LM331

LM131A LM131 LM231A LM231 LM331A LM331 Precision Voltage-to-Frequency Converters December 1994 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 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 670 mW 150 C W 1 25W 100 C W 1 25W 85 C W 260 C 260 C 2000V 500V 500V 260 C 1 25W 100 C W 260 C Electrical Characteristics TA e 25 C unless otherwise specified (Note 2) Parameter VFC Non-Linearity (Note 3) Conditions 4 5V s VS s 20V TMIN s TA s TMAX VFC Non-Linearity In Circuit of Figure 1 Conversion Accuracy Scale Factor (Gain) LM131 LM131A LM231 LM231A LM331 LM331A Temperature Stability of Gain LM131 LM231 LM331 LM131A LM231A LM331A Change of Gain with VS Rated Full-Scale Frequency Gain Stability vs Time (1000 Hrs) Overrange (Beyond Full-Scale) Frequency INPUT COMPARATOR Offset Voltage LM131 LM231 LM331 LM131A LM231A LM331A Bias Current Offset Current Common-Mode Range TMIN s TA s TMAX b0 2 Min Typ g 0 003 g 0 006 g 0 024 Max g 0 01 g 0 02 g 0 14 Units % FullScale % FullScale %FullScale kHz V kHz V ppm C ppm C %V %V kHz % FullScale % VS e 15V f e 10 Hz to 11 kHz VIN e b10V RS e 14 kX 0 95 0 90 TMIN s TA s TMAX 4 5V s VS s 20V 1 00 1 00 g 30 g 20 1 05 1 10 g 150 g 50 4 5VsVS s 10V 10V s VS s 40V VIN e b10V TMIN s TA s TMAX VIN e b11V 10 10 0 0 01 0 006 g 0 02 01 0 06 TMIN s TA s TMAX TMIN s TA s TMAX g3 g4 g3 g 10 g 14 g 10 mV mV mV nA nA V b 80 g8 b 300 g 100 VCCb2 0 2 Electrical Characteristics TA e 25 C unless otherwise specified (Note 2) (Continued) Parameter TIMER Timer Threshold Voltage Pin 5 Input Bias Current Pin 5 All Devices LM131 LM231 LM331 LM131A LM231A LM331A VSAT PIN 5 (Reset) CURRENT SOURCE (Pin 1) Output Current LM131 LM131A LM231 LM231A LM331 LM331A Change with Voltage Current Source OFF Leakage LM131 LM131A LM231 LM231A LM331 LM331A All Devices Operating Range of Current (Typical) REFERENCE VOLTAGE (Pin 2) LM131 LM131A LM231 LM231A LM331 LM331A Stability vs Temperature Stability vs Time 1000 Hours LOGIC OUTPUT (Pin 3) VSAT OFF Leakage SUPPLY CURRENT LM131 LM131A LM231 LM231A LM331 LM331A VS e 5V VS e 40V VS e 5V VS e 40V 20 25 15 20 30 40 30 40 40 60 60 80 mA mA mA mA I e 5 mA I e 3 2 mA (2 TTL Loads) TMINsTAsTMAX 0 15 0 10 g 0 05 0 50 0 40 10 V V mA 1 76 1 70 1 89 1 89 g 60 g0 1 Conditions Min 0 63 Typ 0 667 g 10 Max 0 70 g 100 Units c VS VS e 15V 0VsVPIN 5 s 9 9V VPIN 5 e 10V VPIN 5 e 10V I e 5 mA RS e 14 kX VPIN 1 e 0 126 116 0VsVPIN 1s10V 200 200 0 22 1000 500 05 nA nA nA V 135 136 02 0 01 0 02 20 (10 to 500) 144 156 10 10 10 0 50 0 mA mA mA nA nA nA mA TA e TMAX 2 02 2 08 VDC VDC ppm C % 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 Teflon 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 Teflon 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 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 DETAIL OF OPERATION FUNCTIONAL BLOCK DIAGRAM (FIGURE 1a ) 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 % 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 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 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) 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 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 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 ) 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 0 1 second time constant and settling of 0 7 second to 0 1% accuracy 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 SELECT Rx e (VS b 2V) 0 2 mA RF c (RtCt) RS TL H 5680 – 8 Use stable components with low temperature coefficients FIGURE 5 Simple Frequency-to-Voltage Converter 10 kHz Full-Scale g 0 06% Non-Linearity 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 Long-Term Digital Integrator Using VFC Basic Analog-to-Digital Converter Using Voltage-to-Frequency Converter 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 Metal Can Package Dual-In-Line 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 Life support devices or systems are devices or systems which (a) are intended for surgical implant into the body or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user National Semiconductor Corporation 1111 West Bardin Road Arlington TX 76017 Tel 1(800) 272-9959 Fax 1(800) 737-7018 2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness National Semiconductor Europe Fax (a49) 0-180-530 85 86 Email cnjwge tevm2 nsc com Deutsch Tel (a49) 0-180-530 85 85 English Tel (a49) 0-180-532 78 32 Fran ais Tel (a49) 0-180-532 93 58 Italiano Tel (a49) 0-180-534 16 80 National Semiconductor Hong Kong Ltd 13th Floor Straight Block Ocean Centre 5 Canton Rd Tsimshatsui Kowloon Hong Kong Tel (852) 2737-1600 Fax (852) 2736-9960 National Semiconductor Japan Ltd Tel 81-043-299-2309 Fax 81-043-299-2408 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