4 Interfacing

Interface Circuits:

Hooking Up To TheOutside World

Prof. Greg Kovacs

Department of Electrical Engineering

Stanford University

EE122, Stanford University, Prof. Greg Kovacs 2

Design Note: The Design Process

• Definition of function - what you want.

• Block diagram - translate into circuit functions.

• First Design Review.

• Circuit design - the details of how functions areaccomplished.

– Component selection– Schematic– Simulation– Prototyping of critical sections

• Second Design Review.

• Fabrication and Testing.

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Interface Circuits

• Interface circuits “connect” between conventionalelectronic circuits (op-amps, logic, etc.) to theoutside world.

• They include circuits to buffer, amplify, andprocess sensor signals - INPUT of information.

• Also, they can include circuits to drive actuators,relays, etc. - OUTPUT of information.

• In general, they translate between the “volts andmilliamps” of conventional circuits and theirequivalents within several orders of magnitude.

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Power Driver Circuits

• There are a variety of devices that one might wantto drive that require more current or highervoltages than inexpensive op-amps can produce.

• Of course, one solution is to purchase “specialty”op-amps with high current or high voltageoutputs.

• However, it is very useful to know how to extendthe capabilities of op-amp (and logic circuit)outputs to avoid this, particularly when the moreexpensive approach is not warranted.

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Power Transistors/Heatsinks

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Unipolar Power Switches

• For many output devices, one simply needs toswitch a drive voltage on and off.

• In this case, one can use a bipolar powertransistor with sufficient current gain (or aDarlington configuration) or a power MOSFET.

• Today, the most efficient choice is usually theMOSFET.

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Basic Switch• Can use BJT or

MOSFET.

• If loads areinductive, needflyback protectdiode.

• Can drive directlyfrom TTL/CMOSlogic instead(want logic-driveMOSFET or BJT).

• Use current-limitresistor for BJT.

V1

V

V-

+

V2

FlybackProtectDiode

Rg

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Example Flyback CircuitV+

High voltagepulses out!

2N34401 kΩ

30 mH

Pulse Source

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IRLZ-34 Logic-Level MOSFET60V, 30A, 0.05

5V VGS

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Relay

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Pulse-Width Modulation• Pulse-width modulation, or

PWM, offers a simple,DIGITAL output way ofmodulating power.

• The idea is to vary the dutycycle of pulses from zeroto 100% and if the timeconstants of the devicebeing driven are muchlonger than the pulsetimes, a low-pass filteredequivalent power isobtained.

10%

50%

90%

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Random PWM Ideas

SG3525A/SG3527A

LM3524

Dedicated PWM Chips

Source:NationalSemiconductorLinear 3Databook

TriangleWave

GeneratorV

V-

+

VMOD

VOUT

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DC Motors

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Bipolar Power Switches

V+

LoadDrive Drive

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Modern Vacuum Tube Audio

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Power Inverters

• Digital drive totransformer togenerate higher orlower voltage.

• Can use to powerfluorescent lights, ACappliances, or togenerate higher DCvoltages (needrectifier and filter).

• Can make negativesupply rail.

V+

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MOSFET Power Driver

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HV Inverter

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FashionStatement

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Power Voltage Sources

• In some cases, a “beefy” and variable voltagesource is needed (e.g., audio power amplifiers,signal generator outputs, power supplies, etc.)

• In this case, one can either purchase power op-amps and use them in the standardconfigurations, or use power booster circuits withconventional, low-cost op-amps.

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THE COMPLEMENTARY EMITTERFOLLOWER AMPLIFIER

("PUSH-PULL")• A COMPLEMENTARY PAIR OF

TRANSISTORS ARRANGED ASTWO EMITTER FOLLOWERS CANPROVIDE LOTS OF POWER WITHINEXPENSIVE PARTS!

• VERY EFFICIENT(APPROXIMATELY 80%)

• CAN ALSO PROVIDE ADISTORTED SIGNAL DUE TOCROSSOVER DISTORTION…

• CAN DO THE SAME WITHMOSFETS

-VCC

+VCC

Vin

Vout

THIS IS A "CLASS B" CIRCUIT

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CROSSOVER DISTORTION

-VCC

+VCC

Vin

Vout

• THERE IS A +/- 0.7 V"DEADBAND"WITHIN WHICH THENEITHERTRANSISTOR ISCONDUCTING...

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A CLOSE LOOK AT THE DISTORTION

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OUTPUT SPECTRUM OF CLASS AAMPLIFIER WITH CROSSOVER DISTORTION

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REDUCING CROSSOVER DISTORTIONWITH BIASING DIODES...

-VCC

+VCC

Vin

Vout THIS IS A "CLASS AB" CIRCUIT

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BETTER PERFORMANCE WITHFEEDBACK!

MAGIC SWITCH

R1

R2

-VCC

+VCC

Vin

Vout

• THE +/- 0.7V DEADBAND ISREDUCED TO

• THE SLEW-RATE LIMITATIONS OFTHE OP-AMP MEAN THAT THISDEADBAND WILL STILL BEAPPARENT AT HIGHFREQUENCIES....

± 0.7AVO

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HAYES & HOROWITZ SAY....

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SEE ANY DISTORTION?

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OUTPUT SPECTRUM OF THE SAMEPUSH-PULL AMPLIFIER WITH FEEDBACK

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Peltier Devices

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BeerCooler

#2

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The Bridge Configuration

Source: NationalSemiconductor LM12Application Note.

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High Voltage Amplifiers• For high voltage op-

amp applications,recent pricereductions in HV op-amps make itpossible to usestandardconfigurationseasily.

• www.apexmicrotech.com is a goodsource of chips andapplication notes.

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Current Sources/Sinks/Pumps

• Many transducers require current sources to drivethem (e.g., electromagnetic coils in some settings,lasers, LEDs, etc.).

• There are several simple current driver circuitsthat use op-amps to provide closed-loop control,and the high output impedances required.

• The basic principle is to sense the sourced (orsunk) current and convert it into a signal forfeedback purposes.

• If the desired currents exceed the capabilities ofthe op-amp, external “pass” transistors are used.

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Classic Op-Amp Current Sink

V+

VIN

RF

Load

IL

IL =VIN

RF

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Beer-Locked-Loop

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Types of Sensors

• Electromagnetic Coils

• Strain Gauges

• Accelerometers

• Microphones

• Optical (covered elsewhere)

• Temperature Sensors

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Sensor Signal Processing

• Typical sensor signal processing involves(pre)amplification, filtering and sometimes somedownstream functions.

• Downstream functions may include a comparator(decision) or A/D converter, sometimes precededby a sample-and-hold circuit.

• In some cases (not covered in EE122), the sensorsignal (before or after digitization) is transmittedto another location using telemetry.

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LM334 Temperature Sensor

Source: Linear TechnologyLM334 Datasheet.

Note that current-output sensors allow quite longwire lengths, since they are pretty muchinsensitive to cable resistance.

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Transresistance Amplifiers

• Transresistanceamplifiers simplytranslate currentfrom a sensor into anoutput voltage.

• They are justinverting amplifierswithout the inputresistor. Thetransresistance gainis given in OHMS.

Rf

V

V-

+

V+

Vout

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Transresistance Frequency Response

• Quite often, high DCgain is desiredwithout much ACgain or controlledroll-off.

• These are twoexample approachesto achieve suchcharacteristics.

• In practice the topcircuit is most oftenused.

RF

C1

VOUT

V

V-

+iin

RP

RF

C1

VOUT

V

V-

+i in

AR = −R f

1

R f CS + 1

AR = −RPR fCS + R f

CS R f + RP( ) +1

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New Concepts to Enhance Productivity

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More on Instrumentation Amplifiers

LT1167AD620

Buffered voltagedivider to set“ground.”

Source: Analog Devices andLinear TechnologyDatasheets.

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Nerve Impulse Amplifier

This circuit amplifies the low levelnerve impulse signals received from a patient at Pins 2and 3. RG and the parallel combination of R3 and R4 seta gain of ten. The potential on LT1112’s Pin 1 creates aground for the common mode signal. C1 was chosen tomaintain thestabilityofthe patientground.TheLT1167’shigh CMRR ensures that the desired differential signal isamplified and unwanted common mode signals are at-tenuated. Since the DC portion of the signal is notimportant, R6 and C2 make up a 0.3Hz highpass filter.The AC signal at LT1112’s Pin 5 is amplified by a gain of101 set by (R7/R8) +1. The parallel combination of C3and R7 form a lowpass filter that decreases this gain atfrequencies above 1kHz. The ability to operate at ±3V on0.9mA of supply current makes the LT1167 ideal forbattery-powered applications. Total supply current forthis application is 1.7mA. Proper safeguards, such asisolation, must be added to this circuit to protect thepatient from possible harm.

Source: Linear TechnologyDatasheet.

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Switched Capacitor Filters

• It is possible to buildanalog filters where insteadof resistors to make RCtime constants, analogswitches and capacitorsare used to “simulate”resistances.

• In some filter types (state-variable or biquad), builtwith integrators, theintegrator gain controls thecutoff frequency -therefore, you can sweepthe cutoff frequency withthe clock frequency!

R1

C1

VOUT

V

V-

+

VIN

iin

C2

VOUT

V

V-

+

VIN

C1

Clock

Vout = −1

R1C1

Vindt∫

Vout = − fclock

C1

C2

Vindt∫

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LTC1064 Switched Capacitor Filter

Source: Linear TechnologyLTC1064 Datasheet.

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Analog Multipliers

• These devices can be usedfor modulation (AM), basicmultiplication, and a varietyof other functions.

• The AD633 is a particularlyeasy to use chip.

Source: Analog DevicesDatasheet.

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More AD633 StuffSquaring Circuit

Square Root Circuit

(Note errors!)

Source: Analog DevicesDatasheet.

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Squarer

sin2 t( ) =1

21− cos 2 t( )( )

AD 633

Vsupply = ± 15V

Input f = 200 kHz

Output f = 400 kHz

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Triangle Wave Input

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More AD633 Stuff

Amplitude Modulator

Divider Circuit

Source: Analog DevicesDatasheet.

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TremoloBox

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Multipliersin Filters

Voltage ControlledFilter Circuits

Divider Circuit

Source: Analog DevicesDatasheet.

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Example:Cheesy

Accelerometer

Gluing a 6-32 nutonto aninexpensivepiezoelectricbuzzer yields acheesy, butfunctionalaccelerometer.

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Cheesy Peak-Reading Accelerometer

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Voltage-to-Frequency Converters

Source: NationalSemiconductorLM331 Datasheet.

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Voltage-to-Frequency Converters

Source: NationalSemiconductorLM331 Datasheet.

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Temperature-To-Frequency

Source: NationalSemiconductorLM331 Datasheet.

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Frequency-to-Voltage Converters

Source: NationalSemiconductorLM331 Datasheet.