Texas Instruments LPC661IM/NOPB

Overview

LPC661IM/NOPB from Texas Instruments (formerly National Semiconductor) is a single, low-power CMOS operational amplifier designed for single-supply operation from +5 V to +15 V. It delivers rail-to-rail output swing, 3 mV input offset voltage, ultra-low 2 fA input bias current, and 55 µA supply current - enabling precision high-impedance signal conditioning in battery-powered sensor interfaces and portable instrumentation.

For engineers reviewing the LPC661IM/NOPB datasheet, LPC661IM/NOPB pinout, LPC661IM/NOPB application, or LPC661IM/NOPB equivalent, key selection criteria include its guaranteed rail-to-rail output into 5 kΩ loads, −40°C to +85°C industrial temperature range, SOIC-8 package, and compatibility with photodiode current-to-voltage conversion, long-term integrators, and low-distortion active filters.

Technical Context

The LPC661IM/NOPB employs a non-conventional topology that eliminates the traditional unity-gain buffer stage, instead routing the output directly from a compound integrator with dual feed-forward compensation (C_f and C_ff). This enables rail-to-rail swing while maintaining stability into capacitive loads when paired with series output resistors and feedback capacitors.

Its input stage features ultra-high input resistance (>1 TΩ) and ground-inclusive common-mode range (−0.1 V to V₊ − 2.3 V), making it suitable for direct-sensing applications where input nodes float near ground. Large-signal voltage gain reaches 1000 V/mV into 100 kΩ (sourcing) and remains >200 V/mV into 5 kΩ, supporting precision transimpedance and integrator designs without external gain boosting.

Key Specifications

Pinout & Package

Package: 8-pin SOIC (Small Outline Integrated Circuit), JEDEC MS-012AC, body width 3.9 mm, lead pitch 1.27 mm - compatible with standard surface-mount reflow profiles and automated assembly.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar low-power, rail-to-rail CMOS op amp applications.

Compared with TLC27L1CD and LPV821DRX, the LPC661IM/NOPB uniquely balances rail-to-rail output, 2 fA bias current, and 15 V operation - making it irreplaceable in high-impedance, wide-supply, single-rail sensor signal chains where all three traits are simultaneously required.

Availability

LPC661IM/NOPB is available at Aetrix Electronics and suitable for industrial sensor interfaces, portable medical devices, and battery-powered data loggers requiring stable component supply across extended product lifecycles.

Supply support for LPC661IM/NOPB 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 acquired National Semiconductor in 2011 and maintains full technical and supply chain responsibility for legacy National analog products including the LPC661 family.

The LPC661 belongs to National's precision low-power CMOS op amp product line, engineered specifically for high-impedance, single-supply applications where ultra-low input bias current and rail-to-rail output are mandatory - such as photodiode amplifiers and long-duration integrators.

FAQ

What is the maximum capacitive load the LPC661IM/NOPB can drive stably?

The LPC661IM/NOPB may oscillate with capacitive loads >100 pF in unity-gain follower configuration. Stability is restored by adding a 50–100 Ω series resistor at the output and a 5–10 pF capacitor from inverting input to output. Verified in DS011227-4 through DS011227-6, this method supports >1 nF loads without ringing. The LPC661IM/NOPB itself does not integrate internal compensation for capacitive loading.

Does the LPC661IM/NOPB support dual-supply operation?

Yes, the LPC661IM/NOPB operates with split supplies (e.g., ±7.5 V), as confirmed in Typical Performance Characteristics and Application Circuits (DS011227-23). However, its rail-to-rail output and ground-inclusive input common-mode range are optimized for single-supply use. Dual-supply applications must observe absolute maximum ratings: differential input voltage ≤ ±supply voltage and output short-circuit limits per Note 2.

Is the LPC661IM/NOPB pin-compatible with the LPC661AIM or LPC661IN variants?

Yes - all LPC661 variants (IM, AIM, IN) share identical 8-pin SOIC (M) or DIP (N) footprints and pinouts per NS Package Numbers M08A and N08E. The LPC661IM/NOPB uses the same SOIC-8 layout as LPC661AIM, with identical pin 1 location, thermal pad absence, and NC pin assignments. No PCB changes are required when substituting between IM and AIM grades.

What is the guaranteed minimum open-loop voltage gain of the LPC661IM/NOPB?

The guaranteed minimum large-signal voltage gain is 300 V/mV into 100 kΩ (sourcing) and 120 V/mV into 100 kΩ (sinking) at V₊ = 15 V, per DC Electrical Characteristics table. At V₊ = 5 V and 100 kΩ load, minimum gain is 400 V/mV (sourcing) and 180 V/mV (sinking). These values ensure predictable closed-loop accuracy in precision amplifier configurations.

Can the LPC661IM/NOPB be used in a Howland current pump configuration?

Yes - the LPC661IM/NOPB's ground-inclusive input common-mode range, rail-to-rail output, and ultra-low input bias current make it suitable for Howland current pumps, as illustrated in Figure 8 (DS011227-12). Guard ring implementation is strongly recommended to preserve <2 fA bias performance; the datasheet confirms successful operation with 100 kΩ sense resistors and 10 nA output currents.