LM131A/LM131, LM231A/LM231, LM331A/LM331 Precision Voltage-to-Frequency Converters General Description The LM131/LM231/LM331 family of voltage-to-frequency converters are ideally suited for use in simple low-cost circuits for analog-to-digital conversion, precision frequencyto-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 LM131A/ LM231A/LM331A attains a new high level of accuracy versus temperature which could only be attained with expensive voltage-to-frequency modules. Additionally the LM131 is 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 LM131/LM231/LM331 utilizes a new temperaturecompensated 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 is capable of driving 3 TTL loads, or a high voltage output up to 40V, yet is short-circuit-proof against VCC. Features Y Y Y Y Y Y Y Y Y Y Guaranteed linearity 0.01% max Improved performance in existing voltage-to-frequency conversion applications Split or single supply operation Operates on single 5V supply Pulse output compatible with all logic forms Excellent temperature stability, g 50 ppm/§ C max Low power dissipation, 15 mW typical at 5V Wide dynamic range, 100 dB min at 10 kHz full scale frequency Wide range of full scale frequency, 1 Hz to 100 kHz Low cost Typical Applications fOUT e VIN R 1 # S# 2.09 V RL RtCt TL/H/5680 – 1 *Use stable components with low temperature coefficients. See Typical Applications section. **0.1mF or 1mF, See ‘‘Principles of Operation.’’ FIGURE 1. Simple Stand-Alone Voltage-to-Frequency Converter with g 0.03% Typical Linearity (f e 10 Hz to 11 kHz) C1995 National Semiconductor Corporation TL/H/5680 RRD-B30M115/Printed in U. S. A. LM131A/LM131, LM231A/LM231, LM331A/LM331 Precision Voltage-to-Frequency Converters December 1994 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 (ijA) (H Package) PD ijA (N Package) PD ijA (M Package) PD iJA Lead Temperature (Soldering, 10 sec.) Dual-In-Line Package (Plastic) Metal Can Package (TO-5) ESD Susceptibility (Note 4) Metal Can Package (TO-5) Other Packages LM131A/LM131 40V Continuous Continuous b 0.2V to a VS TMIN TMAX b 55§ C to a 125§ C LM231A/LM231 40V Continuous Continuous b 0.2V to a VS TMIN TMAX b 25§ C to a 85§ C LM331A/LM331 40V Continuous Continuous b 0.2V to a VS TMIN TMAX 0§ C to a 70§ C 1.25W 100§ C/W 1.25W 85§ C/W 1.25W 100§ C/W 260§ C 260§ C 500V 500V 670 mW 150§ C/W 260§ C 260§ C 2000V Electrical Characteristics TA e 25§ C unless otherwise specified (Note 2) Parameter Typ Max Units 4.5V s VS s 20V g 0.003 g 0.01 TMIN s TA s TMAX g 0.006 g 0.02 % FullScale % FullScale VFC Non-Linearity In Circuit of Figure 1 VS e 15V, f e 10 Hz to 11 kHz g 0.024 g 0.14 %FullScale Conversion Accuracy Scale Factor (Gain) LM131, LM131A, LM231, LM231A LM331, LM331A VIN e b10V, RS e 14 kX 1.00 1.00 1.05 1.10 kHz/V kHz/V Temperature Stability of Gain LM131/LM231/LM331 LM131A/LM231A/LM331A TMIN s TA s TMAX, 4.5V s VS s 20V g 30 g 20 g 150 g 50 ppm/§ C ppm/§ C Change of Gain with VS 4.5VsVS s 10V 10V s VS s 40V 0.01 0.006 0.1 0.06 %/V %/V Rated Full-Scale Frequency VIN e b10V Gain Stability vs Time (1000 Hrs) TMIN s TA s TMAX Overrange (Beyond Full-Scale) Frequency VIN e b11V VFC Non-Linearity (Note 3) Conditions Min 0.95 0.90 10.0 kHz g 0.02 % FullScale 10 % INPUT COMPARATOR Offset Voltage LM131/LM231/LM331 LM131A/LM231A/LM331A TMIN s TA s TMAX TMIN s TA s TMAX Bias Current Offset Current Common-Mode Range TMIN s TA s TMAX 2 b 0.2 g3 g4 g3 g 10 g 14 g 10 mV mV mV b 80 b 300 nA g8 g 100 nA VCCb2.0 V Electrical Characteristics TA e 25§ C unless otherwise specified (Note 2) (Continued) Parameter Conditions Min Typ Max Units 0.63 0.667 0.70 c VS g 10 g 100 200 200 1000 500 nA nA nA 0.22 0.5 V 135 136 144 156 mA mA TIMER Timer Threshold Voltage, Pin 5 Input Bias Current, Pin 5 All Devices LM131/LM231/LM331 LM131A/LM231A/LM331A VS e 15V 0VsVPIN 5 s 9.9V VPIN 5 e 10V VPIN 5 e 10V VSAT PIN 5 (Reset) I e 5 mA CURRENT SOURCE (Pin 1) Output Current LM131, LM131A, LM231, LM231A LM331, LM331A RS e 14 kX, VPIN 1 e 0 Change with Voltage 0VsVPIN 1s10V 0.2 1.0 mA TA e TMAX 0.01 0.02 2.0 1.0 10.0 50.0 nA nA nA Current Source OFF Leakage LM131, LM131A LM231, LM231A, LM331, LM331A All Devices 126 116 Operating Range of Current (Typical) (10 to 500) mA REFERENCE VOLTAGE (Pin 2) LM131, LM131A, LM231, LM231A LM331, LM331A 1.76 1.70 1.89 1.89 2.02 2.08 VDC VDC Stability vs Temperature g 60 ppm/§ C Stability vs Time, 1000 Hours g 0.1 % LOGIC OUTPUT (Pin 3) VSAT I e 5 mA I e 3.2 mA (2 TTL Loads), TMINsTAsTMAX OFF Leakage 0.15 0.10 g 0.05 0.50 0.40 1.0 V V mA 3.0 4.0 3.0 4.0 4.0 6.0 6.0 8.0 mA mA mA mA SUPPLY CURRENT LM131, LM131A, LM231, LM231A LM331, LM331A VS e 5V VS e 40V VS e 5V VS e 40V 2.0 2.5 1.5 2.0 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 3 , with 4.0V s VS s 40V, unless otherwise noted. Note 3: Nonlinearity is defined as the deviation of fOUT from VIN c (10 kHz/ b 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, TeflonÉ, or polystyrene. Note 4: Human body model, 100 pF discharged through a 1.5 kX resistor. 3 Functional Block Diagram TL/H/5680 – 2 Pin numbers apply to 8-pin packages only. See connection diagram for LM231WM pin numbers. FIGURE 1a TeflonÉ registered trademark of DuPont 4 Typical Performance Characteristics (All electrical characteristics apply for the circuit of Figure 3 , unless otherwise noted.) Nonlinearity Error, LM131 Family, as Precision V-to-F Converter (Figure 3 ) Nonlinearity Error, LM131 Family Nonlinearity vs Power Supply Voltage Frequency vs Temperature, LM131A VREF vs Temperature, LM131A Output Frequency vs VSUPPLY 100 kHz Nonlinearity Error, LM131 Family (Figure 4 ) Nonlinearity Error, LM131 (Figure 1 ) Input Current (Pins 6, 7) vs Temperature Power Drain vs VSUPPLY Output Saturation Voltage vs IOUT (Pin 3) Nonlinearity Error, Precision F-to-V Converter (Figure 6 ) TL/H/5680 – 3 5 Typical Applications (Continued) PRINCIPLES OF OPERATION OF A SIMPLIFIED VOLTAGE-TO-FREQUENCY CONVERTER DETAIL OF OPERATION, FUNCTIONAL BLOCK DIAGRAM (FIGURE 1a ) The LM131 is a monolithic circuit designed for accuracy and versatile operation when applied as a voltage-to-frequency (V-to-F) converter or as a frequency-to-voltage (F-to-V) converter. A simplified block diagram of the LM131 is shown in Figure 2 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 2 , which consists of the simplified block diagram of the LM131 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 e 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 e i c 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 e i c (1.1 c RtCt) c f, and the current flowing out of CL is exactly Vx/RL j 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. 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 (/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 e 1.90V/RS to flow. For Rs e 14k, i e 135 mA. 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 )/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 )/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 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 50X. 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 mA to 500 mA. 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. 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 2 plus a few components for improved performance. A resistor, RIN e 100 kX g 10%, has been added in the path to pin 7, so that the bias current at pin 7 ( b80 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 kX fixed resistor plus a 5 kX (cermet, preferably) gain adjust rheostat. The function of this adjustment is to trim out the gain tolerance of the LM131, and the tolerance of Rt, RL and Ct. TL/H/5680–4 FIGURE 2. Simplified Block Diagram of Stand-Alone Voltage-to-Frequency Converter Showing LM131 and External Components 6 Typical Applications (Continued) The average current fed into the op amp’s summing point (pin 2) is i c (1.1 RtCt) c f which is perfectly balanced with b VIN/RIN. In this circuit, the voltage offset of the LM131 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 LM131 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 4 operates in the same way as Figure 3 , but with the necessary changes for high speed operation. For best results, all the components should be stable lowtemperature-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 mF to 0.1 mF will be adequate in most cases; however, in cases where better filtering is required, a 1 mF 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 47X resistor, in series with the 1 mF CL, is added to give hysteresis effect which helps the input comparator provide the excellent linearity (0.03% typical). DETAIL OF OPERATION OF PRECISION V-TO-F CONVERTER (FIGURE 3 ) 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 LM131, the timing cycle is initiated. fOUT e b VIN RS 1 # # 2.09 V RIN RtCt TL/H/5680 – 5 *Use stable components with low temperature coefficients. See Typical Applications section. **This resistor can be 5 kX or 10 kX for VS e 8V to 22V, but must be 10 kX for VS e 4.5V to 8V. ***Use low offset voltage and low offset current op amps for A1: recommended types LM108, LM308A, LF411A FIGURE 3. Standard Test Circuit and Applications Circuit, Precision Voltage-to-Frequency Converter 7 Typical Applications (Continued) DETAILS OF OPERATION, FREQUENCY-TOVOLTAGE CONVERTERS (FIGURES 5 AND 6 ) 0.1 second time constant, and settling of 0.7 second to 0.1% accuracy. 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 e i c (1.1 RtCt) c f. In the simple circuit of FIGURE 5 , this current is filtered in the network RL e 100 kX and 1 mF. The ripple will be less than 10 mV peak, but the response will be slow, with a 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 5 . However, for input frequencies below 200 Hz, this circuit will have worse ripple than Figure 5 . 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. *Use stable components with low temperature coefficients. See Typical Applications section. **This resistor can be 5 kX or 10 kX for VS e 8V to 22V, but must be 10 kX for VS e 4.5V to 8V. ***Use low offset voltage and low offset current op amps for A1: recommended types LF411A or LF356. TL/H/5680 – 6 FIGURE 4. Precision Voltage-to-Frequency Converter, 100 kHz Full-Scale, g 0.03% Non-Linearity TL/H/5680–7 VOUT e fIN c 2.09V c RL c (RtCt) RS VOUT e b fIN c 2.09V c *Use stable components with low temperature coefficients. SELECT Rx e FIGURE 5. Simple Frequency-to-Voltage Converter, 10 kHz Full-Scale, g 0.06% Non-Linearity RF c (RtCt) RS TL/H/5680 – 8 (VS b 2V) 0.2 mA *Use stable components with low temperature coefficients. FIGURE 6. Precision Frequency-to-Voltage Converter, 10 kHz Full-Scale with 2-Pole Filter, g 0.01% Non-Linearity Maximum 8 Typical Applications (Continued) Light Intensity to Frequency Converter TL/H/5680 – 9 *L14F-1, L14G-1 or L14H-1, photo transistor (General Electric Co.) or similar Temperature to Frequency Converter TL/H/5680 – 10 Basic Analog-to-Digital Converter Using Voltage-to-Frequency Converter Long-Term Digital Integrator Using VFC TL/H/5680 – 11 TL/H/5680 – 12 9 Typical Applications (Continued) Analog-to-Digital Converter with Microprocessor TL/H/5680 – 13 Remote Voltage-to-Frequency Converter with 2-Wire Transmitter and Receiver TL/H/5680 – 14 Voltage-to-Frequency Converter with Square-Wave Output Using d 2 Flip-Flop TL/H/5680 – 15 Voltage-to-Frequency Converter with Isolators TL/H/5680 – 16 10 Typical Applications (Continued) Voltage-to-Frequency Converter with Isolators TL/H/5680 – 17 Voltage-to-Frequency Converter with Isolators TL/H/5680 – 18 Voltage-to-Frequency Converter with Isolators TL/H/5680 – 19 11 Connection Diagrams Dual-In-Line Package Metal Can Package Note: Metal case is connected to pin 4 (GND.) TL/H/5680–20 TL/H/5680 – 21 Order Number LM131H/883 or LM131AH/883 See NS Package Number H08C Order Number LM231AN, LM231N, LM331AN, or LM331N See NS Package Number N08E Small-Outline Package TL/H/5680 – 24 Top View Order Number LM231WM See NS Package Number M14B 12 Schematic Diagram TL/H/5680 – 22 13 14 Physical Dimensions inches (millimeters) Metal Can Package (H) Order Number LM131H/883 or LM131AH/883 NS Package H08C 14-Pin Small Outline Package (M) Order Number LM231WM NS Package M14B 15 LM131A/LM131, LM231A/LM231, LM331A/LM331 Precision Voltage-to-Frequency Converters Physical Dimensions inches (millimeters) (Continued) 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 OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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