LM231A/LM231/LM331A/LM331 Precision Voltage-to-Frequency Converters General Description The LM231/LM331 family of voltage-to-frequency converters are ideally suited for use in simple low-cost circuits for analog-to-digital conversion, precision frequency-to-voltage conversion, long-term integration, linear frequency modulation or demodulation, and many other functions. The output when used as a voltage-to-frequency converter is a pulse train at a frequency precisely proportional to the applied input voltage. Thus, it provides all the inherent advantages of the voltage-to-frequency conversion techniques, and is easy to apply in all standard voltage-to-frequency converter applications. Further, the LM231A/LM331A attain a new high level of accuracy versus temperature which could only be attained with expensive voltage-to-frequency modules. Additionally the LM231/331 are ideally suited for use in digital systems at low power supply voltages and can provide low-cost analog-to-digital conversion in microprocessor-controlled systems. And, the frequency from a battery powered voltage-to-frequency converter can be easily channeled through a simple photoisolator to provide isolation against high common mode levels. The LM231/LM331 utilize a new temperature-compensated band-gap reference circuit, to provide excellent accuracy over the full operating temperature range, at power supplies as low as 4.0V. The precision timer circuit has low bias currents without degrading the quick response necessary for 100 kHz voltage-to-frequency conversion. And the output are capable of driving 3 TTL loads, or a high voltage output up to 40V, yet is short-circuit-proof against VCC. Features n Guaranteed linearity 0.01% max n Improved performance in existing voltage-to-frequency conversion applications n Split or single supply operation n Operates on single 5V supply n Pulse output compatible with all logic forms n Excellent temperature stability, ± 50 ppm/˚C max n Low power dissipation, 15 mW typical at 5V n Wide dynamic range, 100 dB min at 10 kHz full scale frequency n Wide range of full scale frequency, 1 Hz to 100 kHz n Low cost Typical Applications DS005680-1 *Use stable components with low temperature coefficients. See Typical Applications section. **0.1µF or 1µF, See “Principles of Operation.” FIGURE 1. Simple Stand-Alone Voltage-to-Frequency Converter with ± 0.03% Typical Linearity (f = 10 Hz to 11 kHz) Teflon ® is a registered trademark of DuPont © 1999 National Semiconductor Corporation DS005680 www.national.com LM231A/LM231/LM331A/LM331 Precision Voltage-to-Frequency Converters June 1999 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Output Short Circuit to Ground Output Short Circuit to VCC Input Voltage Operating Ambient Temperature Range Power Dissipation (PD at 25˚C) and Thermal Resistance (θjA) (N Package) PD θjA Lead Temperature (Soldering, 10 sec.) Dual-In-Line Package (Plastic) ESD Susceptibility (Note 4) N Package LM231A/LM231 40V Continuous Continuous −0.2V to +VS TMIN TMAX −25˚C to +85˚C LM331A/LM331 40V Continuous Continuous −0.2V to +VS TMIN TMAX 0˚C to +70˚C 1.25W 100˚C/W 1.25W 100˚C/W 260˚C 260˚C 500V 500V Electrical Characteristics TA = 25˚C unless otherwise specified (Note 2) Parameter VFC Non-Linearity (Note 3) Typ Max Units 4.5V ≤ VS ≤ 20V Conditions Min ± 0.003 ± 0.01 % Full- TMIN ≤ TA ≤ TMAX ± 0.006 ± 0.02 % Full- VS = 15V, f = 10 Hz to 11 kHz ± 0.024 ± 0.14 %Full- Scale Scale VFC Non-Linearity In Circuit of Figure 1 Conversion Accuracy Scale Factor (Gain) Scale VIN = −10V, RS = 14 kΩ LM231, LM231A 0.95 1.00 1.05 kHz/V LM331, LM331A 0.90 1.00 1.10 kHz/V ± 30 ± 20 ± 150 ± 50 ppm/˚C Temperature Stability of Gain TMIN ≤ TA ≤ TMAX, 4.5V ≤ VS ≤ 20V LM231/LM331 LM231A/LM331A ppm/˚C Change of Gain with VS 4.5V ≤ VS ≤ 10V 0.01 0.1 %/V 0.006 0.06 %/V Rated Full-Scale Frequency 10V ≤ VS ≤ 40V VIN = −10V Gain Stability vs Time TMIN ≤ TA ≤ TMAX 10.0 kHz ± 0.02 % Full- (1000 Hrs) Overrange (Beyond Full-Scale) Frequency Scale VIN = −11V % 10 INPUT COMPARATOR ±3 ±4 ±3 ± 10 ± 14 ± 10 mV Bias Current −80 −300 nA Offset Current ±8 ± 100 nA VCC−2.0 V 0.667 0.70 x VS 0V ≤ VPIN 5 ≤ 9.9V 5 = 10V ± 10 ± 100 nA 200 1000 nA = 10V 200 500 nA Offset Voltage LM231/LM331 TMIN ≤ TA ≤ TMAX LM231A/LM331A TMIN ≤ TA ≤ TMAX Common-Mode Range TMIN ≤ TA ≤ TMAX −0.2 mV mV TIMER Timer Threshold Voltage, Pin 5 Input Bias Current, Pin 5 All Devices 0.63 VS = 15V LM231/LM331 VPIN LM231A/LM331A VPIN www.national.com 5 2 Electrical Characteristics (Continued) TA = 25˚C unless otherwise specified (Note 2) Parameter Conditions Min Typ Max Units 0.22 0.5 V TIMER VSAT PIN 5 (Reset) I = 5 mA CURRENT SOURCE (Pin 1) Output Current RS = 14 kΩ, VPIN 1 = 0 LM231, LM231A 126 135 144 µA LM331, LM331A 116 136 156 µA 0.2 1.0 µA 0.02 10.0 nA 2.0 50.0 Change with Voltage 0V ≤ VPIN 1 ≤ 10V Current Source OFF Leakage LM231, LM231A, LM331, LM331A All Devices TA = TMAX Operating Range of Current (Typical) (10 to 500) nA µA REFERENCE VOLTAGE (Pin 2) LM231, LM231A 1.76 1.89 2.02 LM331, LM331A 1.70 1.89 2.08 ± 60 ± 0.1 Stability vs Temperature Stability vs Time, 1000 Hours VDC VDC ppm/˚C % LOGIC OUTPUT (Pin 3) VSAT I = 5 mA I = 3.2 mA (2 TTL Loads), TMIN≤TA≤TMAX OFF Leakage 0.15 0.50 V 0.10 0.40 V ± 0.05 1.0 µA SUPPLY CURRENT LM231, LM231A LM331, LM331A VS = 5V VS = 40V 2.0 3.0 4.0 mA 2.5 4.0 6.0 mA VS = 5V VS = 40V 1.5 3.0 6.0 mA 2.0 4.0 8.0 mA Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. DC and AC electrical specifications do not apply when operating the device beyond its specified operating conditions. Note 2: All specifications apply in the circuit of Figure 4, with 4.0V≤VS≤40V, unless otherwise noted. Note 3: Nonlinearity is defined as the deviation of fOUT from VIN x (10 kHz/−10 VDC) when the circuit has been trimmed for zero error at 10 Hz and at 10 kHz, over the frequency range 1 Hz to 11 kHz. For the timing capacitor, CT, use NPO ceramic, Teflon ® , or polystyrene. Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor. 3 www.national.com Functional Block Diagram DS005680-2 Pin numbers apply to 8-pin packages only. FIGURE 2. www.national.com 4 Typical Performance Characteristics (All electrical characteristics apply for the circuit of Figure 4, unless otherwise noted.) Nonlinearity Error as Precision V-to-F Converter (Figure 4) Nonlinearity Error vs Power Supply Voltage Nonlinearity Error DS005680-26 DS005680-27 DS005680-25 Frequency vs Temperature VREF vs Temperature Output Frequency vs VSUPPLY DS005680-29 DS005680-28 DS005680-30 100 kHz Nonlinearity Error (Figure 5) Nonlinearity Error (Figure 1) DS005680-31 Input Current (Pins 6,7) vs Temperature DS005680-32 5 DS005680-33 www.national.com Typical Performance Characteristics Power Drain vs VSUPPLY (Continued) Output Saturation Voltage vs IOUT (Pin 3) Nonlinearity Error, Precision F-to-V Converter (Figure 7) DS005680-34 DS005680-35 DS005680-36 Typical Applications PRINCIPLES OF OPERATION OF A SIMPLIFIED VOLTAGE-TO-FREQUENCY CONVERTER The LM231/331 are monolithic circuits designed for accuracy and versatile operation when applied as voltage-to-frequency (V-to-F) converters or as frequency-to-voltage (F-to-V) converters. A simplified block diagram of the LM231/331 is shown in Figure 3 and consists of a switched current source, input comparator, and 1-shot timer. The operation of these blocks is best understood by going through the operating cycle of the basic V-to-F converter, Figure 3, which consists of the simplified block diagram of the LM231/331 and the various resistors and capacitors connected to it. The voltage comparator compares a positive input voltage, V1, at pin 7 to the voltage, Vx, at pin 6. If V1 is greater, the comparator will trigger the 1-shot timer. The output of the timer will turn ON both the frequency output transistor and the switched current source for a period t = 1.1 RtCt. During this period, the current i will flow out of the switched current source and provide a fixed amount of charge, Q = i x t, into the capacitor, CL. This will normally charge Vx up to a higher level than V1. At the end of the timing period, the current i will turn OFF, and the timer will reset itself. Now there is no current flowing from pin 1, and the capacitor CL will be gradually discharged by RL until Vx falls to the level of V1. Then the comparator will trigger the timer and start another cycle. The current flowing into CL is exactly IAVE = i x (1.1xRtCt) x f, and the current flowing out of CL is exactly Vx/RL ≅ VIN/RL. If VIN is doubled, the frequency will double to maintain this balance. Even a simple V-to-F converter can provide a frequency precisely proportional to its input voltage over a wide range of frequencies. www.national.com DS005680-4 FIGURE 3. Simplified Block Diagram of Stand-Alone Voltage-to-Frequency Converter and External Components DETAIL OF OPERATION, FUNCTIONAL BLOCK DIAGRAM (Figure 2) The block diagram shows a band gap reference which provides a stable 1.9 VDC output. This 1.9 VDC is well regulated over a VS range of 3.9V to 40V. It also has a flat, low temperature coefficient, and typically changes less than 1⁄2% over a 100˚C temperature change. The current pump circuit forces the voltage at pin 2 to be at 1.9V, and causes a current i = 1.90V/RS to flow. For Rs = 14k, i = 135 µA. The precision current reflector provides a current equal to i to the current switch. The current switch switches the current to pin 1 or to ground depending on the state of the RS flip-flop. The timing function consists of an RS flip-flop, and a timer comparator connected to the external RtCt network. When the input comparator detects a voltage at pin 7 higher than pin 6, it sets the RS flip-flop which turns ON the current switch and the output driver transistor. When the voltage at pin 5 rises to 2⁄3 VCC, the timer comparator causes the RS flip-flop to reset. The reset transistor is then turned ON and the current switch is turned OFF. However, if the input comparator still detects pin 7 higher than pin 6 when pin 5 crosses 2⁄3 VCC, the flip-flop will not be reset, and the current at pin 1 will continue to flow, in its attempt to make the voltage at pin 6 higher than pin 7. This 6 Typical Applications 1 µF capacitor can be used. When the RC time constants are matched at pin 6 and pin 7, a voltage step at VIN will cause a step change in fOUT. If CIN is much less than CL, a step at VIN may cause fOUT to stop momentarily. A 47Ω resistor, in series with the 1 µF CL, is added to give hysteresis effect which helps the input comparator provide the excellent linearity (0.03% typical). (Continued) condition will usually apply under start-up conditions or in the case of an overload voltage at signal input. It should be noted that during this sort of overload, the output frequency will be 0; as soon as the signal is restored to the working range, the output frequency will be resumed. The output driver transistor acts to saturate pin 3 with an ON resistance of about 50Ω. In case of overvoltage, the output current is actively limited to less than 50 mA. The voltage at pin 2 is regulated at 1.90 VDC for all values of i between 10 µA to 500 µA. It can be used as a voltage reference for other components, but care must be taken to ensure that current is not taken from it which could reduce the accuracy of the converter. DETAIL OF OPERATION OF PRECISION V-TO-F CONVERTER (Figure 4) In this circuit, integration is performed by using a conventional operational amplifier and feedback capacitor, CF. When the integrator’s output crosses the nominal threshold level at pin 6 of the LM231/331, the timing cycle is initiated. The average current fed into the op amp’s summing point (pin 2) is i x (1.1 RtCt) x f which is perfectly balanced with −VIN/RIN. In this circuit, the voltage offset of the LM231/331 input comparator does not affect the offset or accuracy of the V-to-F converter as it does in the stand-alone V-to-F converter; nor does the LM231/331 bias current or offset current. Instead, the offset voltage and offset current of the operational amplifier are the only limits on how small the signal can be accurately converted. Since op amps with voltage offset well below 1 mV and offset currents well below 2 nA are available at low cost, this circuit is recommended for best accuracy for small signals. This circuit also responds immediately to any change of input signal (which a stand-alone circuit does not) so that the output frequency will be an accurate representation of VIN, as quickly as 2 output pulses’ spacing can be measured. In the precision mode, excellent linearity is obtained because the current source (pin 1) is always at ground potential and that voltage does not vary with VIN or fOUT. (In the stand-alone V-to-F converter, a major cause of non-linearity is the output impedance at pin 1 which causes i to change as a function of VIN). The circuit of Figure 5 operates in the same way as Figure 4, but with the necessary changes for high speed operation. PRINCIPLES OF OPERATION OF BASIC VOLTAGETO-FREQUENCY CONVERTER (Figure 1) The simple stand-alone V-to-F converter shown in Figure 1 includes all the basic circuitry of Figure 3 plus a few components for improved performance. A resistor, RIN = 100 kΩ ± 10%, has been added in the path to pin 7, so that the bias current at pin 7 (−80 nA typical) will cancel the effect of the bias current at pin 6 and help provide minimum frequency offset. The resistance RS at pin 2 is made up of a 12 kΩ fixed resistor plus a 5 kΩ (cermet, preferably) gain adjust rheostat. The function of this adjustment is to trim out the gain tolerance of the LM231/331, and the tolerance of Rt, RL and Ct. For best results, all the components should be stable low-temperature-coefficient components, such as metal-film resistors. The capacitor should have low dielectric absorption; depending on the temperature characteristics desired, NPO ceramic, polystyrene, Teflon or polypropylene are best suited. A capacitor CIN is added from pin 7 to ground to act as a filter for VIN. A value of 0.01 µF to 0.1 µF will be adequate in most cases; however, in cases where better filtering is required, a 7 www.national.com Typical Applications (Continued) DS005680-5 *Use stable components with low temperature coefficients. See Typical Applications section. **This resistor can be 5 kΩ or 10 kΩ for VS = 8V to 22V, but must be 10 kΩ for VS = 4.5V to 8V. ***Use low offset voltage and low offset current op amps for A1: recommended type LF411A FIGURE 4. Standard Test Circuit and Applications Circuit, Precision Voltage-to-Frequency Converter www.national.com 8 Typical Applications In the precision circuit, an operational amplifier provides a buffered output and also acts as a 2-pole filter. The ripple will be less than 5 mV peak for all frequencies above 1 kHz, and the response time will be much quicker than in Figure 6. However, for input frequencies below 200 Hz, this circuit will have worse ripple than Figure 6. The engineering of the filter time-constants to get adequate response and small enough ripple simply requires a study of the compromises to be made. Inherently, V-to-F converter response can be fast, but F-to-V response can not. (Continued) DETAILS OF OPERATION, FREQUENCY-TOVOLTAGE CONVERTERS (Figure 6 and Figure 7) In these applications, a pulse input at fIN is differentiated by a C-R network and the negative-going edge at pin 6 causes the input comparator to trigger the timer circuit. Just as with a V-to-F converter, the average current flowing out of pin 1 is IAVERAGE = i x (1.1 RtCt) x f. In the simple circuit of Figure 6, this current is filtered in the network RL = 100 kΩ and 1 µF. The ripple will be less than 10 mV peak, but the response will be slow, with a 0.1 second time constant, and settling of 0.7 second to 0.1% accuracy. DS005680-6 *Use stable components with low temperature coefficients. See Typical Applications section. **This resistor can be 5 kΩ or 10 kΩ for VS = 8V to 22V, but must be 10 kΩ for VS = 4.5V to 8V. ***Use low offset voltage and low offset current op amps for A1: recommended types LF411A or LF356. FIGURE 5. Precision Voltage-to-Frequency Converter, 100 kHz Full-Scale, ± 0.03% Non-Linearity 9 www.national.com Typical Applications (Continued) DS005680-7 DS005680-8 *Use stable components with low temperature coefficients. FIGURE 6. Simple Frequency-to-Voltage Converter, 10 kHz Full-Scale, ± 0.06% Non-Linearity *Use stable components with low temperature coefficients. FIGURE 7. Precision Frequency-to-Voltage Converter, 10 kHz Full-Scale with 2-Pole Filter, ± 0.01% Non-Linearity Maximum Light Intensity to Frequency Converter DS005680-9 *L14F-1, L14G-1 or L14H-1, photo transistor (General Electric Co.) or similar Temperature to Frequency Converter DS005680-10 www.national.com 10 Typical Applications (Continued) Basic Analog-to-Digital Converter Using Voltage-to-Frequency Converter Long-Term Digital Integrator Using VFC DS005680-11 DS005680-12 Analog-to-Digital Converter with Microprocessor DS005680-13 Remote Voltage-to-Frequency Converter with 2-Wire Transmitter and Receiver DS005680-14 11 www.national.com Typical Applications (Continued) Voltage-to-Frequency Converter with Square-Wave Output Using ÷ 2 Flip-Flop DS005680-15 Voltage-to-Frequency Converter with Isolators DS005680-16 Voltage-to-Frequency Converter with Isolators DS005680-17 www.national.com 12 Typical Applications (Continued) Voltage-to-Frequency Converter with Isolators DS005680-18 Voltage-to-Frequency Converter with Isolators DS005680-19 Connection Diagram Dual-In-Line Package DS005680-21 Order Number LM231AN, LM231N, LM331AN, or LM331N See NS Package Number N08E 13 www.national.com DS005680-22 Schematic Diagram www.national.com 14 inches (millimeters) unless otherwise noted Dual-In-Line Package (N) Order Number LM231AN, LM231N, LM331AN, or LM331N NS Package N08E LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. 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National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: [email protected] National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. LM231A/LM231/LM331A/LM331 Precision Voltage-to-Frequency Converters Physical Dimensions