Converter Handle Frequency-to-Voltage Needs Simplify your convert

The Datasheet Archive - 100 Million Datasheets from 7500 Manufacturers.

Converter Handle Frequency-to-Voltage Needs Simplify your convert

Datasheet Thumbnail

Top Searches for this datasheet

Converter Handle Frequency-to-Voltage Needs
Converter Handle Frequency-to-Voltage Needs
Simplify your converter designs with versatile ICs. Starting with basic converter circuit, modify meet almost application requirement. spare yourself some hard labor when designing frequency-tovoltage (F/V) converters using voltage-to-frequency your designs. These form basis series accurate, economical, converters suiting variety applications. Figure shows LM331 LM131 military temperature range) basic converter configuration (sometimes termed stand-alone converter because requires amps other active devices other than IC). (Comparable ICs, 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 µs).
National Semiconductor Application Note Robert Pease August 1980
RtC. capacitor filters this pulsating current from current's average value flows through load resistor result, input, circuit outputs across with good (0.06% typical) nonlinearity. problems remain, however: output includes about mVp-p ripple, also lags second behind input frequency step change, settling 0.1% full-scale 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 VDC. output voltage extend within supply voltage, choose maintain that output range. Adding k/0.1 postfilter circuit slows response slightly, also reduces ripple less than mVp-p frequencies from kHz. reduction ripple achieved adding this passive filter, while good that obtainable using active filter, could suffice some applications.
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/RS flows time
00874101
FIGURE Simple Stand-Alone Converter Forms Basis Many Other Converter-Circuit Configurations
AN-C
2002 National Semiconductor Corporation
AN008741
www.national.com
AN-C
Improving Basic Circuit
Further modifications additions basic converter shown Figure adapt specific performance requirements. Figure shows such modification, which improves converter's nonlinearity 0.006% 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 one. improved circuit, other hand, transistor acts cascade, output impedance sees constant voltage that won't modulate gain. Also, with alpha ranging between 0.998 0.990, transistor exhibits temperature coefficient between ppm/°C ppm/°C fairly minor effect. Thus, this circuit's nonlinearity does exceed 0.01% maximum output range shown normally worse than 0.01% supply voltage between 40V.
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, amp; types such LF351B LM308A, which have input currents, provide best accuracy. output buffer Figure also acts active filter, furnishing 2-pole response from single amp. This filter provides general response VOUT/IOUT RL/(1 K2p2). differential operator d/dt.) shown, controls filter's gain. high frequency response rolls dB/octave. Near circuit's natural resonant frequency, choose damping give little overshoot none, desired.
00874102
FIGURE Adding Cascade Transistor LM331's Output Improves Nonlinearity 0.006%
www.national.com
AN-C
Output Buffer
(Continued)
00874103
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. Figure Figure show filter circuits suitable cascading. inverting filter Figure requires closely matched resistors with over their temperature range best accuracy. lowest error, choose (RIN|RF). This circuit's response -VOUT/VIN n/(1 nR2)C4p RFR2C3C4p2). where gain. -VOUT/VIN 1/(1 2R2)C4p RFR2C3C4p2).
00874104
FIGURE Output-Ripple Performance Several Different Converter Configurations Illustrates Effect Voltage Ripple
www.national.com
AN-C
Dealing with Converter Ripple
(Continued)
having only overshoot step response. maintaining Figure ratio C1:C2 R2:RL, 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 0.05 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.
Avoid Low-Leakage Limitations
00874105
FIGURE Cascade This 2-Pole Inverting Filter onto Converter's Output
Note that most ordinary applications, this 2-pole filter performs well with 0.02 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 -110 ppm/°C. When such component timing capacitor converter (such figure) circuit's output voltage gain terms volts kilohertz also exhibits -110 ppm/°C resistor-diode network (RX, connected from ground figure cancel effect timing capacitor's large When current flowing through will then have overall ppm/°C, 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.
00874106
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/VIN 1/(1 R1R2C1C2p2). best results, choose
Components Determine Response
specific response circuit Figure VOUT/IOUT RL/(1 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
www.national.com
AN-C
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 0.1% 30°C ppm/°C) during oven test, adding series with cancels temperaturecaused deviation. full-scale output decreases -0.04% 20°C (-20 ppm/°C), just parallel with Note that allow trimming both directions, must start with finite fixed (such -110 ppm/°C Ct), which then nominally cancels addition finite adjustable Only using this procedure compensate whatever polarity found oven test. utilize this technique obtain ppm/°C, perhaps even ppm/°C, take passes zero-in best value optimum results, consider following guidelines: good capacitor cheapest polystyrene capacitors shift value 0.05% 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 75°C 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 -25°C trim ppm/°C temperatures from +25°C 60°C. None components figure's circuit offer linearity much better than ppm/°C ppm/°C cold, trimmed zero warm temperatures. Even using these techniques obtain data converter with better than 0.02% accuracy 0.003% linearity, 20°C range around room temperature. Start trimming with installed value near design-center value (e.g., will reasonably close zero will usually find process slower start without resistor, because trimming converges more slowly. 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 ppm/°C maximum) RN55D film resistors (each ppm/°C) probably won't able trim resulting ppm/°C worst-case Resistors with specification ppm/°C usually work well. Finally, same resistor value (e.g., 12.1 both when these resistors come from same manufacturer's batch, their tracking will usually rate better than ppm/°C. Whenever used buffer Figure offset voltage current maximum respectively, most inexpensive devices) cause 17.5 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.
00874107
Diodes Resistor Help Decrease Converter's Temperature Coefficient excellent linearity 0.003% typical, 0.01% maximum). because LM331 always remains VDC, this
Need Negative Output?
your converter application requires negative output voltage, circuit shown Figure provides solution with
www.national.com
AN-C
Need Negative Output?
(Continued)
circuit needs cascade transistor. (Note, however, that while circuit's nonlinearity error negligible, ripple not.) 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.
VOUT/IOUT RF/(1 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, but, despite these speed-ups, LM331's ppm/°C 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 firstorder compensation, trim higher lower need more precise correction.
Familiar Response
circuit Figure exhibits same 2-pole response with heavy output ripple attenuation noninverting filter Figure Specifically,
00874108
FIGURE This Circuit, Output-Buffer Derives Offset Voltage from Precision Voltage Source LM331
www.national.com
AN-C
Familiar Response
(Continued)
00874109
FIGURE LM334 Temperature Sensor Compensates Circuit's Temperature Coefficient
Detect Frequencies Accurately
Using converter combined with comparator frequency detector obvious application these devices. when converter utilized this way, 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 kHz, converter's output responds within about When input falls from kHz, however, output responds only after lag, utilize this circuit only applications that tolerate circuits' inherent delays ripples.
www.national.com
Converter Handle Frequency-to-Voltage Needs
Detect Frequencies Accurately
(Continued)
00874110
FIGURE Combining with Comparators Produces Slow-Response Frequency Detector
Author's Biography
Pease staff scientist Advanced Linear Integrated Circuit Group National Semiconductor Corp., Santa
Clara, Holder four patents, earned BSEE from MIT. lists tracking abandoned railroad roadbeds designing converters hobbies.
LIFE SUPPORT POLICY NATIONAL'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT GENERAL COUNSEL NATIONAL SEMICONDUCTOR CORPORATION. used herein: Life support devices systems devices systems which, intended surgical implant into body, support sustain life, whose failure perform when properly used accordance with instructions provided labeling, reasonably expected result significant injury user.
National Semiconductor Corporation Americas Email: support@nsc.com National Semiconductor Europe Fax: 180-530 Email: europe.support@nsc.com Deutsch Tel: 9508 6208 English Tel: 2171 Tel: 8790
critical component component life support device system whose failure perform reasonably expected cause failure life support device system, affect safety effectiveness.
AN-C
National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: ap.support@nsc.com
National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507
www.national.com
National does assume responsibility circuitry described, circuit patent licenses implied National reserves right time without notice change said circuitry specifications.

Converter Handle Frequency-to-Voltage Needs Simplify your convert

Top Searches for this datasheet

Other recent searches