LM131/LM231/LM331 Voltage-to-Frequency Converter

Overview

The LM131, LM231, and LM331 are precision voltage-to-frequency converters designed for a wide range of analog signal processing applications. These devices provide an output pulse train with a frequency that is precisely proportional to the applied input voltage, making them ideal for analog-to-digital conversion, frequency-to-voltage conversion, long-term integration, and frequency modulation/demodulation. The LM131 family is known for its high accuracy, excellent temperature stability, and ease of use in both split and single-supply configurations.

Features

• Exceptional Linearity: Guaranteed linearity of 0.01% maximum ensures high accuracy in voltage-to-frequency conversion.
• Improved Performance: Enhanced performance in existing voltage-to-frequency conversion applications.
• Flexible Power Supply: Operates on split or single supply, including a single 5V supply.
• Logic-Compatible Output: Pulse output is compatible with all logic families.
• Temperature Stability: Excellent temperature stability with a maximum drift of 50 ppm/°C.
• Low Power Consumption: Typical power dissipation of only 15 mW at 5V.
• Wide Dynamic Range: Minimum dynamic range of 100 dB at 10 kHz full-scale frequency.
• Adjustable Full-Scale Frequency: Configurable for full-scale frequencies ranging from 1 Hz to 100 kHz.
• Cost-Effective: Provides high performance at a low cost.

Applications

The LM131 family is versatile and can be used in a variety of applications, including but not limited to:

• Analog-to-Digital Conversion (ADC): Converts analog signals to digital pulse trains for microcontroller or digital signal processing.
• Precision Frequency-to-Voltage Conversion: Useful in tachometers, speed measurement, and other frequency-based sensing applications.
• Long-Term Integration: Suitable for integrating signals over extended periods, such as in energy measurement or data logging.
• Frequency Modulation/Demodulation: Enables linear frequency modulation (FM) or demodulation in communication systems.
• Signal Isolation: Provides galvanic isolation when used with optocouplers or transformers.
• Low-Cost Signal Conditioning: Ideal for cost-sensitive applications requiring precise voltage-to-frequency conversion.

Electrical Characteristics

The following table summarizes the key electrical specifications of the LM131 family under typical operating conditions (unless otherwise noted):

• Supply Voltage Range: 4V to 40V (single supply) or ±2V to ±20V (split supply)
• Operating Temperature Range: 0°C to +70°C (LM131), -25°C to +85°C (LM231), -40°C to +85°C (LM331)
• Input Voltage Range: 0V to V_CC - 2V
• Output Frequency Range: 1 Hz to 100 kHz (adjustable)
• Linearity Error: 0.01% maximum
• Temperature Coefficient: 50 ppm/°C maximum
• Power Dissipation: 15 mW typical at 5V
• Output Pulse Width: Adjustable via external components

Package Information

The LM131 family is available in multiple package options to suit different application requirements. The following packages are commonly available:

• 8-Pin PDIP (Plastic Dual In-Line Package): Industry-standard through-hole package for easy prototyping and testing.
• 8-Pin SOIC (Small Outline Integrated Circuit): Surface-mount package for space-constrained applications.
• TO-99 Metal Can Package: Hermetically sealed package for high-reliability applications.

Each package option provides robust performance and is designed to meet industry standards for reliability and durability.

Typical Application Circuit

A basic voltage-to-frequency converter circuit using the LM131 is shown below. Key components include:

• Input Voltage (V_IN): Applied to the input pin through a current-setting resistor.
• Timing Resistor (R_T) and Capacitor (C_T): Determine the output frequency range.
• Output Pulse: Taken from the open-collector output pin, which can be pulled up to a logic supply voltage.

For optimal performance, it is recommended to use stable components with low temperature coefficients, such as metal-film resistors and polystyrene or polypropylene capacitors.

Conclusion

The LM131/LM231/LM331 voltage-to-frequency converters offer a cost-effective, high-performance solution for a wide range of analog signal processing tasks. With their excellent linearity, temperature stability, and flexible power supply options, these devices are well-suited for applications requiring precise voltage-to-frequency conversion. Whether used in data acquisition, instrumentation, or communication systems, the LM131 family provides reliable performance and ease of integration.