Feeding Reading Williams, Linear Technology Corporation INTRODUCTION A
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Application Note November 2002 Bias Voltage Current Sense Circuits Avalanche Photodiodes
Feeding Reading Williams, Linear Technology Corporation INTRODUCTION Avalanche photodiodes (APDs) widely utilized laser based fiberoptic systems convert optical data into electrical form. usually packaged with signal conditioning amplifier small module. receiver module attendant circuitry appears Figure module (figure right) contains transimpedance (e.g., current-to-voltage) amplifier. optical port permits interfacing fiberoptic cable APD's photosensitive portion. module's compact construction facilitates direct, loss connection between amplifier, necessary because extremely high speed data rates involved. receiver module needs support circuitry. requires relatively high voltage bias (figure left) operate, typically 90V. This voltage bias supply's programming port. This programming voltage also include corrections APD's temperature dependent response. Additionally, desirable monitor APD's average current (figure center), which indiTYPICALY BIAS INPUT
cates optical signal strength. This information combined with feedback techniques maintain optical signal strength optimal level. feedback loop's operating characteristics also determine deleterious degradation optical components occurred, permitting corrective measures taken. current typically between 100nA 1mA, dynamic range 10,000:1. This measurement, which should taken with accuracy inside normally must occur APD's "high side," complicating circuit design. This restriction applies because APD's anode committed receiver amplifier's summing point. module, expensive electrically delicate device, must protected from damage under conditions. support circuitry must never produce spurious outputs which could destroy module. Particular attention must devoted bias supply's dynamic response under programming power-up/down conditions. Finally, desirable power support circuitry from single rail.
RECEIVER MODULE
5VIN
BIAS POWER SUPPLY
CURRENT MONITOR
AMPLIFIER OUTPUT AMPLIFIER
PROGRAMMING INPUT
OUTPUT GROUND REFERRED BIAS CURRENT MONITOR. CURRENT TYPICALLY 100nA
OPTICAL INPUT
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Figure Avalanche Photodiode (APD) Module (Figure Right) Contains APD, Amplifier Optical Port. Power Supply (Figure Left) Provides Bias Voltage. Current Monitor (Figure Center) Operates High Common Mode Voltage, Complicating Signal Conditioning
registered trademarks Linear Technology Corporation.
AN92-1
Application Note
bias voltage current measurement requirements described above constitute significant design challenge addressed following text. Simple Current Monitor Circuits (with Problems) Figure straightforward approaches attempt address current monitor problem. Figure uses instrumentation amplifier powered separate rail measure across current shunt. Figure similar derives power supply from bias line. Although both approaches function, they meet current sensing requirements. bias voltages range 90V, exceeding amplifier's supply common mode voltage limits. Additionally, measurement's wide dynamic range requires single rail powered amplifier swing within 100µV zero, which impractical. Finally, desirable amplifiers operate from single, voltage rail. Figure circuit divides down high common mode current shunt voltage, theoretically permitting powered amplifier extract current measurement over bias range. practice, this arrangement introduces prohibitive errors, primarily because desired signal also divided down. current measurement information buried divider resistor's tolerance, even with 0.01% components. desired accuracy over 100mA range cannot achieved. Finally, although amplifier operates from single supply, cannot swing zero. clear from preceding circuits that common circuit approaches will meet signal conditioning requirements. More sophisticated techniques necessary.
990k 0.01% 990k 0.01% 2.23k CURRENT MONITOR OUTPUT BIAS OUTPUT
LT1789
0.01% 0.01%
AN92
Figure Dividing Down High Common Mode Voltage Introduces Huge Errors, Even with Precision Components. Desired Accuracy Over 100nA Current Monitor Range Buried Resistor Mismatch, Even with 0.01% Resistors. Single Rail Powered Amplfier Cannot Swing Close Enough Zero. Approach Impractical
Carrier Based Current Monitor Figure utilizes carrier modulation techniques meet current monitor requirements. features 0.4% accuracy over sensed current range, runs from supply high noise rejection characteristics carrier based "lock measurements. LTC1043 switch array clocked internal oscillator. Oscillator frequency, capacitor about 150Hz. clocking biases level shifter chops voltage across current shunt, modulating into differential square wave signal which
BIAS OUTPUT
1N4684 3.3V
LT1789
CURRENT MONITOR OUTPUT
AN92 F02a
BIAS OUTPUT
LT1789
CURRENT MONITOR OUTPUT
AN92 F02b
(2a)
(2b)
Figure Instrumentation Amplifiers Extract Current Measurement from Modest Common Mode Voltages. Figure Requires Separate Amplifier Power Bias Supply Connections; Figure Derives Both Connections from Single Point. Zener Level Shift Accomodates Amplifier Input Common Mode Range. Circuits Cannot Operate from Single, Voltage Rail, Swing Close Zero Accomodate High Bias Voltages
AN92-2
Application Note
feeds through 0.2µF coupling capacitors. A1's single-ended output biases demodulator which presents output buffer amplifier A2's output circuit output. Switch clocks negative output charge pump which supplies amplifier's pins, permitting output swing (and below) zero volts. 100k resistors minimize on-resistance error contribution prevent destructive potentials from reaching (and rail) either 0.2µF capacitor fails. A2's gain corrects slight attenuation introduced A1's input resistors. practice, desirable derive bias voltage regulator's feedback signal from indicated point, eliminating shunt resistor's voltage drop.1 Verifying accuracy involves loading bias line with 100nA noting output agreement.2 Coupled Current Monitor Figure coupled current monitor eliminates previous circuit's trim pulls more current from bias supply. floats, powered bias rail. zener diode current source ensure never exposed destructive voltages. current shunt's voltage drop sets A1's positive input potential. balances inputs feedback controlling negative input such, Q1's source voltage equals A1's
HIGH VOLTAGE BIAS INPUT
100V 100k* 100k* 100V
OPTIONAL "ZERO CURRENT" FEEDBACK BIAS REGULATOR, APPENDIX VOUT
1N4690 5.6V
0.2µF
MPSA42 0.2µF LT1789
LT1006
-3.5V
20k* -3.5V 200k*
OUTPUT
22µF
-3.5V AMPLIFIERS
1N4148 TP0610L 0.056µF
AN92
Figure Lock-In Amplifier Technique Permits Accurate Current Measurement Over 100nA Range. Current Modulated Single-Ended Demodulated S2-A2
Note Appendix "Low Error Feedback Signal Derivation Techniques," details.
Note Appropriate high value load resistors, perhaps augmented with monitoring current meter, available from Victoreen other suppliers. Tight resistor tolerance, while convenient, strictly required, output target value current meter indication.
AN92-3
0.1% METAL FILM RESISTOR 100V TECATE CMC100105MX1825 CIRCLED NUMBERS LTC1043 NUMBER
22µF
Application Note
positive input voltage drain current sets voltage across source resistor. Q1's drain current produces voltage drop across ground referred resistor identical drop across current shunt and, hence, current. This relationship holds across bias voltage range. 5.6V zener assures A1's inputs always within their common mode operating range resistor maintains adequate zener current when current very levels. output options shown. chopper stabilized amplifier, provides analog output. output able swing (and below) zero because supplied with negative voltage. This potential generated using A2's internal clock activate charge pump which, turn, biases A2's pin.3 second output option substitutes A-to-D converter, providing serial format digital output. supply required, LTC2400 A-to-D will convert inputs (and slightly below) zero volts. Resistors strategic locations prevent destructive failures. unit protects bias line shorts ground. resistor limits current safe value fails 100k resistor serves similar purpose malfunctions. previous figure, voltage regulator feedback taken current shunt's output maintain optimal regulation.4 stated, this circuit does require trimming maintains 0.5% accuracy. does, however, pull current approximately equalling current delivered APD, addition Q2's collector current. This issue bias supply restricted current capability.
HIGH VOLTAGE BIAS INPUT
1N4690 5.6V
CURRENT SHUNT
OPTIONAL "ZERO CURRENT" FEEDBACK BIAS REGULATOR, APPENDIX VOUT
LT1077
1N4702
MPSA42 Hi-Z OUTPUT 100k LT1460 2.5V
ZVP0545A
0.1% METAL FILM RESISTOR BAT85 BUFFERED OUTPUT -3.5V HERE 100k OPTIONAL BUFFERED OUTPUT
AN92
VREF
DIGITAL INTERFACE
10µF 2N3904
LTC1150
LTC2400 A-TO-D OPTIONAL DIGITAL OUTPUT
10µF
Figure A1-Q1 Float High Voltage Rail, Measuring Current Shunt. Q1's Ground Referred Drain Current Provides Hi-Z Output. Buffer Options Include Analog (Figure Bottom Left) Digital (Figure Bottom Right)
Note Circuit veterans will exercise extreme wariness when confronted with bootstrapped biasing scheme such this. Appendix Single Rail Amplifier with True Zero Volt Output Swing," should soothe anxieties. Note Appendix "Low Error Feedback Signal Derivation Techniques."
AN92-4
Application Note
Bias Supply previous examples have been current monitors. Figure developed Michael Negrete, high voltage bias supply.5 LT1930A switching regulator form flyback based boost stage. flyback events pump diode-capacitor network tripler, producing high voltage output. Feedback from output R1R2 combination stabilizes regulator's operating point. protect switch feedback pins, respectively, from parasitic negative excursions resistors prevent excessive switch current. series connected high voltage capability, minimize output noise. 4.5V programming voltage results corresponding output accuracy) with about current capacity. Circuit output noise quite low. Figure taken with 500µA loading VOUT 50V, shows about 200µV ripple harmonic residue 10MHz bandwidth. This adequate most receivers.6
0.1µF 100V
VOUT
0.15µF 0.15µF
2.2µH 4.7µF 6.3V LT1930A SHDN VPROGRAM 4.5V 90VOUT 30VOUT
35.7k
35.7k
METAL FILM RESISTORS TAIYO YUDEN JMK212BJ475MG MURATA GRM42-2X7R105K050 TAIYO YUDEN HMK316BJ104ML VISHAY 695D154X9050AZ CENTRAL SEMI CMDSH2-3TR MURATA LQH32C2R2M24
5.49k
0.15µF 1.24k
0.15µF
Figure Boost Regulator/Charge Pump Supplies Bias with Only 200µVP-P Noise
100µV/DIV COUPLED
200ns/DIV
Figure Figure Bias Supply Shows 200µVP-P Ripple Harmonic Residue 10MHz Bandwidth
Note Reference Note Faithful noise measurements these levels requires considerable care. Appendices practical details.
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AN92 F07.tif
AN92-5
Application Note
Bias Supply Current Monitor Figure Martin Configuration, combines previous circuit with Figure current monitor, providing complete signal conditioner.7 programmable bias supply before, except that feedback comes sensing after current shunt, isolates R1-R2 path loading, preventing from influencing shunt's voltage drop. A2's action also insures tight output regulation, despite current shunt's presence.8 current monitor, borrowing from Figure measures across current shunt, presenting output Q1's drain line. shown, output about output impedance, although either Figure output options employed. When considering circuit operation, note that both amplifiers powered charge pump's high voltage output, with their returned "2/3 VOUT" point. This biasing permits amplifiers process high voltage signals, although voltage across them never exceeds 30V.
METAL FILM RESISTORS 0.1% METAL FILM RESISTORS TAIYO YUDEN JMK212BJ475MG MURATA GRM42-2X7R105K050 TAIYO YUDEN HMK316BJ104ML VISHAY 695D154X9050AZ CENTRAL SEMI CMDSH2-3TR MURATA LQH32C2R2M24 VOUT VOUT VOUT 5.49k 35.7k 32.3k
Transformer Based Bias Supply Current Monitor Figure circuit, another complete bias supply current monitor, uses different techniques than previous example. Advantages include 0.25% bias voltage current monitoring accuracy, small size fewer high voltage components greater reliability. LT1946 switching regulator form flyback type boost configuration. T1's turn ratio provides voltage gain high voltage flyback events rectified smoothed diode capacitor T1's secondary. This potential divided down back compares this signal bias programming input sets LT1946's operating point, closing control loop. Loop compensation furnished local rolloff lead network across feedback resistor. This loop establishes maintains bias output accordance with programming input's value. active VSUPPLY 1.2V, prevents output overshoot power turn grounding programming input command while
1N4690 5.6V 0.1µF 100V
1K** 1k**
0.15µF 0.15µF
1N4148
BIAS
LT1078
2.2µH 4.7µF 6.3V LT1930A SHDN
ZVP0545A
LT1078
1k**
ZOUT FIGURE OUTPUT OPTIONS
VPROGRAM 4.5V 85VOUT 30VOUT
0.15µF 1.24k
0.15µF
Figure Figure Current Monitor Combines with Figure Bias Supply, Providing Bias Current Measurement. Buffers LT1930A's Feedback Path Loading from Bias Supply Output, Eliminating Current Error. Amplifiers Process Signals, Although Voltage Across Them Never Exceeds
Note This circuit based work Alan Martin. Note Appendix "Low Error Feedback Signal Derivation Techniques," further discussion.
AN92-6
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Application Note
simultaneously forcing A1's output low. This shuts switching regulator high voltage produced. When power turn reaches changes state A1's positive input ramps programming voltage. switching regulator's output follows this turn-on profile overshoot occurs. LT1004 clamps spurious programming inputs beyond 2.5V, preventing excessive high voltage outputs.9 circuit's current monitor portion takes full advantage T1's floating secondary. Here, current shunt resides T1's secondary return path (Pin eliminating high common mode voltages encountered previous "high side" sensed examples. Circuit ground declared shunt's uncommitted terminal, meaning T1's will undergo increasing negative excursion with greater current. Inverter converts shunt's negative voltage buffered positive output. gain, scaled above unity, compensates input resistor's shunt loading error. Swing zero facilitated returning A2's small negative potential derived from LT1946's switching. 10M-287k divider's current loading error prevented from appearing A2's output compensatory current from bias programming input. This compensating current, arriving 100k-3.65k-1M network, scaled precisely balance shunt's output portion 10M-287k path's loading error. Appendix detailed discussion this technique.
LT1077 0.01µF 1N970 MBR0540 100k BAV21 100V 10M* 100k 0.01µF 0.1µF 100V BIAS OUTPUT
BIAS PROGRAMMING INPUT 0.55V 2.5V
LT1004 2.5V
200k
0.1µF
100k 0.1% METAL FILM RESISTOR METAL FILM RESISTOR SRW5EE-V01H003 1N4148 2N3904
Figure Controls LT1946 Boost Regulator Supply Bias. Prevents Output Overshoot Power Turn-On. Senses Current Across Shunt T1's Output Return. Programming Input Feedforward Cancels 10M-287k Feedback Divider's Loading Error, Preserving Current Monitor Accuracy
0.1µF
10µF LT1946 0.1µF 100k* 100k 3.65k* 101k*
287k*
LT1077
CURRENT MONITOR OUTPUT
LT1946
LT1635
0.1µF
39k** 2k**
LT1635
0.2V REFERENCE
AN92
Note Optional circuitry allows input clamping desired voltage. Appendix "APD Protection Circuits."
AN92-7
Application Note
Output noise this circuit, shown Figure about 1mVP-P 10MHz bandwidth. This characteristic flyback regulators somewhat higher than Figure charge pump based arrangement. still acceptable most receivers, although special switching regulator techniques (read on!) considerably reduce this figure. stabilizes output, which varied appropriate biasing VPROGRAM input. Components LT1946 compensate loop. Over output range, circuit remains within VPROGRAM input dictated output voltage. Figure shows switching related output noise about millivolts peak-to-peak 10MHz bandwidth.
6.8µH
BAV21
100V 100V
500µV/DIV
1N4148
BIAS OUTPUT
MBR0540
200ns/DIV
AN92 F08.tif
LT1946
0.1µF
Figure Figure Output Noise Measures 1mVP-P 10MHz Bandwidth
1500pF
14k*
Inductor Based Bias Supply Figure borrows from Figure flyback technique form simple, small area bias supply. Figure current monitor function been deleted-this circuit provides only bias supply. Additionally, Figure transformer been replaced with 2-terminal inductor. circuit basic inductor flyback boost regulator with single important deviation. high voltage device, been interposed between LT1946 switching regulator inductor. This permits regulator control Q1's high voltage switching without undergoing high voltage stress. operating "cascode" with LT1946's internal switch, withstands L1's high voltage flyback events.10 Diodes associated with Q1's source terminal clamp originated spikes arriving Q1's junction capacitance. high voltage rectified filtered forming circuit's output. Feedback regulator
VPROGRAM 90VOUT 20VOUT
100V
METAL FILM RESISTOR ZETEX ZTN4424 SUMIDA CDRH4D28-6R8 TECATE CMC100105MX1825
AN92
Figure Cascoded with LT1946 Switches Providing Bias Output. Q1's Source Diodes Clamp Parasitic Conducted Spikes Safe Levels
500µV/DIV
200ns/DIV
AN92 F12.tif
Figure Cascode Based Bias Supply Noise 10MHz Bandwidth About 1.3mVP-P
Note Reference
AN92-8
Application Note
200µV Output Noise Bias Supply Some receiver applications require extremely noise extended bandwidth. Figure 13's bias supply uses special switching regulator techniques achieve 200µV noise 100MHz bandwidth. LT1533 "push-pull" output switching regulator with controllable switch transition times. Output harmonic content ("noise") notably reduced with slower switch transition times.11 Switch current voltage transition times
510pF LT1533 0.02µF RVSL RCSL 2.49k* 100V METAL FILM RESISTOR TECATE CMC100105MX1825 COOPER SD12-330 COILCRAFT B07T COOPER CTX-02-16004 BAV21 133k* PGND 22nH 22µF 33µH 33µH BIAS 100V
controlled resistors RCSL RVSL pins, respectively. other respects, circuit behaves classical push-pull, transformer based, step-up converter. VPROGAM input biases feedback loop, setting output anywhere between 90V. controlled transition times result dramatic decrease output noise. Figure shows ripple switching related residue 200µV 100MHz bandwidth. This below conventional regulators, meeting most stringent noise requirement.
100V
VPROGRAM INPUT 0VIN 4VIN 90VOUT 20VOUT
7.5k*
AN92
Figure Transformer Coupled Bias Supply Controls Switch Transition Time Extremely Output Noise
200µV/DIV
2µs/DIV
AN92 F14.tif
Figure LT1533's Controlled Transition Times Achieve Spectacularly Output Harmonic Residue. Switching Related Noise Below 100µV, Fundamental Ripple About 200µV. Measurement Bandwidth 100MHz
Note Noise contains regularly occurring coherent components. such, switching regulator output "noise" misnomer. Unfortunately, undesired switching related components regulated output almost always referred "noise." Accordingly, although technically incorrect, this publication treats undesired output signals "noise." Reference
AN92-9
Application Note
Noise Bias Supply Current Monitor Figure builds previous circuit's performance, forming complete, high performance signal conditioner. bias supply identical Figure 13's noise example, with addition based feedback buffer. This stage, similar Figure isolates regulator's feedback path current from shunt, preserving current monitor accuracy. A1's zener-current source power biasing scheme permits process high voltage signals even though voltage device.12 current monitor, shown block form, selected from choices indicated depending upon requirements. 0.02% Accuracy Current Monitor Some current monitor applications call high accuracy stability. Figure 16's unusual optical switching based approach achieves 0.02% accuracy over sensed 100nA range. This scheme measures shunt current switching (S1A, S1B) capacitor across shunt ("ACQUIRE"). After time capacitor charges voltage across shunt. open close ("READ"). This grounds capacitor plate
510pF LT1533 0.02µF RVSL RCSL 2.49k* 133k* 1N969 0.1µF PGND 22nH 22µF 33µH 33µH 1N4670 5.6V CURRENT MONITOR FIGURES OPTIONS
capacitor discharges into grounded unit S2B. This switching cycle continuously repeated, resulting A1's ground referred positive input assuming same voltage that across floating shunt. driven MOSFET switches specified have junction potentials optical drive contributes charge injection error. nonoverlapping clock prevents simultaneous conduction which would result charge loss, causing errors possible circuit damage. 5.1V zener prevents switched capacitor failure bias output shorted ground. chopper stabilized amplifier, clock output. This clock, level shifted buffered drives logic divider chain. first flip-flop activates charge pump, pulling A1's negative, permitting amplifier swing (and below) zero volts.13 divider chain terminates into logic network. This network provides phase opposed charging 0.02µF capacitors (Traces Figure 17). gating associated with these capacitors arranged logic provides nonoverlapping, complementary biasing These transistors supply this nonoverlapping drive actuating LEDs (Traces
100V 100V
BIAS
LT1077
100V METAL FILM RESISTOR TECATE CMC100105MX1825 COOPER SD12-330 COILCRAFT B07T COOPER CTX-02-16004 BAV21 100k
AN92
VPROGRAM INPUT 0VIN 4VIN 90VOUT 20VOUT
7.5k*
MPSA42
Figure Figure Augmented with Feedback Divider Buffer Current Monitor Provides Complete 100µV Noise Signal Conditioner
Note feedback buffer considered detail Appendix "Low Error Feedback Signal Derivation Techniques." Note This scheme, variant described back Figure detailed Appendix Single Rail Amplifier with True Zero Volt Output Swing."
AN92-10
Application Note
extremely small parasitic error terms driven MOSFET switches results nearly theoretical circuit performance. However, residual error (0.1%) caused S1A's high voltage switching pumping S2B's junction capacitance. This results slight quantity unwanted charge being transferred capacitor S2B. amount charge transferred varies with bias voltage (20V 90V) and, lesser extent, varactor-like response S2B's off-state capacitance.
0.01% 20VIN 90VIN 5.1k "ACQUIRE" "READ" 100k IOUT 10µF BAT85s -3.5V HERE 10µF 1N4689 5.1V
These terms partially cancelled feedforward A1's negative input feedforward from Q1's gate S2B. corrections compensate error factor five, resulting 0.02% accuracy. Optical switch failure could expose high voltage, destroying possibly presenting destructive voltages rail. This most unwelcome state affairs prevented resistors A1's positive input.
OPTIONAL "ZERO CURRENT" CONNECTION BIAS REGULATOR, APPENDIX BIAS 2N3906
CLKOUT
LTC1150
750k
TP0610L "ACQUIRE" (S1) 130k TP0610L "READ" (S2) 0.02µF 130k 0.02µF
74C74
140Hz
74C90
74C74
PANASONIC ECP-U1C105MA5 OPTICALLY DRIVEN MOSFETS. AROMAT AQW227NA (DUAL) VOLTAGES <80V, AQS225SX (QUAD, SO-16 PACKAGE) 74C02
AN92
Figure 0.02% Accurate Current Monitor Utilizes Optically Driven FETs Flying Capacitor. Logic Driven Q1-Q2 Provides Nonoverlapping Clocking S1-S2 LEDs. Clock Derives from A1's Internal Oscillator
5V/DIV
5V/DIV
5mA/DIV
5mA/DIV
2ms/DIV
AN92 F17.tif
Figure Clocked, Cross Coupled Capacitors (Traces 74C02 Based Network Result Nonoverlapping Drive (Traces S1-S2 Actuating LEDs
150pF
AN92-11
Application Note
Digital Output 0.09% Accuracy Current Monitor Figure modifies optically based current monitor supply digital output. schematic essentially identical Figure 16's, with significant differences. Here, digital output supplied LTC2431 A-to-D converter. converter's differential inputs allow same feedforward based error correction used previous example. divider chain countdown ratio changed accomodate higher speed clock, sourced LTC1799 oscillator. This higher speed clock, which times A-to-D operation, centers A-to-D's internal notch filter optical switches commutation frequency, maximizing rejection.14 This circuit's 0.09% accuracy does equal previous analog ouput's version because LT1460 reference's 0.075% tolerance, which trimmable. circuit adjusted 0.02% accuracy trimming shunt measured output current directly corresponds A-to-D output. Digital Output Current Monitor Previous current monitor examples furnish digital output from ground referenced A-to-D converters from analog level shifting stages. Figure directly digitizes shunt current floating A-to-D converter bias line. A-to-D output level shifted digital
OPTIONAL "ZERO CURRENT" FEEDBACK CONNECTION BIAS REGULATOR, APPENDIX BIAS 5.1k "ACQUIRE" "READ" S12B VREF LTC2431 A-TO-D 179kHz 1N4689 5.1V
0.01% 20VIN 90VIN
LT1460 2.5V
750k
DIGITAL INTERFACE
TP0610L 0.02µF
150pF
130k TP0610L 0.02µF 130k
"ACQUIRE" (S1)
74C74
140Hz
LTC1799 74C90 CD4024 RSET 56.2k*
"READ" (S2)
METAL FILM RESISTOR PANASONIC ECP-U1C105MA5 OPTICALLY DRIVEN MOSFETS. AROMAT AQW227NA (DUAL) VOLTAGES <80V, AQS225SX (QUAD, SO-16 PACKAGE) 74C02 AN92
Figure Figure 16's Optically Driven Based Current Monitor Modified Digital Output. LTC1799 Clocks A-to-D Optical Switch LEDs. 0.09% Accuracy, Trimmable 0.02%
Note LTC2431's internal digital filter's first null occurs 1/2560 frequency applied pin. details, LTC2431 data sheet.
AN92-12
Application Note
domain, presenting ground referred digital data. This simple approach attractive, although available bias supply must supply about A-to-D attendant circuitry. LTC2410 LT1029 reference powered directly from high voltage bias supply input. Current sink LT1029 bias LTC2410 pin, maintaining across A-to-D over bias rail range. A-to-D's differential inputs measure across current shunt. Resistors zener clamp protect A-to-D from excessive voltages bias line shorted ground. A-to-D's digital outputs, floating high voltage, drive level shifts which provide ground referred data. identical stages shown; other indicated conceptual form. stage designed quiescent dynamic current consumption while maintaining data fidelity. This necessary minimize current drain from bias supply avoid modulating with transient loading artifacts. High voltage common emitter sources current which provides ground referred logic compatible output. Capacitive feedforwards maintain data edge speed while minimizing standing current requirements. This circuit's 0.25% untrimmed accuracy shunt LT1029 tolerances. Trimming LT1029 (see schematic note) permits 0.05% accuracy.
5.1k
0.05% 5.1k
OPTIONAL "ZERO CURRENT" FEEDBACK CONNECTION BIAS REGULATOR, APPENDIX BIAS
1N4689 5.1V
VREF- VREF LTC2410 LT1029 10µF TRIM (SEE NOTES)
LEVEL SHIFT IDENTICAL Q1-Q2 STAGE
MPSA42 2.2k
470pF 2N2369
AN92
2N5400
LT1029 TRIM PERMITS HIGHER ACCURACY. DATA SHEET
Figure A-to-D Converter Floats High Voltage, Forming Digital Output Current Monitor. Q1-Q2 Level Shift Provides Ground Referenced Digital Output. 0.25% Accuracy Trimmable 0.05%
AN92-13
Application Note
Digital Output Current Monitor Bias Supply Figure also floats A-to-D converter across shunt, while including bias supply. bias supply derived from LT1946 switching regulator operating nearly identical fashion Figure 11's circuit. primary difference that Figure 11's inductor replaced here with transformer. transformer's primary winding furnishes high voltage step-up, similar Figure floating secondary drives isolated LT1120 based 3.75V regulator. This floating regulator's output, stacked bias line, powers LTC2400 A-to-D converter. isolated 3.75V supply permits Ato-D measure across shunt without pulling operating power from supply. Resistive current limiting 5.1V zener protect A-to-D from high voltage bias output shorted ground. power optoisolators provide ground referred digital output while eliminating floating supply "starve out" cross regulation interaction with regulation loop. Specifically, very power bias outputs could result insufficient transformer flux furnish floating supply loading requirements. Common optoisolators require significant current, mandating power types specified. previous circuit's discrete level shift stage would draw even less power optoisolators simple adequate.
0.001µF 510k* 3.75V REFERRED
1N4148 BAV21 ZVN4424 1N4148 MBR0520 200pF 100V
LT1120A
2.5V
10µF
10µF 1N4689 5.1V
10µF
NOTES 100V
BIAS OUTPUT
5.1k INPUT LTC2400 A-TO-D
LT1946
18.3k*
SHDN 0.1µF
1500pF VPROGRAM 4.7k 4.7k
100V OPTOISOLATORS
METAL FILM RESISTOR TECATE CMC100105MX1825 AGILENT HCPL-2300 COOPER CTX02-16003X1 0.1% ACCURACY, LT1460 2.5V REFERENCE LTC2400 CHANGE SHUNT TOLERANCE 0.025% OPTIONAL "ZERO CURRENT" FEEDBACK CONNECTION, APPENDIX
AN92
Figure Bias Supply with Digital Output Current Monitor. T1's Primary Supplies High Voltage Source, Similar Figure Secondary Furnishes Power Floating Circuitry. Shunt Voltage Drop Compensatible Using Optional Feedback Circuitry. Optoisolators Provide Ground Referred Digital Output. Current Monitor Accuracy Trimmable 0.1%
AN92-14
Application Note
LT1120 2.5V reference shunt tolerances dictate circuit accuracy. tighter tolerance components noted schematic used, 0.1% accuracy practical. Summary Figure 21's chart attempt summarize circuits presented, although such brevity breeds oversimplification. such, although chart reviews salient features, there substitute thorough investigation particular application's requirements.
BIAS FIGURE SUPPLY NUMBER CAPABILITY
ANALOG OUTPUT CURRENT MONITOR (100nA 1mA)
DIGITAL OUTPUT CURRENT MONITOR (100nA 1mA)
COMMENTS 0.4% Accuracy. High Noise Rejection 0.5% Accuracy. Draws Current from Bias Supply Approximately Equalling Current Delivered Addition Housekeeping Current 200µV Noise 10MHz Bandwidth. Accuracy Bias Voltage Accuracy. 0.5% Current Monitor Accuracy. Current Monitor Output Impedance 0.25% Bias Voltage Accuracy. Output Noise 10MHz Bandwidth. 0.25% Current Monitor Accuracy. Small Size. Large Value, High Voltage Capacitors Improves Reliability. Current Drain from Rail Permits Smaller High Voltage Capacitors Given Ripple Level Bias Voltage Accuracy. 1.5mV Output Noise 10MHz Bandwidth. Small Size, Simple Bias Voltage Accuracy. 200µV Ripple Noise 100MHz Bandwidth. Relatively Large Solution Size 250kHz Oscillator Frequency Bias Voltage Accuracy. 200µV Ripple Noise 100MHz Bandwidth. Current Monitor Accuracy Depends Option Selected. Relatively Large Solution Size 250kHz Oscillator Frequency 0.02% Accuracy. Current Drain from Rail Permits Smaller High Voltage Capacitors Given Ripple Level 0.09% Accuracy. 0.02% Achievable with Shunt Trimming. Current Drain from Rail Permits Smaller High Voltage Capacitors Given Ripple Level 0.25% Accuracy. Trimmable 0.05% Adjusting Reference Bias Voltage Accuracy. Current Monitor Accuracy. 0.1% Accuracy Obtainable with Optional LT1460 Reference. Current Drain from Rail Permits Smaller High Voltage Capacitors Given Ripple Level
Figure Summarized Characteristics Techniques Presented. Applicable Circuit Depends Application Specifics
Note: This application note derived from manuscript originally prepared publication magazine.
AN92-15
Application Note
REFERENCES Meade, M.L., "Lock-In Amplifiers Applications," London, Peregrinus, Ltd. Williams, Monolithic Switching Regulator with 100µV Output Noise," Linear Technology Corporation, Application Note October 1997. Williams, "Measurement Control Circuit Collection," "VBE Based Thermometer," Linear Technology Corporation, Application Note June 1991, Williams, "Applications Switched Capacitor Instrumentation Building Block," Linear Technology Corporation, Application Note July 1985. Williams, "Monolithic CMOS-Switch Suits Diverse Applications," Magazine, October 1984. Williams, Fourth Generation Backlight Technology," "Floating Lamp Circuits," Linear Technology Corporation, Application Note November 1995, 40-43, Figure Negrete, "Fiberoptic Communication Systems Benefit from Tiny, Noise Avalanche Photodiode Bias Supply," Linear Technology Corporation, Design Note 273, December 2001. Martin, "Charge Pump Based Circuits," Private Communication, 2002. Williams, "Applications Precision Amps," "Instrumentation Amplifier with 300V 160dB CMRR," Linear Technology Corporation, Application Note January 1985, 1-2. Williams, "Bridge Circuits," "Optically Coupled Switched Capacitor Instrumentation Amplifier," Linear Technology Corporation, Application Note June 1990, 9-10 Hickman, Hunt, Electronic Voltage Stabilizers," "Cascode," Review Scientific Instruments, January 1939, 6-21,
APPENDIX ERROR FEEDBACK SIGNAL DERIVATION TECHNIQUES Various text circuits either detail make reference counteracting loading effects bias supply's output feedback divider. divider located before current shunt, current drain included current monitor's output error incurred. potential difficulty with this approach that shunt appears series with bias supply output, degrading load regulation. maximum shunt current produces output regulation drop. some cases this permissible further consideration required. Circumstances dictating tighter load regulation require compensation techniques. Divider Current Error Compensation-"Low Side" Shunt Case When shunt transformer's return path ("low side shunt"), divider error cancelled introducing compensatory term into current monitor circuitry. Figure shows details. output voltage divider's current loading error prevented from appearing A1's output feeding forward compensatory current from bias programming input. This compensating current, arriving RLARGE, scaled precisely balance portion shunt output contributed voltage divider's loading error. Divider Current Error Compensation-"High Side" Shunt Case Figure addresses situations where shunt resides "high side." Such arrangements involve high common mode voltages, seemingly mandating high voltage buffer amplifier isolate divider's current loading. Figure shows around this, using standard voltage amplifiers process high voltage signals. sensing after shunt, isolates feedback divider's loading while permitting bias regulator include shunt within feedback loop. powered directly from bias regulator's high voltage output
AN92-16
Application Note
zener clamped with respect pin. Current sink maintains this bias over wide range possible regulator outputs. Although processes high voltage signals, voltage across held safe levels. 5.6V zener bias line ensures A1's inputs always inside their common mode operating range. resistor maintains adequate zener bias when currents extremely low. resistor protects from destructive high voltage bias output shorted ground. Similarly, 100k resistor prevents high voltage from appearing supply fails.
THIS POINT GOES NEGATIVE WITH INCREASING CURRENT
OUTPUT VOLTAGE DIVIDER
IAPD
CURRENT SHUNT CONTROLLER BIAS PROGRAMMING INPUT RLARGE FEEDFORWARD COMPENSATION CURRENT RLARGE SCALED VPROGRAM INDUCED CURRENT DIVIDER ERROR CURRENT
287k
DIVIDER ERROR CURRENT EAPD RDIVIDER (E.G., EAPD DIVIDER ERROR CURRENT 8.75µA)
DIVIDER ERROR 100k 101k
CURRENT MONITOR OUTPUT (IAPD DIVIDER ERROR)
AN92 FA01
Figure Output Voltage Divider Current Loading Error Compensated with Feedforward from Programming Input. Algebraically Sums Feedforward Term Current Shunt Information, Presents Corrected Output
CURRENT MONITOR FROM BIAS REGULATOR 25VIN 95VIN 1N4690 5.6V
VOUT
1N969 FEEDBACK DIVIDER 0.1µF LT1077
BIAS REGULATOR FEEDBACK INPUT
MPSA42
100k
AN92 FA02
Figure Follower Floats from High Voltage Rail, Eliminates Feedback Divider Current Loading Error. Current Source Zener Maintain Voltage Across Amplifier; 5.6V Zener Accomodates A1's Input Range
AN92-17
Application Note
APPENDIX PREAMPLIFIER OSCILLOSCOPE SELECTION level measurements described require some form preamplification oscilloscope. Current generation oscilloscopes rarely have greater than 2mV/DIV sensitivity, although older instruments offer more capability. Figure lists representative preamplifiers oscilloscope plug-ins suitable noise measurement. These units feature wideband, noise performance. particularly significant that many these instruments longer produced. This keeping with current instrumentation trends, which emphasize digital signal acquisition opposed analog measurement capability. monitoring oscilloscope should have adequate bandwidth exceptional trace clarity. latter regard high quality analog oscilloscopes unmatched. exceptionally small spot size these instruments well-suited level noise measurement.1 digitizing uncertainties raster scan limitations DSOs impose display resolution penalties. Many displays will even register small levels switching-based noise.
INSTRUMENT TYPE Amplifier Differential Amplifier Differential Amplifier Differential Amplifier Differential Amplifier Differential Amplifier Differential Amplifier
MANUFACTURER Hewlett-Packard Tektronix Tektronix Tektronix Tektronix Preamble Preamble
MODEL MAXIMUM NUMBER BANDWIDTH SENSITIVITY/GAIN 461A 7A13 11A33 P6046 1855 1822 150MHz 50MHz 100MHz 150MHz 100MHz 100MHz 10MHz Gain 1mV/DIV 1mV/DIV 1mV/DIV 1mV/DIV Gain Gain 1000
AVAILABILITY Secondary Market Secondary Market Secondary Market Secondary Market Secondary Market Current Production Current Production
COMMENTS Input, Standalone Requires Series Mainframe Requires 7000 Series Mainframe Requires 11000 Series Mainframe Standalone Standalone, Settable Bandstops Standalone, Settable Bandstops
Figure Some Applicable High Sensitivity, Noise Amplifiers. Trade-Offs Include Bandwidth, Sensitivity Availability
Note work have found Tektronix types 453, 453A, 454, 454A, excellent choices. Their pristine trace presentation ideal discerning small signals interest against noise floor limited background.
AN92-18
Application Note
APPENDIX PROBING CONNECTION TECHNIQUES LEVEL, WIDEBAND SIGNAL INTEGRITY1 most carefully prepared breadboard cannot fulfill mission signal connections introduce distortion. Connections circuit crucial accurate information extraction. level, wideband measurements demand care routing signals test instrumentation. Ground Loops Figure shows effects ground loop between pieces line-powered test equipment. Small current flow between test equipment's nominally grounded chassis creates 60Hz modulation measured circuit output. This problem avoided grounding line powered test equipment same outlet strip otherwise ensuring that chassis same ground potential. Similarly, test arrangement that permits circuit current flow chassis interconnects must avoided. Pickup Figure also shows 60Hz modulation noise measurement. this case, 4-inch voltmeter probe feedback input culprit. Minimize number test connections circuit keep leads short. Poor Probing Technique Figure C3's photograph shows short ground strap affixed scope probe. probe connects point which provides trigger signal oscilloscope. Circuit output noise monitored oscilloscope coaxial cable shown photo.
100µV/DIV
500µV/DIV
2ms/DIV
AN92
5ms/DIV
AN92
Figure Ground Loop Between Pieces Test Equipment Induces 60Hz Display Modulation
Figure 60Hz Pickup Excessive Probe Length Feedback Node
Note Veterans Application Notes, hardened crew, will recognize this Appendix from AN70 (see Reference Although that publication concerned considerably more wideband noise measurement, material directly applicable this effort. such, reproduced here reader convenience.
AN92-19
AN92-20
Application Note
Figure Poor Probing Technique. Trigger Probe Ground Lead Cause Ground Loop-Induced Artifacts Appear Display
Application Note
Figure shows results. ground loop board between probe ground strap ground referred cable shield causes apparent excessive ripple display. Minimize number test connections circuit avoid ground loops. Violating Coaxial Signal Transmission-Felony Case Figure coaxial cable used transmit circuit output noise amplifier-oscilloscope been replaced with probe. short ground strap employed probe's return. error inducing trigger channel probe previous case been eliminated; 'scope triggered noninvasive, isolated probe.2 Figure shows excessive display noise breakup coaxial signal environment. probe's ground strap violates coaxial transmission signal corrupted Maintain coaxial connections noise signal monitoring path. Violating Coaxial Signal Transmission- Misdemeanor Case Figure C7's probe connection also violates coaxial signal flow, less offensive extent. probe's ground strap eliminated, replaced grounding attachment. Figure shows better results over preceding case, although signal corruption still evident. Maintain coaxial connections noise signal monitoring path. Proper Coaxial Connection Path Figure coaxial cable transmits noise signal amplifier-oscilloscope combination. theory, this affords highest integrity cable signal transmission. Figure C10's trace shows this true. former example's aberrations excessive noise have disappeared. switching residuals faintly outlined amplifier noise floor. Maintain coaxial connections noise signal monitoring path. Direct Connection Path good verify there cable-based errors eliminate cable. Figure C11's approach eliminates cable between breadboard, amplifier oscilloscope. Figure C12's presentation indistinguishable from Figure C10, indicating cable-introduced infidelity. When results seem optimal, design experiment test them. When results seem poor, design experiment test them. When results expected, design experiment test them. When results unexpected, design experiment test them. Test Lead Connections theory, attaching voltmeter lead regulator's output should introduce noise. Figure C13's increased noise reading contradicts theory. regulator's output impedance, albeit low, zero, especially frequency scales noise injected test lead works against finite output impedance, producing 200µV noise indicated figure. voltmeter lead must connected output during testing, should done through 10k-10µF filter. Such network eliminates Figure C13's problem while introducing minimal error monitoring DVM. Minimize number test lead connections circuit while checking noise. Prevent test leads from injecting into test circuit.
Note discussed. Read
100µV/DIV (INVERTED)
5µs/DIV
AN85
Figure Apparent Excessive Ripple Results from Figure C3's Probe Misuse. Ground Loop Board Introduces Serious Measurement Error
AN92-21
Application Note
Figure Floating Trigger Probe Eliminates Ground Loop, Output Probe Ground Lead (Photo Upper Right) Violates Coaxial Signal Transmission
500µV/DIV
5µs/DIV
AN92
Figure Signal Corruption Figure C5's Noncoaxial Probe Connection
AN92-22
Application Note
Figure Probe with Grounding Attachment Approximates Coaxial Connection
100µV/DIV
5µs/DIV
AN92
Figure Probe with Grounding Attachment Improves Results. Some Corruption Still Evident
AN92-23
Application Note
Figure Coaxial Connection Theoretically Affords Highest Fidelity Signal Transmission
100µV/DIV
5µs/DIV
AN92
Figure C10. Life Agrees with Theory. Coaxial Signal Transmission Maintains Signal Integrity. Switching Residuals Faintly Outlined Amplifier Noise
AN92-24
Application Note
Figure C11. Direct Connection Equipment Eliminates Possible Cable-Termination Parasitics, Providing Best Possible Signal Transmission
100µV/DIV
5µs/DIV
AN92
Figure C12. Direct Connection Equipment Provides Identical Results Cable-Termination Approach. Cable Termination Therefore Acceptable
AN92-25
Application Note
200µV/DIV
5µs/DIV
AN92
Figure C13. Voltmeter Lead Attached Regulator Output Introduces Pickup, Multiplying Apparent Noise Floor
Isolated Trigger Probe text associated with Figure somewhat cryptically alluded "isolated trigger probe." Figure reveals this simply choke terminated against ringing. choke picks residual radiated field, generating isolated trigger signal. This arrangement furnishes 'scope trigger signal with essentially measurement corruption. probe's physical form appears Figure C15. good results, termination should adjusted minimum ringing while preserving highest possible amplitude output. Light compensatory damping produces Figure C16's output, which will cause poor 'scope triggering. Proper adjustment results more favorable output (Figure C17), characterized minimal ringing welldefined edges. Trigger Probe Amplifier field around switching magnetics small adequate reliably trigger some oscilloscopes. such cases, Figure C18's trigger probe amplifier useful. uses adaptive triggering scheme compensate variations probe output amplitude. stable trigger output maintained over 50:1 probe output range. operating gain 100, provides wideband gain. output this stage biases 2-way peak detector through Q4). maximum peak stored Q2's emitter capacitor, while minimum excursion retained Q4's
emitter capacitor. value midpoint A1's output signal appears junction 500pF capacitor units. This point always sits midway between signal's excursions, regardless absolute amplitude. This signal-adaptive voltage buffered trigger voltage LT1394's positive input. LT1394's negative input biased directly from A1's output. LT1394's output, circuit's trigger output, unaffected >50:1 signal amplitude variations. X100 analog output available Figure shows circuit's digital output (Trace responding amplified probe signal (Trace Figure typical noise testing setup. includes breadboard, trigger probe, amplifier, oscilloscope coaxial components.
PROBE SHIELDED CABLE TERMINATION OUTPUT
CONNECTION TERMINATION J.W. MILLER #100267
DAMPING ADJUST 4700pF
AN92 FC14
Figure C14. Simple Trigger Probe Eliminates Board Level Ground Loops. Termination Components Damp L1's Ringing Response
AN92-26
Application Note
AN92-27
Figure C15. Trigger Probe Termination Box. Clip Lead Facilitates Mounting Probe, Electrically Neutral
Application Note
10mV/DIV
10mV/DIV
10µs/DIV
AN92
10µs/DIV
AN92
Figure C16. Misadjusted Termination Causes Inadequate Damping. Unstable Oscilloscope Triggering Result
Figure C17. Properly Adjusted Termination Minimizes Ringing with Small Amplitude Penalty
ANALOG OUTPUT 'SCOPE TRIGGER INPUT
0.005µF 0.005µF 500pF
LT1227
10µF
0.1µF
100µF
0.1µF
0.1µF
CA3096 ARRAY: SUBSTRATE (PIN GROUND 1N4148 TRIGGER PROBE TERMINATION (SEE FIGURE DETAILS)
Figure C18. Trigger Probe Amplifier Analog Digital Outputs. Adaptive Threshold Maintains Digital Output Over 50:1 Probe Signal Variations
1V/DIV COUPLED
5V/DIV
10µs/DIV (UNCALIB)
AN92
Figure C19. Trigger Probe Amplifier Analog (Trace Digital (Trace Outputs
AN92-28
LT1006
DIGITAL TRIGGER 'SCOPE
AN92
LT1394
Application Note
AN92-29
Figure C20. Typical Noise Test Setup Includes Trigger Probe, Amplifier, Oscilloscope Coaxial Components
Application Note
APPENDIX SINGLE RAIL AMPLIFIER WITH TRUE ZERO VOLT OUTPUT SWING Performance requirements necessitate analog output current monitors swing within 100µV zero. This difficult because circuits from single, positive rail. single rail amplifier swing this close zero while maintaining accurate outputs. Figure D1's power supply bootstrapping scheme achieves desired characteristics with minimal component addition. chopper stabilized amplifier, clock output. This output switches providing drive diode-capacitor charge pump. charge pump output feeds A1's terminal, pulling below zero, permitting output swing (and below) ground. desired, negative output excursion limited either clamp option shown. Reliable start-up this bootstrapped power supply scheme valid concern, warranting investigation. Figure amplifier's (Trace initially rises supply turn-on (Trace heads negative when amplifier clocking (Trace commences about midscreen. circuit provides simple obtain output swing zero volts, permitting true "live zero" output.
LTC1150
10µF CLKOUT 100k DASHED LINE CIRCUITRY CLAMP OPTIONS. TEXT BAT85
AN92 FD01
-3.5V HERE
Figure Single Rail Powered Amplifier True Zero Volt Output Swing. A1's Clock Output Switches Driving DiodeCapacitor Charge Pump. A1's Assumes Negative Voltage, Permitting Zero (and Below) Volt Output Swing
5V/DIV
5V/DIV
0.2V/DIV
5ms/DIV
AN92
Figure Amplifier Bootstrapped Supply Start-Up. Amplifier (Trace Initially Rises Positive Supply (Trace Turn-On. When Amplifier Internal Clock Starts (Trace Vertical Division), Charge Pump Activates, Pulling Negative
AN92-30
2N3904
10µF
Application Note
APPENDIX PROTECTION CIRCUITS receiver modules electrically delicate expensive devices. Because this, Figure E1's protection circuits interest. They designed protect module from bias programming overvoltage error (Figure E1a), excessive current (E1b) destructive voltage (Figure E1c). Figure E1a, normally programming voltage passes bias regulator voltage programming input. Abnormally high inputs, defined potentiometer's setting, cause swing low, biasing closing A1's feedback loop. This causes Q1's emitter clamp potentiometer wiper's voltage, safely limiting bias regulator's programming input. Figure current limiter. This particular circuit designed with "low side" shunts transformer coupled supplies, such text Figure although technique generally applicable. long shunt current's absolute value below current limit point, saturated high associated bias regulator functions normally. Shunt overcurrent forces A2's output lower, pulling regulator's control (VC) lower limiting current. 100pF-1M combination stabilizes bias regulator assumes characteristics current source. Figure overvoltage crowbar. intended last line defense against uncontrolled bias supply high voltage outputs. Normally, LTC1696 crowbar below 0.88V trigger threshold off. bias rises high LTC1696 triggers, firing SCR. turn-on "crowbars" bias line, arresting high voltage maintaining short across line latch characteristic. bias supply significant output impedance, prolonged loading deleterious; not, bias supply should fused.
INPUT FROM PROGRAMMING VOLTAGE
BIAS REGULATOR VOLTAGE PROGRAMMING INPUT
4.7k LT1006 VOLTAGE CLAMP ADJUST
2N3906
AN92 FE01a
(E1a) Programming Voltage Clamp
UNGROUNDED SIDE SHUNT TRANSFORMER SHUNT DERIVED CURRENT BIAS OUTPUT BIAS REGULATOR 0.001µF
LIMIT CURRENT CURRENT LIMIT INPUT
AN92 FE01b
LT1077
7.5M* TRIG 0.88V 88.7k* VTRIG 75.3V
2N5063
100pF
LTC1696
AN92 FE01c
(E1b) Current Limiter
METAL FILM RESISTOR 1N4148
(E1c) Bias Voltage Crowbar
Figure Protection Circuits Prevent Destruction Hardware Software Failures. Options Include Programming Voltage Clamp (Figure E1a), Current Limiter (Figure E1b) Bias Voltage Crowbar (Figure E1c)
Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
AN92-31
Application Note
AN92-32
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408) 432-1900 FAX: (408) 434-0507
an92f LT/TP 1102 PRINTED
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2002
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