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

where v is the speed of propagation.f

v

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 - 1m) 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 configurationallows the load (motor) tobe driven both ways.

Switches 1-4 are closedduring the low “0” part ofthe pulse signal, and 2-3are 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 aremicro-farad/voltage)

PWM

Low-pass filters can also be built with resistor-capacitor combinations:

The corner frequency is given by f = 1/(2RC) 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

LCf

2

1

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 foreither 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

HzVolts /

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…