VOLTAGE-TO-FREQUENCY/FREQUENCY-TO-VOLTAGE CONVERTERS

Transcription

1 VOLTAGE-TO-FREQUENCY/ FEATURES Voltage-to-Frequency Choice of Guaranteed Linearity: % %... 0.% DC to 00 khz (F/V) or Hz to 00kHz (V/F) Low Power Dissipation... mw Typ Single/Dual Supply Operation... + V to + V or ± V to ±.V Gain Temperature Stability... ± ppm/ C Typ Programmable Scale Factor Frequency-to-Voltage Operation... DC to 00 khz Choice of Guaranteed Linearity: % %... 0.% Programmable Scale Factor APPLICATIONS µp Data Acquisition -Bit Analog-to-Digital Converters Analog Data Transmission and Recording Phase-Locked Loops Frequency Meters/Tachometer Motor Control FM Demodulation FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION The // are low-cost voltage-tofrequency (V/F) converters utilizing low power CMOS technology. The converters accept a variable analog input signal and generate an output pulse train whose frequency is linearly proportional to the input voltage. The devices can also be used as highly-accurate frequency-to-voltage (F/V) converters, accepting virtually any input frequency waveform and providing a linearly-proportional voltage output. A complete V/F or F/V system only requires the addition of two capacitors, three resistors, and reference voltage. ORDERING INFORMATION Linearity Temperature Part No. (V/F) Package Range COD 0.0% -Pin 0 C to +0 C SOIC (Narrow) CPD 0.0% -Pin 0 C to +0 C Plastic DIP EJD 0.0% -Pin 0 C to + C CerDIP CPD 0.0% -Pin 0 C to +0 C Plastic DIP EJD 0.0% -Pin 0 C to + C CerDIP CPD 0.% -Pin 0 C to +0 C Plastic DIP EJD 0.% -Pin 0 C to + C CerDIP Input Voltage Integrator Capacitor R IN Reference Capacitor Integrator OpAmp Threshold Detector One Shot Pulse Output Pulse/ Output I REF Reference Voltage //- //9 -

2 VOLTAGE-TO-FREQUENCY/ ABSOLUTE MAXIMUM RATINGS* V SS... +V...0mA V OUT Max V OUT Common...V V SS....V Storage Temperature Range... C to +0 C Operating Temperature Range C Device... 0 C to +0 C E Device... 0 C to + C Package Dissipation (T A 0 C) -Pin CerDIP...00mW -Pin Plastic DIP...0mW -Pin SOIC...0mW Lead Temperature (Soldering, 0 sec) C *Static-sensitive device. Unused devices must be stored in conductive material. Protect devices from static discharge and static fields. Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only and functional operation of the device at these or any other conditions above those indicated in the operational sections of the specifications is not implied. Exposure to Absolute Maximum Rating Conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS: = +V, V SS = V, V GND = 0V, = V, R BIAS = 00kΩ, Full Scale = 0kHz, unless otherwise specified. T A = + C, unless temperature range is specified ( 0 C to + C for E device, 0 C to +0 C for C device). VOLTAGE-TO-FREQUENCY Parameter Definition Min Typ Max Min Typ Max Min Typ Max Unit Accuracy Linearity 0 khz Output Deviation From Straight % Full Line Between Normalized Zero Scale and Full-Scale Input Linearity 00 khz Output Deviation From Straight % Full Line Between Normalized Zero Scale Reading and Full-Scale Input Gain Temperature Variation in Gain A Due to ± ± 0 ± ± 0 ± 0 ± 00 ppm/ C Drift (Note ) Temperature Change Full Scale Gain Variance Variation From Ideal Accuracy ± 0 ± 0 ± 0 % of Nominal Zero Offset (Note ) Correction at Zero Adjust for Zero ± 0 ± 0 ± 0 ± 0 ± 0 ± 00 mv Output When Input is Zero Zero Temperature Variation in Zero Offset Due to ± ± 0 ± ± 0 ± 0 ± 00 µv/ C Drift (Note ) Temperature Change Analog Input Full Scale Full-Scale Analog Input Current to µa Achieve Specified Accuracy Overrange Overrange Current µa Response Time Settling Time to 0.% Full Scale Cycle Digital Section V I OL = 0mA Logic "0" Output Voltage (Note ) V V OUT Max V OUT Voltage Range Between Output V Common (Note ) and Common Pulse Frequency µsec Output Width -

3 VOLTAGE-TO-FREQUENCY/ ELECTRICAL CHARACTERISTICS: (Cont.) = +V, V SS = V, V GND = 0, = V, R BIAS = 00kΩ, Full Scale = 0kHz, unless otherwise specified. T A = + C, unless temperature range is specified 0 C to + C for E device, 0 C to +0 C for C device. FREQUENCY-TO-VOLTAGE Parameter Definition Min Typ Max Min Typ Max Min Typ Max Unit Supply Current I DD Quiescent Current Required From Positive (Note ) Supply During Operation.. 0 ma I SS Quiescent Current Required From Negative (Note ) Supply During Operation.. 0 ma Supply Operating Range of Positive Supply... V V SS Supply Operating Range of Negative Supply... V Reference Voltage V SS Range of Voltage Reference Input... V Accuracy Nonlinearity (Note 0) Deviation From Ideal Transfer % Full Function as a Percentage Scale Full-Scale Voltage Input Frequency Frequency Range for Specified 0 00k 0 00k 0 00k Hz Range (Note and ) Nonlinearity Frequency Input Positive Excursion Voltage Required to Turn V Threshold Detector On Negative Excursion Voltage Required to Turn V Threshold Detector Off Minimum Positive Time Between Threshold µsec Pulse Width (Note ) Crossings Minimum Negative Time Between Threshold µsec Pulse Width (Note ) Crossings Input Impedance MΩ Analog Outputs Output Voltage Voltage Range of Op Amp Output V (Note 9) for Specified Nonlinearity Output Loading Resistive Loading at Output of kω Op Amp Supply Current I DD Quiescent Current Required From Positive (Note 0) Supply During Operation.. 0 ma I SS Quiescent Current Required From Negative (Note 0) Supply During Operation.. 0 ma Supply Operating Range of Positive Supply... V V SS Supply Operating Range of Negative Supply... V Reference Voltage V SS Range of Voltage Reference Input... V NOTES:. Full temperature range. Guaranteed, Not Tested.. = 0.. Full temperature range, I OUT = 0mA.. I OUT = 0µA.. Threshold Detect = V, Amp Out = 0V, Full Temperature Range. 0Hz to 00kHz.; Guaranteed, Not Tested. µsec minimum positive pulse width and 0. µsec minimum negative pulse width.. t R = t F = 0 nsec. 9. R L kω.; 0kΩ 0. Full temperature range, V IN = 0.V. -9

4 VOLTAGE-TO-FREQUENCY/ PIN CONFIGURATIONS -Pin Plastic DIP/CerDIP -Pin SOIC (Narrow) I BIAS I BIAS ZERO ADJ NC ZERO ADJ NC V SS OUT 0 AMPLIFIER OUT THRESHOLD DETECTOR FREQ/ OUT V SS V OUT REF 0 AMPLIFIER OUT THRESHOLD DETECTOR FREQ/ OUT GND 9 OUTPUT COMMON GND 9 OUTPUT COMMON PULSE FREQ OUT PULSE FREQ OUT NC = NO INTERNAL CONNECTION PIN DESCRIPTIONS Pin No. Symbol Description I BIAS This pin sets bias current in the. Connect to V SS through a 00 kω resistor. See text. Zero Adj Low frequency adjustment input. See text. Input current connection for the V/F converter. V SS Negative power supply voltage connection, typically V. OUT Reference capacitor connection. GND Analog ground. Voltage reference input, typically V. Pulse Freq Out Frequency output. This open drain output will pulse LOW each time the Freq threshold detector limit is reached. The pulse rate is proportional to input voltage. 9 Output Common Source connection for the open drain output FETs. See text. 0 Freq/ Out This open drain output is a square wave at one half the frequency of the pulse output (pin ). Output transitions of this pin occur on the rising edge of pin. Threshold Detect Input to the threshold detector. This pin is the frequency input during F/V operation. Amplifier Out Output of the integrator amplifier. NC No internal connection Positive power supply connection, typically +V. -90

5 VOLTAGE-TO-FREQUENCY/ INPUT V IN 0V 0V 0kΩ C INT 0pF R IN MΩ +V 0kΩ V OFFSET Figure. + +V THRESHOLD DETECT µsec DELAY C REF 0pF 0kΩ V AMP OUT OUT pf ZERO VOLTAGE-TO-FREQUENCY (V/F) CIRCUIT DESCRIPTION SELF- START R BIAS 00kΩ THRESHOLD DETECTOR 0kΩ 0pF OpAmp I BIAS The V/F converter operates on the principal of charge balancing. The operation of the is easily understood by referring to Figure. The input voltage (V IN ) is converted to a current ( ) by the input resistor. This current is then converted to a charge on the integrating capacitor and shows up as a linearly decreasing voltage at the output of the op amp. The lower limit of the output swing is set by the threshold detector, which causes the reference voltage to be applied to the reference capacitor for a time period long enough to charge the capacitor to the reference voltage. This action reduces the charge on the integrating capacitor by a fixed amount (q = C REF ), causing the op amp output to step up a finite amount. V SS V 0 Hz to 0 khz V/F Converter REFERENCE VOLTAGE (TYPICALLY V) GND f OUT f OUT / 0 9 OUTPUT COMMON +V +V R L 0kΩ R L 0kΩ At the end of the charging period, C REF is shorted out. This dissipates the charge stored on the reference capacitor, so that when the output again crosses zero the system is ready to recycle. In this manner, the continued discharging of the integrating capacitor by the input is balanced out by fixed charges from the reference voltage. As the input voltage is increased, the number of reference pulses required to maintain balance increases, which causes the output frequency to also increase. Since each charge increment is fixed, the increase in frequency with voltage is linear. In addition, the accuracy of the output pulse width does not directly affect the linearity of the V/F. The pulse must simply be long enough for full charge transfer to take place. -9

6 VOLTAGE-TO-FREQUENCY/ f OUT µsec TYP f OUT / /f AMP OUT 0V C REF C INT NOTES:. To adjust f MIN, set V IN = 0mV and adjust the 0kΩ offset for 0Hz output.. To adjust f MAX, set V IN = 0V and adjust R IN or for 0 khz output.. To increase f OUT MAX to 00kHz, change C REF to pf and C INT to pf.. For high-performance applications, use high-stability components for R IN, C REF, (metal film resistors and glass capacitors). Also, separate output ground (pin 9) from input ground (pin ). Figure. Output Waveforms The contains a "self-start" circuit to ensure the V/F converter always operates properly when power is first applied. In the event that, during power-on, the Op Amp output is below the threshold and C REF is already charged, a positive voltage step will not occur. The op-amp output will continue to decrease until it crosses the.0v threshold of the "self-start" comparator. When this happens, an internal resistor is connected to the op-amp input, which forces the output to go positive until the is in its normal operating mode. The utilizes low power CMOS processing for low input bias and offset currents with very low power dissipation. The open-drain N-channel output FETs provide high voltage and high current sink capability. VOLTAGE-TO-TIME MEASUREMENTS The output can be measured in the time domain as well as the frequency domain. Some microcomputers, for example, have extensive timing capability but limited counter capability. Also, the response time of a time domain measurement is only the period between two output pulses, while the frequency measurement must accumulate pulses during the entire counter timebase period. Time measurements can be made from either the 's Pulse Freq Out output or from the Freq/ output. The Freq/ output changes state on the rising edge of Pulse Freq Out, so Freq/ is a symmetrical square wave at one half the pulse output frequency. Timing measurements can therefore be made between successive Pulse Freq Out pulses, or while Freq/ is high (or low). PIN FUNCTIONS Threshold Detector Input In the V/F mode, this input is connected to the amplifier output (pin ) and triggers a µsec pulse when the input voltage passes through its threshold. In the F/V mode, the input frequency is applied to this input. The nominal threshold of the detector is halfway between the power supplies, or ( + V SS )/ ±00mV. The 's charge balancing V/F technique is not dependent on a precision comparator threshold, because the threshold only sets the lower limit of the op-amp output. The op-amp's peak-to-peak output swing, which determines the frequency, is only influenced by external capacitors and by. Pulse Freq Out This output is an open-drain N-channel FET which provides a pulse waveform whose frequency is proportional to the input voltage. This output requires a pull-up resistor and interfaces directly with MOS, CMOS, and TTL logic. Freq/ Out This output is an open-drain N-channel FET which provides a square wave one-half the frequency of the pulse frequency output. The Freq/ output will change state on the rising edge of Pulse Freq Out. This output requires a pullup resistor and interfaces directly with MOS, CMOS, and TTL logic. -9

7 VOLTAGE-TO-FREQUENCY/ Output Common The sources of both the Freq/ out and the Pulse Freq Out are connected to this pin. An output level swing from the drain voltage to ground or to the V SS supply may be obtained by connecting this pin to the appropriate point. R BIAS An external resistor, connected to V SS, sets the bias point for the. Specifications for the are based on R BIAS = 00kΩ ±0%, unless otherwise noted. Increasing the maximum frequency of the beyond 00kHz is limited by the pulse width of the Pulse Output (typically µsec). Reducing R BIAS will decrease the pulse width and increase the maximum operating frequency, but linearity errors will also increase. R BIAS can be reduced to 0kΩ, which will typically produce a maximum full scale frequency of 00kHz. Amplifier Out The output stage of the operational amplifier. During V/F operation, a negative-going ramp signal is available at this pin. In the F/V mode, a voltage proportional to the frequency input is generated. Zero Adjust This pin is the noninverting input of the operational amplifier. The low-frequency set point is determined by adjusting the voltage at this pin. The inverting input of the operational amplifier and the summing junction when connected in the V/F mode. An input current of 0µA is specified, but an overrange current up to 0µA can be used without detrimental effect to the circuit operation. connects the summing junction of an operational amplifier. Voltage sources cannot be attached directly, but must be buffered by external resistors. A reference voltage from either a precision source or the V SS supply is applied to this pin. Accuracy of the is dependent on the voltage regulation and temperature characteristics of the reference circuitry. Since the is a charge balancing V/F converter, the reference current will be equal to the input current. For this reason, the DC impedance of the reference voltage source must be kept low enough to prevent linearity errors. For linearity of 0.0%, a reference impedance of 00Ω or less is recommended. A 0.µF bypass capacitor should be connected from to ground. Out The charging current for C REF is supplied through this pin. When the op amp output reaches the threshold level, this pin is internally connected to the reference voltage and a charge, equal to x C REF, is removed from the integrator capacitor. After about µsec, this pin is internally connected to the summing junction of the op amp to discharge C REF. Break-before-make switching ensures that the reference voltage is not directly applied to the summing junction. V/F CONVERTER DESIGN INFORMATION Input/Output Relationships The output frequency (f OUT ) is related to the analog input voltage (V IN ) by the transfer equation: V Frequency out = IN R IN ( ) (C REF ) External Component Selection R IN The value of this component is chosen to give a fullscale input current of approximately 0µA: R IN V IN Full Scale. 0µA Example: 0V R IN = MΩ. 0µA Note that the value is an approximation and the exact relationship is defined by the transfer equation. In practice, the value of R IN typically would be trimmed to obtain fullscale frequency at V IN full scale (see "Adjustment Procedure"). Metal film resistors with % tolerance or better are recommended for high-accuracy applications because of their thermal stability and low-noise generation. C INT The exact value is not critical but is related to C REF by the relationship: C REF C INT 0 C REF. Improved stability and linearity are obtained when C INT C REF. Low-leakage types are recommended, although mica and ceramic devices can be used in applications where their temperature limits are not exceeded. Locate as close as possible to pins and. -9

8 VOLTAGE-TO-FREQUENCY/ C REF The exact value is not critical and may be used to trim the full-scale frequency (see "Input/Output Relationships"). Glass film or air trimmer capacitors are recommended because of their stability and low leakage. Locate as close as possible to pins and., V SS Power supplies of ±V are recommended. For highaccuracy requirements, 0.0% line and load regulation and 0.µF disc decoupling capacitors located near the pins are recommended. Adjustment Procedure Figure shows a circuit for trimming the zero location. Full scale may be trimmed by adjusting R IN,, or C REF. Recommended procedure for a 0kHz full-scale frequency is as follows: () Set V IN to 0 mv and trim the zero adjust circuit to obtain a 0Hz output frequency. () Set V IN to 0V and trim either R IN,, or C REF to obtain a 0kHz output frequency. If adjustments are performed in this order, there should be no interaction and they should not have to be repeated. Improved Single Supply V/F Converter Operation A which operates from a single to V variable power source is shown in Figure. This circuit uses two Zener diodes to set stable biasing levels for the. The Zener diodes also provide the reference voltage, so the output impedance and temperature coefficient of the Zeners will directly affect power supply rejection and temperature performance. Full scale adjustment is accomplished by trimming the input current. Trimming the reference voltage is not recommended for high accuracy applications unless an op amp is used as a buffer, because the requires a low impedance reference (see the pin description section for more information). The circuit of Figure will directly interface with CMOS logic operating at V to V. TTL or V CMOS logic can be accommodated by connecting the output pullup resistors to the +V supply. An optoisolator can also be used if an isolated output is required. C REF (pf) +pf khz = +V V SS = V R IN = MΩ V IN = +0V T A = + C 00 00kHz 0 (V) Figure. Recommended C REF vs -9

9 VOLTAGE-TO-FREQUENCY/ V IN 0.9 R GAIN OFFSET R IN MΩ V. kω kω 0. R 0 pf R V 0.0 µf 0.0 µf 0 pf V + = V TO V (FIXED) 0 0kΩ 0kΩ f OUT f OUT / 0V 0V 00 kω 9 V + 0V V V R R MΩ. MΩ MΩ 0kΩ kω 0kΩ f OUT = (V V ) (C REF ) (V IN V ) (V + V ) = + R IN (0.9 R +0. R ) Figure. Fixed Voltage Single Supply Operation.k* + to +V R 90k R 90k INPUT VOLTAGE (0 to 0V) R GAIN R 00k 00k R 9k Rp OFFSET 0k µf D.VZ D.VZ ANALOG GROUND C INT 0.µ C REF 00k THRESHOLD DETECT AMP OUT C REF IIN fout ZERO GND fout/ 0 VREF IBIAS OUTPUT COMMON VSS 9 0k 0k OUTPUT FREQUENCY DIGITAL GROUND COMPONENT SELECTION F/S FREQ. khz 0 khz 00 khz CREF 00pF 0pF pf CINT 00pF 0pF pf Figure. Voltage to Frequency -9

10 VOLTAGE-TO-FREQUENCY/ FREQUENCY-TO-VOLTAGE (F/V) CIRCUIT DESCRIPTION When used as an F/V converter, the generates an output voltage linearly proportional to the input frequency waveform. Each zero crossing at the threshold detector's input causes a precise amount of charge (q = C REF ) to be dispensed into the op amp's summing junction. This charge in turn flows through the feedback resistor, generating voltage pulses at the output of the op amp. A capacitor (C INT ) across R INT averages these pulses into a DC voltage which is linearly proportional to the input frequency. F/V CONVERTER DESIGN INFORMATION Input/Output Relationships The output voltage is related to the input frequency (f IN ) by the transfer equation: V OUT = [ C REF R INT ] f IN. The response time to a change in f IN is equal to (R INT C INT ). The amount of ripple on V OUT is inversely proportional to C INT and the input frequency. C INT can be increased to lower the ripple. Values of µf to 00µF are perfectly acceptable for low frequencies. When the is used in the single-supply mode, is defined as the voltage difference between pin and pin. Input Voltage Levels The input frequency is applied to the Threshold Detector input (Pin ). As discussed in the V/F circuit section of this data sheet, the threshold of pin is approximately ( + V SS ) / ±00mV. Pin 's input voltage range extends from to about. V below the threshold. If the voltage on pin goes more than. volts below the threshold, the V/F mode startup comparator will turn on and corrupt the output voltage. The Threshold Detector input has about 00 mv of hysteresis. In ± V applications, the input voltage levels for the are ±00mV, minimum. If the frequency source being measured is unipolar, such as TTL or CMOS operating from a +V source, then an AC coupled level shifter should be used. One such circuit is shown in Figure a. The level shifter circuit in Figure b can be used in single supply F/V applications. The resistor divider ensures that the input threshold will track the supply voltages. The diode clamp prevents the input from going far enough in the negative direction to turn on the startup comparator. The diode's forward voltage decreases by. mv/ C, so for high ambient temperature operation two diodes in series are recommended. +V +V to +V 0k Frequency Input k 0.0µF DET Frequency Input k 0.0µF DET +V IN9.0M +V IN9.0M 0V 0V GND V SS 0.µF 0k V SS V (A) ±V Supply (B) Single Supply Figure. Frequency Input Level Shifter -9

11 VOLTAGE-TO-FREQUENCY/ +V V + * A A A THRESHOLD DETECT f IN SEE FIGURE µsec DELAY THRESHOLD DETECTOR f OUT/ 0 OUTPUT COMMON f OUT OUT 9 * V + * * OPTIONAL IF BUFFER IS NEEDED kω OFFSET +V 00kΩ ZERO OP AMP + pf 0pF AMP OUT C REF pf R INT MΩ + SEE EQUATION, PAGE C INT 000pF V O.kΩ I BIAS V SS GND V 0 kω Figure. (TYPICALLY V) DC 0 khz F/V Converter Input Buffer 0.µsec MIN INPUT f OUT.0µsec MIN DELAY = µsec f OUT and f OUT / are not used in the F/V mode. However, these outputs may be useful for some applications, such as a buffer to feed additional circuitry. Then, f OUT will follow the input frequency waveform, except that f OUT will go high µsec after f IN goes high; f OUT / will be squarewave with a frequency of one-half f OUT. If these outputs are not used, pins, 9 and 0 should be connected to ground. f OUT / Figure. F/V Digital Outputs -9

12 VOLTAGE-TO-FREQUENCY/ V + = 0V to V 0k GND.V.0µF 0k 00k Frequency Input Offset Adjust k 00k 0.0µF IN9 V +.0k.0M ZERO DET I BIAS OUT AMP OUT V SS GND pf M.00µF V OUT 0.µF.0k 00k Note: The output is referenced to pin, which is at.v (Vz). For frequency meter applications, a ma meter with a series-scaling resistor can be placed across pins and. Figure 9. F/V Single Supply F/V Converter Output Filtering The output of the has a sawtooth ripple superimposed on a DC level. The ripple will be rejected if the output is converted to a digital value by an integrating analog to digital converter, such as the TC0 or TC09. The ripple can also be reduced by increasing the value of the integrating capacitor, although this will reduce the response time of the F/V converter. The sawtooth ripple on the output of an F/V can be eliminated without affecting the F/V's response time by using the circuit in Figure 0. The circuit is a capacitance multiplier, where the output coupling capacitor is multiplied by the AC gain of the op amp. A moderately fast op amp, such as the TL0, should be used. OUT AMP OUT GND pf M.00µF 00.0µF M 0.µF + M + TL0 V OUT Figure 0. Ripple Filter -9

13 VOLTAGE-TO-FREQUENCY/ F/V POWER-ON RESET In F/V mode, the output voltage will occasionally be at its maximum value when power is first applied. This condition remains until the first pulse is applied to f IN. In most frequency-measurement applications this is not a problem, because proper operation begins as soon as the frequency input is applied. 000pF In some cases, however, the output must be zero at power-on without a frequency input. In such cases, a capacitor connected from pin to will usually be sufficient to pulse the and provide a power-on reset (see Figure A). Where predictable power-on operation is critical, a more complicated circuit, such as Figure B, may be required. f IN kω THRESHOLD DETECTOR (A) V CC B R C CLRA (B) 00kΩ µf A CD Q V SS f IN Figure. Power-On Operation/Reset To TC