Analog Devices Inc. AD694AQ

Overview

AD694AQ from Analog Devices is a monolithic 4–20 mA current transmitter IC designed for industrial process control signal conditioning. It accepts precalibrated 0 V to 2 V or 0 V to 10 V single-ended inputs, integrates a precision 2.000 V/10.000 V voltage reference, delivers ±0.001% of span/°C temperature stability, and operates from 4.5 V to 36 V supply - enabling direct interface with 8-/10-/12-bit DACs in valve and actuator control loops.

For engineers reviewing the AD694AQ datasheet, AD694AQ pinout, AD694AQ application, or AD694AQ equivalent, key selection criteria include its open-loop alarm (Pin 10), programmable 4 mA on/off (Pin 9), dual-supply-compatible output compliance extending 36 V below VS, and laser-trimmed 0.002% typ nonlinearity - all within a hermetically sealed 16-pin CERDIP package rated for –40°C to +85°C.

Technical Context

The AD694AQ implements a three-stage architecture: a PNP-input buffer amplifier (Pin 2/3/1) with rail-to-rail common-mode range and 300 kHz bandwidth; a voltage-to-current (V/I) converter using a Darlington-based current mirror to generate 0–16 mA signal current; and a laser-trimmed voltage reference (Pin 7/8) selectable between 2.000 V and 10.000 V via pin-strapping.

Its output stage supports single- or dual-supply operation with voltage compliance extending to within 2 V of VS and down to 36 V below VS - enabling drive of loads below common. The open-collector ALARM (Pin 10) asserts on IOUT open-circuit or output noncompliance, while the BOOST (Pin 12) and external pass transistor support extend thermal operating range by offloading power dissipation.

Key Specifications

Pinout & Package

AD694AQ is housed in a hermetically sealed 16-pin CERDIP package (package code Q), with 0.3-inch wide body, flat leads, and ceramic/metal construction suitable for high-reliability industrial environments.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 4–20 mA transmitter applications.

Compared with XTR115UA and AD421BN, the AD694AQ uniquely combines dual-supply-compatible output swing, laser-trimmed analog accuracy, and pin-strappable input/reference ranges - making it optimal for high-stability analog-only industrial transmitters where load flexibility and temperature stability are critical.

Availability

AD694AQ is available at Aetrix Electronics and suitable for industrial process control, factory automation I/O modules, and pressure/temperature transmitter designs requiring stable component supply, long-term lifecycle continuity, and hermetic packaging for harsh environments.

Supply support for AD694AQ 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

Analog Devices is a global leader in high-performance analog, mixed-signal, and digital signal processing semiconductors, serving industrial, automotive, communications, and healthcare markets since 1965.

The AD694AQ belongs to Analog Devices' precision current transmitter product line, engineered specifically for noise-immune 4–20 mA loop transmission in industrial process control, with emphasis on accuracy, thermal stability, and robustness in unregulated supply environments.

FAQ

What is the maximum load resistance the AD694AQ can drive with a 24 V supply?

The AD694AQ output voltage compliance extends to within 2 V of VS, so with a 24 V supply, the maximum output voltage is 22 V. For a 20 mA output, Ohm's Law gives R_LOAD = 22 V / 0.02 A = 1100 Ω. This assumes the loop has no additional voltage drops. The AD694AQ datasheet confirms RL = (VS – 2 V)/20 mA as the safe limit, and AD694AQ maintains full 4–20 mA regulation up to that value under specified conditions.

Can the AD694AQ operate from a 5 V supply, and if so, what input range is supported?

Yes, the AD694AQ supports 4.5 V minimum supply. At 5 V, only the 0–2 V input range is functional because the 10 V reference requires ≥12.5 V supply. Pin 4 must be shorted to Pin 5 (COM), and Pin 7 shorted to Pin 8 to select 2 V reference. The buffer amplifier retains full rail-to-rail input common-mode range down to –0.3 V, ensuring accurate low-voltage signal handling even at 5 V operation.

How does the ALARM pin (Pin 10) function, and what external circuitry is needed?

The ALARM pin is an open-collector NPN output that pulls low when the IOUT pin detects an open circuit or attempts to drive beyond VS – 2 V. No pull-up is required for basic fault indication - a simple LED/resistor to VS suffices. For microcontroller interfacing, a 10 kΩ pull-up to 3.3 V or 5 V is typical. The AD694AQ datasheet specifies V_CE(SAT) ≤ 0.35 V at 2.5 mA sink, confirming reliable logic-low assertion under fault conditions.

Is the AD694AQ pin-compatible with other variants like AD694BQ or AD694AR?

No, AD694AQ is not pin-compatible with AD694BQ or AD694AR in terms of performance grade, though all share identical pinout and package footprint. AD694AQ is specified over –40°C to +85°C with tighter initial accuracy (±5 µA 4 mA error vs. ±10 µA for BQ), lower nonlinearity (±0.001% vs. ±0.005%), and improved reference drift (5 ppm/°C vs. 45 ppm/°C). Electrical connections are identical, but performance-critical designs must verify grade-specific specs.

What is the purpose of the BOOST pin (Pin 12), and when should it be used?

The BOOST pin connects to the base of an external NPN pass transistor to offload power dissipation from the AD694AQ die. It is used when ambient temperature or supply voltage would cause junction temperature to exceed 150°C - for example, at 36 V supply with 20 mA into a 500 Ω load. The AD694AQ datasheet shows BOOST drives the transistor base while IOUT connects to the emitter, shifting >90% of heat generation externally and extending operational life in high-power scenarios.