Texas Instruments ACF2101BU

Overview

ACF2101BU from Burr-Brown (now Texas Instruments) is a dual precision switched integrator IC designed for low-noise current-to-voltage conversion in scientific instrumentation. It integrates input current using internal 100pF capacitors, features TTL-compatible digital control (Hold/Reset/Select), and delivers ±100µA input range, 100fA bias current, and 10µVrms output noise in Hold mode.

For engineers reviewing the ACF2101BU datasheet, ACF2101BU pinout, ACF2101BU application, or ACF2101BU equivalent, this page provides verified technical context, package-specific pin mapping, real-world integration use cases, and validated alternative options for photodiode front ends, CT scanner signal chains, and precision charge measurement systems.

Technical Context

The ACF2101BU implements two independent switched-capacitor integrators with Difet® amplifiers, each supporting Integrate/Hold/Reset modes via three TTL-level digital inputs per channel. Its architecture eliminates external feedback resistors by relying on capacitor-based integration, achieving 120dB dynamic range and sub-1nV/µs droop with internal 100pF capacitors.

Each channel includes dedicated switches: Hold (1.5kΩ ON resistance), Reset (1.5kΩ ON), and dual Select switches (250Ω ON) for multiplexed output routing. The device operates from ±4.5V to ±18V supplies, with specified performance at +5V/–15V, and supports both internal and external integration capacitors.

Key Specifications

Pinout & Package

ACF2101BU is housed in a 24-pin plastic SOIC package (package code DW), with pin 1 at top-left corner of top view. Pin functions are electrically identical to ACF2101BP but physically rearranged for surface-mount compatibility.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar switched integrator applications.

Compared with LTC1043CSW#PBF and AD7745ARUZ, the ACF2101BU uniquely delivers dual-channel analog integration with embedded 100pF capacitors, TTL-controlled hold/reset, and multiplexed outputs - making it irreplaceable for legacy analog front ends requiring synchronized, low-noise charge accumulation without external timing logic.

Availability

ACF2101BU is available at Aetrix Electronics and suitable for CT scanner front ends, photodiode integrators, and medical instrumentation requiring stable component supply throughout long production lifecycles.

Supply support for ACF2101BU 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

Burr-Brown Corporation, acquired by Texas Instruments in 2000, specialized in high-precision analog ICs for test and measurement, medical, and industrial markets.

The ACF2101BU belongs to Burr-Brown's precision switched integrator product line, engineered specifically for low-noise current-to-voltage conversion in scientific instrumentation and medical imaging signal chains.

FAQ

What is the maximum input current the ACF2101BU can handle without damage?

The ACF2101BU has an absolute maximum input current rating of ±5mA per channel, but its specified linear input range is ±100µA. Exceeding ±100µA causes output saturation and potential integration error accumulation. For reliable operation within datasheet specifications, keep input currents between –100µA and +100µA. The device includes internal protection that prevents latch-up or permanent damage up to the ±5mA absolute limit, but sustained operation above ±100µA degrades accuracy and increases charge transfer errors. Always verify input current compliance using the Sw In or In pins per channel in the ACF2101BU design.

Does the ACF2101BU require external passive components to operate with its internal 100pF capacitor?

No, the ACF2101BU operates fully functional with only power supplies (+5V/–15V), ground, and digital control signals - the internal 100pF capacitors are laser-trimmed and ready for use. External components are optional: bypass capacitors (1µF tantalum) are recommended at V+ and V– pins for noise suppression, and guarding traces are advised around Sw In/In pins to mitigate PCB leakage. If using external integration capacitors, connect them to Cap A/Cap B pins and tie unused Cap pins to Com A/Com B. The ACF2101BU does not require external resistors, op-amps, or timing circuitry to perform basic integrate/hold/reset functions.

How does the ACF2101BU achieve 120dB dynamic range despite its 10µVrms noise floor?

The ACF2101BU achieves 120dB dynamic range by combining ultra-low input bias current (100fA), precise 100pF integration capacitance, and wide output voltage swing (–10V to +0.5V). With 10µVrms noise in Hold mode and a 10.5V full-scale range, the theoretical SNR is ~120dB. This is realized in practice because the integrating architecture rejects high-frequency noise, and the Difet® amplifiers minimize 1/f noise. The range holds across the entire ±100µA input span - for example, 100pA input yields 10mV output (100pA × 100ms × 10⁹ Ω equivalent), while 100µA yields –10V, all within <0.01% nonlinearity. This makes the ACF2101BU effective for detecting femtoamp signals superimposed on microamp backgrounds.

Can the ACF2101BU be used with a single +5V supply instead of ±15V?

Yes, the ACF2101BU operates from +4.5V to +18V on V+ and –10V to –18V on V–, so a +5V/–15V supply meets specifications. However, a single +5V supply (with V– tied to ground) is not supported - the device requires a negative rail to achieve its –10V output swing and maintain 100fA bias current. Attempting single-supply operation causes output clipping above 0V and increased bias current drift. For true single-supply designs, consider modern alternatives like the AD8615-based integrator circuits, but the ACF2101BU must use split supplies. Verified operation at +5V/–15V delivers full performance; reducing V– to –10V still maintains 100fA bias but slightly compresses output range to –8.5V.

What is the purpose of the Sw Com A and Sw Com B pins on the ACF2101BU?

Sw Com A and Sw Com B serve as dedicated common terminals for the respective channel's switched outputs (Sw Out A/Sw Out B) and Select switches. They provide a low-impedance return path during multiplexed operation, isolating switching transients from the main analog common (Com A/Com B) and preventing crosstalk between channels. When the Select switch closes, current flows between Sw Out and Sw Com, routing the held output voltage onto a shared bus. Keeping Sw Com A and Sw Com B separate from Com A/Com B minimizes injected charge from digital switching from disturbing the integration node - a key design feature confirmed in the ACF2101BU's layout guidelines and noise characterization curves. These pins must be connected to system ground or a clean analog reference, not left floating.