used to isolate component power supplies:
Printed circuit boards
 The most widely-used connection system for
electronics.
 Typically epoxy or other plastic board with
copper conductors.
 Usually two or more layers of conductor.
 Holes are drilled and copper-plated to allow
component insertion + connects between layers.
 There are other prototyping systems for circuits,
but its often best to go straight to board design:
 Start dealing with layout issues immediately.
 Avoids difficulties due to the prototyping hardware.
PCB tips
 Main idea is to join the component pins that
need to be joined, but there are some tips:
 Ground and power conductors should be large,
as straight and direct as possible.
 All conductors should be as short and direct as
possible (avoid sharp turns which increase
inductance).
 For two-sided boards, it often helps to prefer
horizontal runs on one side, vertical on the other.
PCB tips
 Keep large signals away from small ones.
 Place bypass capacitors physically close to the
pins being bypassed.
 Use sockets for expensive components, or
components that may need to be replaced.
PCB systems
 ExpressPCB is a software system for fabricating
small boards, which can be sent directly to the
vendor for fab.
 Also draws schematics.
 EX USB sensor board.
To hand in
 Project proposal…