Introduction

An intact dry 1 cm2 patch of skin has a typical electrical resistance of about 100 kΩ, which is primarily provided by epithelium, the horny outermost layer of skin. Under normal circumstances, the skin provides considerable protection against brief contact with electrical potentials even as high as 120 V. However, if the skin is wet or perspiring, the electrical resistance is greatly reduced and dangerous currents can be conducted. If the skin has a cut, or if Ag(AgCl) electrodes with electrode paste are used, the resistance can decrease to below 1 kΩ. Under these conditions, dangerously high currents can be produced by potentials as low as 12 V. See Table G.1 for the physiological effects of various current levels. With a skin resistance of 100 kΩ, 500 V would be required to produce a current of 5 mA, but with a skin resistance of 1 kΩ, only 5 V will produce the same current. At the other extreme, high-voltage generators with potentials of 100 kV are used in science museums to show safely the effect of static electricity on people whose heads are endowed with large quantities of hair. The important lesson here is that the primary factor in electrical hazards is the current, not the voltage, passing directly through the heart. This is of particular concern to designers of equipment that uses electrodes placed near or on the surface of the heart, such as coronary catheters, or during surgery.

Introduction

In the past few years, enormous advances have been made in the cost, power, and ease of use of microcomputers and associated analog and digital circuits. It is now possible, with a relatively small expenditure, to purchase a microcomputer system that will take data, quickly analyze them, and display the results or control a process. This has been made possible by the development of technology that can fabricate millions of transistors, diodes, resistors, capacitors, and conductors on a single silicon integrated circuit chip.

Normally, the microcomputer is equipped with a number of standard items: the microprocessor chip and associated circuits, random-access memory chips, removable floppy and cartridge disk drives, magnetic hard disk drives, optical disk drives, keyboards, video display screens, serial interfaces, printers, and x–y entry devices such as the mouse, trackball, joystick, bitpad, and touch-sensitive display screen. However, data acquisition and control require additional components, such as digital and analog input/output (I/O) ports, and counters/timers. Analog input ports contain analog multiplexers, sample-and-hold (S/H) amplifiers, and analog-to-digital (A/D) converters. Analog output ports contain digital-to-analog (D/A) converters.

Even for designs requiring only a microprocessor and a few additional circuits, there are considerable advantages to using the resources of the microcomputer during the development stage. These include program code editors and compilers, an operating system for the storage and manipulation of code and data files, and ample random-access memory.

Introduction

The instructions below describe how to use the HP VEE panel driver for the HP 54600 digital oscilloscope to record and print waveforms.

Capturing the waveform

• Open HP VEE from the shortcut on the desktop.

• Open the “Instrument Manager” from the “I/O” pull-down menu.

• Under “HP-IB7” select “Digital Scope (hp54600b@707)” (just select it, don't double click on it).

• Click on the “Panel Driver” button.

• A panel driver will appear that allows you to control the oscilloscope.

• Select the timebase and channel sensitivity on the oscilloscope to get a good waveform.

• Press the “Stop” button on the oscilloscope to capture the waveform on the oscilloscope screen.

• To transfer the digitized waveform from the oscilloscope into the panel driver memory, click once on the black window in the panel driver window.

Printing the waveform

• Right click on the panel driver and select “Add Terminal: Data Output.”

• Select “WF_Ch1 (waveform)” if you want to print the waveform on channel 1.

• If you are sampling both channels 1 and 2, also select “WF_Ch2 (waveform).”

• One or two output labels (depending on the number of channels selected) will appear on the blue part of the panel driver located on its right end.

• Select “Waveform (time)” from the “Display” pull-down menu.

• Place the display in the HP VEE window.

• The default setting allows one input trace to go into the display. To add a second waveform, right click on the display window and select “Add Terminal: Data Input.”

• Connect the output channel of the panel driver to the desired input channel of the display by clicking the black dots of the desired channel ports.

• […]

Introduction

Noise in electronic circuits includes three basic categories:

1. noise received with the original signal and indistinguishable from it,

2. intrinsic noise in the circuit (Johnson, shot, etc.), and

3. interference noise generated by components in the circuit or picked up from outside the circuit.

This appendix considers only the last category, which is the only form of noise that can be influenced by choices of wiring and shielding. The sections below discuss interference noise from common impedance paths and from capacitive coupling, and list general rules to follow.

Interference noise due to common impedance

Whenever two circuit elements share a common current path, the impedance of that path can couple signals between them. This most commonly occurs when several circuits use a single conductor to return current back to their power supplies. This is shown in Figure A.1, where analog and digital circuits are connected to their power supplies through conductors having impedance Z. The + 5- and ±15-V current supply wires do not share a common impedance and are relatively free from interference noise. On the other hand, the current return path (usually referred to as “ground”) has a common impedance for all circuits and power supplies. A current transient in a digital circuit will briefly shift the “ground” reference for the analog circuits. This problem will generally not be visible using a voltage meter because the effect is brief.

Introduction

In this chapter, we discuss the components that convert data between the digital world of the microcomputer and its I/O ports and the analog world of continuously varying voltages. These are the digital-to-analog (D/A) converter, the analog-to-digital (A/D) converter, the sample-and-hold amplifier, and the comparator. We then go on to discuss some of the fundamental limits to sampling time-varying analog signals, including the role of the sample-and-hold amplifier, and the minimum sampling frequency.

Laboratory Exercise 7 is designed as an introduction to the characteristics of the D/A and A/D converters and uses an analog I/O circuit board. Laboratory Exercise 8 interfaces a D/A converter to the binary output port, measures its transfer characteristics, and uses it in waveform generation. Laboratory Exercise 9 interfaces an A/D converter to the binary input port and performs periodic sampling of sine waves. Laboratory Exercise 10 samples and recovers sine waves of various frequencies and explores the aliasing problems that arise when the input frequency is greater than one-half of the sampling frequency.

Digital-to-analog converter circuits

The digital-to-analog (D/A) converter changes an N-bit binary number to an analog output voltage that can have 2N distinct values. It consists of resistors, switches, and an amplifier. Frequently, input latches are provided to retain the input number. Usually, the relationship between the input number and the output voltage is linear, but other relationships (e.g. logarithmic) are also used.

Introduction

The transducer is a device that converts one form of energy to another. For interfacing to a microcomputer, we are primarily interested in the electronic transducer, which has an input or an output that is electrical in nature, such as a voltage, current, or resistance. The sensor is an electronic transducer that converts a physical quantity into an electrical signal. An actuator is an electronic transducer that converts electrical energy into a physical quantity, and is an essential element in control systems.

Sensors are used to detect displacement, temperature, strain, force, light, etc. Almost all sensors require additional circuits to produce the voltage and current needed for analog-to-digital conversion. As we shall see in this chapter, the thermistor changes its electrical resistance as a function of temperature, and a bridge is needed to produce a corresponding voltage, whereas the silicon photodiode produces a current, and a stage of amplification is needed to produce a voltage. Often, the term “sensor” includes both the transducer and the circuits needed to produce an output voltage.

Laboratory Exercise 11 uses a circular resistor and a computer to record angle and the oscillations of a damped pendulum. Laboratory Exercise 12 explores the measurement of temperature using the dial thermometer, a platinum resistance thermometer, the thermocouple, and the thermistor. Laboratory Exercise 13 measures force, using four metal foil strain gauges bonded to a plastic rod and wired in opposing pairs to form a bridge circuit.

Standard resistor values and color codes

Carbon resistors are the most frequently used in printed circuits and come in a limited selection of values. The tables below list those values that resistor manufacturers commonly provide. In this book, all circuits used in the laboratory exercises specify resistor values taken from this list.

For carbon resistors with tolerances of 2% or larger, the first three colors are used to determine the value of the resistor. The first two colors (Table H.1) indicate the first two digits and the third color (Table H.2) indicates a multiplier. The fourth color (Table H.3) indicates the resistor tolerance. Values of commonly available 2, 5, and 10% resistors and their first three color codes are listed in Table H.4.

For metal film resistors with tolerances of 1% or smaller, the first four colors are used to determine the value of the resistor. The first three colors indicate the first three digits and the fourth color indicates a multiplier. The fifth color indicates the resistor tolerance.

Silver is used to indicate a multiplier of 0.01. For resistor values below 10 ω, the values corresponding to 11, 13, 16, 20, 24, and 30 are not available.

Tolerances of 1% or better are available in metal film resistors.

Standard capacitor values and codes

Capacitors are manufactured in a number of types and are available in a limited selection of values. The tables below list values that capacitor manufacturers commonly provide.

Purpose

To use a compiler/editor to write and compile a simple C program. To investigate the 2's complement unsigned, and hexadecimal representation of 8-, 16-, and 32-bit numbers, to use the printf function for printed output, and the scanf function for interactive program control and data entry.

Equipment

• IBM-compatible Pentium microcomputer with Windows NT operating system and Microsoft Visual C++ compiler

• Printer (shared with other laboratory stations)

Additional reading

1. Appendix C: C programming hints

2. Section 1.3 Number systems

3. B. W. Kernighan and D. M. Ritchie, The C Programming Language, Prentice Hall, Englewood Cliffs, NJ, 1988.

Procedure

Microsoft Visual C++ programming environment

You will need a separate project file and folder for each laboratory exercise that requires programming. Rather than creating your own project folder from scratch, copy a “starter project” folder into your file storage area. The laboratory assistant or teaching associate will tell you where to find it on your local network. This folder contains a starter project file with all the library modules that your project will require, and C code files you may need, such as InitAll and fft.c.

Rename the starter project folder and the .dsw project file in it, but be sure to keep the .dsw extension. Start the Microsoft Visual C++ developer studio by double-clicking the shortcut on the desktop. Open the project by clicking on the “File: Open Workspace” command and select your project .dsw file. “File: Close Workspace” will close the project you are currently working on.

Multimeter accuracy

The following is the typical accuracy for a student-type digital multimeter, information often provided on the back of the meter. The shorthand notation (A + B) means that the uncertainty of the reading is guaranteed to be less than ±(A% of reading + B least significant digits):

1. Max counts 2,000 (e.g. 2.000 V or 200.0 mV or 2,000 ω)

2. <200 V dc (0.1 + 1) 45 Hz to 10 kHz (0.5 + 2)

3. <200 mA dc (0.3 + 1) 45 Hz to 10 kHz (1.0 + 2)

4. <200 kΩ (0.2 + 1)

5. >2 MΩ (0.5 + 1)

For example, for a reading of 1.453 V dc, the uncertainty is less than ±(1.453 × 0.1% + 0.0001) V. For a reading of 1.453 V ac, the uncertainty is less than ±(1.453 × 0.5% + 0.002) = ±0.009 V.

Note that in the latter case, the accuracy (adherence to the accepted standard) is ± 0.009 V, but the precision (ability to detect small changes) is one least significant digit, or ±0.001 V.

A triangular distribution with full width ±W has a standard deviation σ = 0.408W. So as a rule of thumb, the multimeter uncertainty can be thought of as representing approximately 2.5 standard deviations.