Analog Devices Inc. AD694BQ

Overview

AD694BQ 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 AD694BQ datasheet, AD694BQ pinout, AD694BQ application, or AD694BQ equivalent, key selection criteria include its 16-pin CERDIP package, –40°C to +85°C industrial temperature rating, open-loop alarm (Pin 10), programmable 4 mA on/off (Pin 9), and compliance to 4–20 mA loop standards without external calibration components.

Technical Context

The AD694BQ implements a three-stage architecture: a PNP-input buffer amplifier (Pin 1 FB output, ±2.5 mV trim range) drives a ground-referenced V/I converter whose transfer function is IOUT = 20 × VPIN1/(R1 + R2); a laser-trimmed voltage reference (2 V or 10 V, ±50 ppm/°C max drift) feeds a 4 mA offset generator; and an open-collector alarm circuit monitors IOUT compliance against VS – 2 V or open-loop conditions.

Its output stage uses a Darlington-based design allowing IOUT (Pin 11) to drive loads down to 36 V below VS - enabling dual-supply DAC interfacing without level shifting - while supporting optional external NPN pass transistors to offload power dissipation and extend thermal operating range beyond +85°C.

Key Specifications

Pinout & Package

AD694BQ is housed in a hermetically sealed 16-pin CERDIP package (package code Q), rated for –40°C to +85°C operation, with substrate tied to Pin 5 (COM). The package provides high reliability in harsh industrial environments and supports through-hole mounting.

Key Features

Applications

Equivalent & Alternatives

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

Compared with XTR115UA and MAX16992ATP+, the AD694BQ offers superior absolute accuracy (±0.001% vs. ±0.01%) and eliminates reference sourcing complexity, but requires external supply - making it ideal for fixed-location industrial modules rather than battery- or loop-powered field devices.

Availability

AD694BQ is available at Aetrix Electronics and suitable for industrial process control, factory automation, and remote sensor transmitter applications requiring stable component supply, long-term obsolescence management, and hermetic packaging for harsh environments.

Supply support for AD694BQ 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, communications, and instrumentation markets since 1965.

The AD694BQ belongs to Analog Devices' precision current transmitter product line, engineered specifically for noise-immune 4–20 mA loop interfaces in process control systems - emphasizing laser-trimmed accuracy, wide supply range, and robust fault detection.

FAQ

What is the maximum load resistance the AD694BQ can drive in a 4–20 mA loop with a 24 V supply?

The AD694BQ output voltage compliance extends from VS – 36 V to VS – 2 V. With a 24 V supply, minimum compliance is 22 V. At 20 mA, maximum RL = 22 V / 0.02 A = 1100 Ω. This assumes no additional voltage drop across protection diodes or wiring - actual design margin should reserve ≥1 V headroom. The AD694BQ datasheet confirms RL ≤ (VS – 2 V)/20 mA for linear operation.

Can the AD694BQ operate from a 5 V supply, and if so, what configuration is required?

Yes, the AD694BQ supports 4.5 V minimum supply - but only in 2 V full-scale input mode with 2 V reference output. Pin 4 (2V FS) must be shorted to Pin 5 (COM), and Pins 7–8 (10V FORCE–2V SENSE) must be shorted to select 2 V reference. This configuration reduces headroom requirements for internal amplifiers. Operation at 5 V is validated in the AD694 datasheet Rev. C, Figure 3 and Table 4.

How does the AD694BQ's alarm function detect loop faults, and what is the electrical behavior of Pin 10?

Pin 10 (ALARM) is an open-collector NPN output that pulls low when either the IOUT loop opens (no current flow) or the output voltage exceeds VS – 2 V - indicating loss of compliance. Leakage current is ±11 µA max; saturation voltage is ≤0.35 V at 2.5 mA sink. An external pull-up resistor (typically 10 kΩ to VS) is required to generate a logic-high signal during normal operation.

Is the AD694BQ pin-compatible with other variants like AD694JN or AD694AQ?

No - while all AD694 variants share identical pinout and functionality, the AD694BQ (16-pin CERDIP) differs physically from AD694JN (16-pin plastic DIP) and AD694AQ (16-pin CERDIP with different temp grade). Package dimensions, thermal characteristics (θJC = 30°C/W for Q vs. 50°C/W for N), and specified temperature ranges differ. PCB layout must match the exact package footprint and thermal pad requirements of AD694BQ.

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

Pin 12 (BOOST) connects to the base of an external NPN transistor to increase current drive capability of the 2 V reference output - useful when powering multiple downstream circuits (e.g., sensor excitation, ADC reference) from the same 2 V source. It is not used in standard 4–20 mA transmitter configurations and has no effect on the main IOUT path or buffer amplifier performance.