Steven Green +15V 78L05 5.6V Left Analog Input Right Analog Input

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Steven Green +15V 78L05 5.6V Left Analog Input Right Analog Input

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INPUT BUFFER PROTECTION TECHNIQUES
Steven Green
+15V 78L05 5.6V Left Analog Input Right Analog Input
BAT-85 HP5082-2810 AINL CS5336 CS5338 CS5339 AINR AGND
-15V 79L05 5.6V
Figure Input Protection Op-Amp
INTRODUCTION
design input buffer protection circuits analog-to-digital-converters (ADC) critical optimized reliable data acquisition system. Crystal Semiconductor application note "ADC Input Buffers" covered this area well system designer should review this information. Since publication "ADC Input Buffers" there have been many requests additional information circuits relating input protection. This application note describes suitable buffer/protection circuits CS5336 family converters. techniques described equally applicable other families Crystal analogto-digital converters.
Cirrus Logic, Inc. Crystal Semiconductor Products Division P.O. 17847, Austin, Texas 78760 (512) 7222 FAX: (512) 7581 http://www.crystal.com
LATCH-UP
latch-up been defined "the creation impedance path between power supply rails triggering parasitic, four- layer bipolar structures (SCR's) inherent CMOS input output circuitry." This self-sustaining condition once latched, CMOS device will remain regardless voltages until power supply voltages removed. excessive power dissipation during latch-up also damage device. Latch-up most often caused forcing current into inputs outputs CMOS device applying voltages greater than power supply rails. When powered, Crystal Semiconductor ADC's extremely immune latch-up because
Copyright Cirrus Logic, Inc. 1998 (All Rights Reserved)
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amount current required initiate latch. Problems arise when input voltages greater than instantaneous power supply voltages applied during power-up. less common equally damaging condition occur when power-supply voltages exceed absolute maximum specified value. There several protection techniques available designer each with their advantages disadvantages.
discussed detail references 1-7. Figure shows diode-clamped input buffer circuit utilizing multiple supplies. type diode selected CR1-CR4 crucial must evaluated using following criteria: Forward-biased voltage characteristics. Schottky diodes preferred their forward-biased voltage characteristics. Reverse-bias leakage current. effects voltage dependent leakage currents proportional circuit impedences cause distortion. Leakage currents will also vary with temperature must evaluated over intended temperature operating range. Reverse-bias capacitance. Voltage dependent junction capacitance cause distortion must insignificant comparison circuit component values. Figure comparison %THD plot circuit Figure with without 1N5818 Schottky diodes installed CR1-CR4. Note increase %THD resulting from diode capacitance. Figure comparison %THD plot circuit Figure with without Philips BAT-85 Schottky diodes. Note lack distortion produced addition suitable protection components. Hewlett-Packard 5082-2810 diodes give similar results.
PROTECTION TECHNIQUES
goal input protection guarantee that input voltage never exceeds supply voltages converter. This accomplished with op-amp buffer between "outside" world input, then limiting input voltage excursions range bounded converter power supply voltages.
Method
There many high quality op-amps available design engineer input buffers majority these have been designed operate from power supplies greater than required multiple supplies presents potential problems. possible analog input experience voltages greater than supplies either during signal amplitude excursions, transient power-on conditions op-amp failure. Several methods available clamp input volt-
Figure %THD Effects 1N5818 Schottky Diodes with NE5532 Op-amp
Figure %THD Effects BAT-85 Schottky Diodes with NE5532 Op-amp
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Notes Method
values were selected optimize source impedance CS5336 utilize current limiting characteristics op-amp. Clamping circuits with diodes feedback loop op-amp work well signal clamping effective power-on transient op-amp failure conditions. These circuits recommended protection.
Method
goals input protection also achieved powering input buffer from same supplies converter shown Figure This circuit requires fewer components than circuit Figure common power supplies guarantees that op-amp output will exceed supply voltages. However, required analog voltage achieve full scale digital output CS5336 typically ±3.68 majority op-amps have this output capability with supplies. Motorola MC33078/9 viable contender this application. Figure shows %THD Frequency MC33078 with supplies operating 3.68 Note lack performance degradation resulting from reduced supplies.
Power Supply Overvoltage
Standard 3-terminal regulators designed either source (78L05) sink (79L05) current both. possible raise supply voltage above regulation voltage through Schottky diodes during error conditions. zener diodes included prevent supply voltages from exceeding maximum specified value damaging converter.
+15V 78L05
Left Analog Input Right Analog Input VA0.1 must capable ±3.7 output with supplies AGND CS5336
AINL
AINR
-15V
79L05
Figure Input Protection Op-Amp
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Figure %THD Effects Power Supply Variation MC33078
Figure %THD Effects Power Supply Variation MC33078 ±4.75
power supply voltages could 4.75 tolerance 78L05/79L05. Figure shows increased %THD MC33078 this supply voltage. transient nature audio signals, digital audio systems generally operated average levels below full scale. This allow sufficient headroom handle high amplitude transient signals. increase distortion full scale regulator tolerances could considered insignificant. required, regulators will avoid this increase distortion.
REFERENCES
Johnston, Jerome: Input Buffers, Crystal Semiconductor Corp. Fredriksen, Thomas Intuitive Amps, National Semiconductor Technology Series 1984 Pease, Robert: Bounding, clamping techniques improve circuit performance, Nov. 1983 Pease, Robert: Active-component problems yield painstaking probing, Aug. 1989 Hewlett/Packard Components: Application Bulletin Waveform Clipping With Schottky Diodes. Hewlett/Packard Components: Application Bulletin Waveform Clipping With Schottky Diodes. Hewlett/Packard Components: Application Note 942, Schottky Diodes High Volume, Cost Applications Application Note MSAN-107: Understanding Eliminating Latch-up CMOS Applications, MITEL August 1982
CONCLUSION
circuits have been described which utilize effective protection techniques. either these circuits techniques described will insure that performance reliability data acquisition system will limited input buffer protection circuits.
AN20REV2
Notes

Steven Green +15V 78L05 5.6V Left Analog Input Right Analog Input

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