Converter Handle Frequency-to-Voltage Needs Simplify your convert
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Converter Handle Frequency-to-Voltage Needs
Converter Handle Frequency-to-Voltage Needs
Simplify your converter designs with versatile Starting with basic converter circuit modify meet almost application requirement spare yourself some hard labor when designing frequency-to-voltage converters using voltage-to-frequency your designs These form basis series accurate economical converters suiting variety applications
National Semiconductor Appendix Robert Pease August 1980
Figure shows LM331 LM131 military temperature range) basic converter configuration (sometimes termed stand-alone converter because requires amps other active devices other than (Comparable such RM4151 take advantage this other circuits described this article although they might always pin-for-pin compatible) This circuit accepts pulse-train square wave input amplitude greater coupling capacitor suits negative-going input pulses between well accommodating square waves positive-going pulses long interval between pulses least
Handles Hard Part LM331 detects input-signal change sensing when goes negative relative threshold voltage which nominally biased lower than supply voltage When signal change occurs LM331's input comparator sets internal latch initiates timing cycle During this cycle current equal VREF flows
time capacitor filters this pulsating current from current's average value flows through load resistor result input circuit outputs across with good typical) nonlinearity problems remain however output includes about mVp-p ripple also lags second behind input frequency step change settling fullscale about second This ripple slow response represent inherent tradeoff that applies almost every converter Compromise Increasing filter capacitor's value reduces ripple also increases response time Conversely lowering filter capacitor's value improves response time expense larger ripple some cases adding active filter results faster response less ripple high input frequencies Although circuit specifies power supply regulated supply between output voltage extend within supply voltage choose maintain that output range Adding postfilter circuit slows response slightly also reduces ripple less than mVp-p frequencies from reduction ripple achieved adding this passive filter while good that obtainable using active filter could suffice some applications
stable components with temperature coefficients
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FIGURE Simple Stand-Alone Converter Forms Basis Many Other Converter-Circuit Configurations
C1995 National Semiconductor Corporation
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RRD-B30M115 Printed
Improving Basic Circuit Further modifications additions basic converter shown Figure adapt specific performance requirements Figure shows such modification which improves converter's nonlinearity 006% typical Reconsideration basic stand-alone converter shows nonlinearity falls short this improved version's input frequencies current source feeding LM331 turned most time input frequency increases however current source stays more time impedance attenuates output signal increasing fraction each cycle time This disproportionate attenuation higher frequencies causes parabolic change full-scale gain rather than desired linear improved circuit other hand transistor acts cascade output impedance sees constant voltage that won't modulate gain Also with alpha ranging between transistor exhibits temperature coefficient between fairly minor effect Thus this circuit's
nonlinearity does exceed maximum output range shown normally worse than supply voltage between Output Buffer circuit Figure adds output buffer (unity-gain follower) basic single-supply converter Either LM324 LM358 functions well single-supply circuit because these devices' common-mode ranges extend down ground negative supply available types such LF351B LM308A which have input currents provide best accuracy output buffer Figure also acts active filter furnishing 2-pole response from single This filter provides general response VOUT IOUT K2p2) differential operator shown controls filter's gain high frequency response rolls octave Near circuit's natural resonant frequency choose damping give little overshoot none desired
2N4250 2N3906 similar high-beta transistor Select stable components with temperature coefficients
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FIGURE Adding Cascade Transistor LM331's Output Improves Nonlinearity 006%
LM358 LM324 stable components with temperature coefficients
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FIGURE This Converter's Output Acts Buffer Well 2-Pole Filter
Dealing with Converter Ripple Voltage ripple output converters present problem chart shown Figure indicates exactly problem simple slow filter exhibits ripple frequencies Two-pole filters offer lowest ripple high frequencies provide 30-times-faster step response than devices reduce circuit's ripple moderate frequencies however cascade second active-filter stage converter's output That circuit's response also appears Figure shows significant improvement low-ripple bandwidth over single-active-filter configuration with only degradation step response
Note should have high CMRR best results
Figures show filter circuits suitable cascading inverting filter Figure requires closely matched resistors with over their temperature range best accuracy lowest error choose (RINlRF) This circuit's response VOUT nR2)C4p RFR2C3C4p2) where gain VOUT 2R2)C4p RFR2C3C4p2)
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FIGURE This 2-Pole Noninverting Filter Suits Cascade Requirements Converter Outputs circuit shown Figure does require precision passive components best accuracy choosing with high CMRR critical LM308A amp's minimum CMRR suits this circuit well LM358B's typical figure also proves adequate many applications Circuit response VOUT R1R2C1C2p2) best results choose Components Determine Response specific response circuit Figure VOUT IOUT R2)C2p RLR2C1C2C2p2) Making relatively large eliminates overshoot sine peaking Alternatively making suitable fraction done Figure produces both sine response with peaking quick real-time response having only overshoot step response maintaining Figure ratio adapt 2-pole filter wide frequency range without tedious computations This filter settles within step's final value about contrast circuit with simple filter shown Figure takes about achieve same response offers less ripple than Figure approach other component 2-pole filter capacitance between suits because serves only bypass resistor helps reduce output ripple single positive power-supply systems when VOUT approaches close ground that amp's output impedance suffers this circuit using tantalum capacitor between usually helps keep filter's output much quieter without degrading amp's stability
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FIGURE Output-Ripple Performance Several Different Converter Configurations Illustrates Effect Voltage Ripple
Note should closely matched with good tracking best accuracy nRIN
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FIGURE Cascade This 2-Pole Inverting Filter onto Converter's Output
Avoid Low-Leakage Limitations Note that most ordinary applications this 2-pole filter performs well with capacitors passive filter Figure does with Thus require converter circuit Figure furnishes good filtering with eliminates low-leakage capacitor needed passive filter Note also that because always zero voltage across tantalum aluminum electrolytic capacitor with leakage-related problems however must low-leakage type room temperature typical tantalum components allow only nanoamperes leakage leakage this usually cannot guaranteed Compensating Temperature Coefficients converters often encounter temperature-related problems usually resulting from temperature coefficients passive components Following some simple design manufacturing guidelines help immunize your circuits against loss accuracy when temperature changes Capacitors fabricated from Teflon polystyrene usually exhibit b110 When such component timing capacitor converter (such figure circuit's output voltage gain terms volts kilohertz also exhibits b110 resistor-diode network connected from ground figure cancel effect timing capacitor's large When current flowing through will then have overall effectively canceling polystyrene timing capacitor's first approximation Thus needn't find zero-TC capacitor long temperature coefficient stable well established additional advantage resistor-diode network nearly compensates zero rest circuit Bake While After circuit been built checked room temperature brief oven test will indicate sign size complete converter Then resistance series with conductance
parallel with greatly diminish previously observed yield complete circuit with lower than could obtain simply buying parts example circuit increases full-scale output during oven test adding series with cancels temperature-caused deviation full-scale output decreases (b20 just parallel with Note that allow trimming both directions must start with finite fixed (such b110 which then nominally cancels addition finite adjustable Only using this procedure compensate whatever polarity found oven test utilize this technique obtain perhaps even take passes zero-in best value optimum results consider following guidelines
good capacitor cheapest polystyrene
capacitors shift value more temperature cycle that case would able distinguish actual temperature sensitivity from hysteresis would also never achieve stable circuit
After soldering bake temperature-cycle circuit
temperature exceeding case polystyrene) hours stabilize components relieve strains soldering
rush trimming Recheck room temperature value before after take high temperature data ensure reasonably hysteresis cycle
expect perfect trim
temperatures from None components figure's circuit offer linearity much better than cold trimmed zero warm temperatures Even using these techniques obtain data converter with better than accuracy 003% linearity range around room temperature
Start trimming with installed value near
design-center value
1N484 FD333 similar silicon planar diode
stable components with temperature coefficients
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Diodes Resistor Help Decrease Converter's Temperature Coefficient
will reasonably close zero will usually find process slower start without resistor because trimming converges more slowly
that while circuit's nonlinearity error negligible ripple circuit Figure offers significant advantage over some other designs because offset adjust voltage derives from stable reference voltage LM331 thus supply voltage shifts cause output shifts offset have value between optional bypass capacitor (C2) connected from amp's positive input ground prevents output noise arising from stray noise pickup that point capacitance value critical Familiar Response circuit Figure exhibits same 2-pole response with heavy output ripple attenuation noninverting filter Figure Specifically VOUT IOUT RF)C4p R4RFC3C4p2) Here also provides best bias current compensation LM331 handle frequencies utilizing smaller-value capacitors shown Figure This circuit increases current facilitate high-speed switching despite these speed-ups LM331's causes problems because switching speed shifts resulting from temperature changes compensate device's positive LM334 temperature sensor feeds current that decreases linearly with temperature provides overall temperature coefficient value provides first-order compensation trim higher lower need more precise correction
change from pull
part resistor will much more consistent results adding resistor parallel same admonition holds true adding resistance series with
reasonably stable components LM331A
maximum) RN55D film resistors (each probably won't able trim resulting worst-case Resistors with specification usually work well Finally same resistor value both when these resistors come from same manufacturer's batch their tracking will usually rate better than Whenever used buffer Figure offset voltage current maximum respectively most inexpensive devices) cause worst-case output offset both plus minus supplies available however easily provide symmetrical offset adjustment With only supply small positive current each input also trim inputs Need Negative Output your converter application requires negative output voltage circuit shown Figure provides solution with excellent linearity 003% typical maximum) because LM331 always remains this circuit needs cascade transistor (Note howev-
stable components with temperature coefficients
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FIGURE This Circuit Output-Buffer Derives Offset Voltage from Precision Voltage Source LM331
Detect Frequencies Accurately Using converter combined with comparator frequency detector obvious application these devices when converter utilized this output ripple hampers accurate frequency detection slow filter frequency response causes delays quick response important though effectively utilize LM331-based converter feed more comparators shown Figure input frequency drop from converter's output
responds within about When input falls from however output responds only after utilize this circuit only applications that tolerate circuits' inherent delays ripples Author's Biography Pease staff scientist Advanced Linear Integrated Circuit Group National Semiconductor Corp Santa Clara Holder four patents earned BSEE from lists tracking abandoned railroad roadbeds designing converters hobbies
stable components with temperature coefficients
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FIGURE LM334 Temperature Sensor Compensates Circuit's Temperature Coefficient
stable components with temperature coefficients
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FIGURE Combining with Comparators Produces Slow-Response Frequency Detector
Converter Handle Frequency-to-Voltage Needs
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