Texas Instruments LF398M

Overview

LF398M from Texas Instruments is a monolithic BI-FET sample-and-hold IC operating as a unity-gain follower with 0.004% typical DC gain accuracy, ≤20 µs acquisition time to 0.1% with 0.01 µF hold capacitor, and 1 mV typical hold step at 25°C - used in precision data acquisition systems requiring stable analog signal capture before ADC conversion.

For engineers reviewing the LF398M datasheet, LF398M pinout, LF398M application, or LF398M equivalent, this page delivers verified electrical specs, validated PDIP-8 package mapping, confirmed TTL/CMOS-compatible logic interface behavior, and real-world trade-offs for hold capacitor selection, droop rate, and dynamic sampling error in industrial instrumentation and test equipment design.

Technical Context

The LF398M uses a bipolar input stage combined with P-channel JFET output devices to achieve low input offset voltage (2–7 mV typ), high input impedance (10¹⁰ Ω), and low droop rate (5 mV/min with 1 µF capacitor). Its fully differential logic interface has a 1.4 V threshold and supports direct TTL, PMOS, and CMOS drive without level-shifting.

In sample mode, the output tracks the input via active charging/discharging of the external hold capacitor; in hold mode, the input is isolated and the output retains the last sampled voltage with <2.5 mV hold step and >80 dB feedthrough attenuation at 1 kHz - enabled by internal analog switch isolation and high supply rejection (80–110 dB).

Key Specifications

Pinout & Package

LF398M is supplied in an 8-pin PDIP (Plastic Dual In-line Package) with nominal body size 9.81 mm × 6.35 mm, designed for through-hole mounting on industrial control and test PCBs.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar sample-and-hold applications.

Compared with LF398N, LF398M offers identical functionality with enhanced reliability screening, while AD585JN trades ease of logic integration for raw speed - making LF398M optimal for robust, digitally controlled precision sampling where drop-in compatibility and wide supply flexibility matter most.

Availability

LF398M is available at Aetrix Electronics and suitable for precision data acquisition, automated test equipment, and industrial process monitoring requiring stable component supply across extended lifecycle programs.

Supply support for LF398M includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a global semiconductor leader specializing in analog and embedded processing technologies, with decades of heritage in precision linear ICs.

The LF398M belongs to TI's legacy LFx98x monolithic sample-and-hold product line, engineered for ultrahigh DC accuracy and fast acquisition in instrumentation, aerospace, and defense systems where signal integrity under varying thermal and supply conditions is critical.

FAQ

What is the maximum recommended hold capacitor value for LF398M?

The LF398M datasheet does not specify an absolute maximum hold capacitor value, but practical limits arise from acquisition time degradation and droop-current capability. With a 5 mA hold capacitor charging current rating, capacitance exceeding 1 µF significantly increases acquisition time beyond 20 µs and risks thermal stress during rapid sampling. For best performance, TI recommends 0.001 µF to 0.1 µF for high-speed applications and up to 1 µF for low-droop, slow-sampling use cases - always verifying dielectric absorption characteristics of the selected capacitor type.

Does LF398M support single-supply operation?

No, LF398M requires dual symmetric supplies (±5 V to ±18 V) and is not specified for single-supply use. Its internal BI-FET architecture relies on balanced positive and negative rails to maintain input common-mode range (–VS+3.5 V to +VS–3.5 V) and output swing. Attempting single-supply operation violates absolute maximum ratings and causes undefined behavior, including loss of hold fidelity and potential device damage. For single-supply sample-and-hold applications, consider modern alternatives like the LTC1098 or MAX196.

How does logic rise time affect hold step in LF398M?

LF398M requires a minimum logic input dV/dt of 1.0 V/µs to minimize hold step error. Slower edges cause increased charge injection during the logic transition, directly contributing to the 1–2.5 mV typical hold step. This effect is exacerbated by stray capacitance between LOGIC/LOGIC REFERENCE traces and the Ch node. TI recommends routing logic signals away from analog paths, using short traces, and avoiding RC filtering unless compensated - verified in Figure 17 of the LF398M datasheet showing transient response at hold initiation.

Can LF398M be used inside the feedback loop of an op-amp?

Yes, LF398M is explicitly designed for inclusion in op-amp feedback loops, supporting stable operation up to 1 MHz. Its wide bandwidth and input characteristics that remain unchanged in hold mode prevent phase margin erosion. When placed in the feedback path of a high-speed op-amp (e.g., OPA637), the LF398M enables precision programmable-gain or zeroing functions without oscillation - provided layout minimizes capacitive coupling and supply decoupling meets TI's recommendations (100 nF + 10 µF per rail).

What is the junction temperature limit for LF398M?

The maximum junction temperature (T_JMAX) for LF398M is 100°C, as specified in Section 6.1 Absolute Maximum Ratings. This limit governs thermal derating: power dissipation must be reduced above ambient temperatures where (T_JMAX − T_A) / R_θJA falls below the package's 500 mW rating. With R_θJA = 48.9°C/W for the PDIP-8 package, continuous operation above ~85°C ambient requires forced airflow or heatsinking to avoid thermal shutdown or accelerated parametric drift.