LF398N

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LF398N - Monolithic Sample-and-Hold Circuit

Overview

The LF198/LF298/LF398 series are precision monolithic sample-and-hold circuits that utilize advanced BI-FET technology to deliver exceptional performance characteristics. These devices combine the advantages of bipolar and FET technologies to achieve ultra-high DC accuracy, fast signal acquisition, and extremely low droop rates. Designed to operate as unity gain followers, these components offer outstanding DC gain accuracy of 0.002% (typical) with acquisition times as low as 6 µs to reach 0.01% accuracy.

The LF398N features a bipolar input stage that provides low offset voltage and wide bandwidth, making it suitable for high-precision applications. The device's architecture ensures minimal feed-through from input to output in hold mode, even when input signals approach the supply voltages. With an impressive input impedance of 10¹⁰Ω, the LF398N can interface with high-impedance sources without compromising measurement accuracy.

Key Features

• Operates from ±5V to ±18V power supplies
• Ultra-high DC accuracy: 0.002% typical gain error
• Fast acquisition time: 6 µs to 0.01%
• Exceptionally low droop rate: 5 mV/min with 1 µF hold capacitor
• Low offset voltage with single-pin adjustment
• Wide bandwidth allows use in 1 MHz op amp feedback loops
• Extremely high input impedance: 10¹⁰Ω
• P-channel JFET output stage for low noise performance
• No feed-through in hold mode, even at supply voltage levels
• Superior temperature stability compared to MOS designs
• Input offset adjust doesn't degrade offset drift

Applications

The LF398N is designed for precision applications requiring accurate signal sampling and hold functionality:

• High-speed data acquisition systems
• Precision analog-to-digital converter interfaces
• Digital voltmeters and multimeters
• Medical instrumentation
• Industrial process control systems
• Automatic test equipment
• Signal reconstruction circuits
• Analog delay lines
• Peak detectors
• Oscilloscope sampling circuits
• Radar and sonar signal processing
• Telecommunications systems

Electrical Characteristics

(All specifications at V_S = ±15V, T_A = 25°C unless otherwise noted)

DC Characteristics

• Input Offset Voltage: 2 mV (max), 0.5 mV (typ)
• Input Offset Current: 50 pA (max), 10 pA (typ)
• Input Bias Current: 100 pA (max), 30 pA (typ)
• DC Gain Accuracy: 0.01% (max), 0.002% (typ)
• Gain Nonlinearity: 0.001% (max)
• Power Supply Rejection Ratio: 86 dB (min)

Dynamic Characteristics

• Acquisition Time (to 0.01%): 10 µs (max), 6 µs (typ) with 1 µF hold cap
• Aperture Time: 150 ns (max)
• Aperture Jitter: 20 ns (max)
• Hold Step: 2 mV (max)
• Droop Rate: 0.5 µV/µs (max) with 1 µF hold cap
• Feedthrough Rejection: -90 dB (min) at 1 kHz
• Bandwidth (-3 dB): 1 MHz (typ)

Power Requirements

• Supply Voltage Range: ±5V to ±18V
• Supply Current: 6 mA (max), 3.5 mA (typ) per supply
• Power Consumption: 180 mW (max) at ±15V

Package Information

The LF398N is available in multiple package options to suit different application requirements:

8-Pin Plastic DIP (N Package)

• Package Dimensions: 9.81mm x 6.35mm x 3.3mm
• Lead Pitch: 2.54mm
• Operating Temperature Range: -25°C to +85°C
• Storage Temperature Range: -65°C to +150°C
• Moisture Sensitivity Level (MSL): 1 (Unlimited floor life)

8-Pin SOIC (D Package)

• Package Dimensions: 4.9mm x 3.91mm x 1.58mm
• Lead Pitch: 1.27mm
• Operating Temperature Range: -25°C to +85°C
• Storage Temperature Range: -65°C to +150°C
• Moisture Sensitivity Level (MSL): 1 (Unlimited floor life)

Design Considerations

When implementing the LF398N in circuit designs, several important factors should be considered:

Hold Capacitor Selection

The choice of hold capacitor significantly impacts performance:

• Use high-quality polypropylene, polystyrene, or Teflon capacitors
• Typical values range from 0.01 µF to 1 µF
• Larger values reduce droop but increase acquisition time
• Ensure capacitor has low dielectric absorption

Layout Guidelines

• Minimize stray capacitance at the hold pin
• Use ground planes to reduce noise pickup
• Keep hold capacitor leads as short as possible
• Separate analog and digital grounds
• Use bypass capacitors (0.1 µF) close to power pins

Thermal Considerations

• Maintain ambient temperature within specified range
• Avoid placing near heat-generating components
• Consider thermal vias for SOIC package in high-temperature environments

Performance Graphs

(Note: The following performance characteristics are typical)

• Acquisition Time vs. Hold Capacitance
• Droop Rate vs. Temperature
• Feedthrough vs. Frequency
• Gain Error vs. Input Voltage
• Supply Current vs. Supply Voltage

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