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AD693 in Practice: Solving Self-Heating and Navigating High-Cost 4-20mA Design

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Quick-Reference Card: AD693 at a Glance

Attribute Detail
Component Type Monolithic 4-20 mA Transmitter / Signal Conditioner
Manufacturer Analog Devices Inc.
Key Spec < 0.05% Residual Nonlinearity
Supply Voltage Loop-powered (refer to datasheet for range)
Package Options PDIP, CERDIP, SOIC (Check availability)
Lifecycle Status Legacy / NRND (Not Recommended for New Designs)
Best For High-precision RTD and bridge sensor signal transmission

AD693 product photo in ceramic or plastic DIP package


1. What Is the AD693? (Definition + Architecture)

The AD693 is a monolithic signal conditioning circuit from Analog Devices Inc. that accepts low-level inputs from transducers to control a standard 4-20 mA, two-wire current loop. Unlike discrete solutions that require multiple op-amps and precision resistors, the AD693 integrates an instrumentation amplifier, a voltage reference, and a current output stage onto a single chip.

1.1 Core Architecture & Design Philosophy

The AD693 was designed to be a "complete" solution for the industrial signal chain. Its architecture centers on a high-impedance instrumentation amplifier (InAmp) front end. This is followed by a voltage-to-current converter stage. The internal design philosophy prioritizes stability and precalibration; by laser-trimming internal resistors, Analog Devices provides specific input spans (30 mV and 60 mV) out of the box, which significantly reduces the calibration burden for the end user.

1.2 Where It Fits in the Signal Chain

The AD693 sits at the very edge of the field instrumentation. It takes raw, millivolt-level signals from sensors—such as RTDs, thermocouples, or strain gauges—and converts them into a robust 4-20 mA current signal. This current signal is then transmitted over long distances (hundreds of meters) to a PLC or DCS, where it is converted back to a voltage via a precision shunt resistor.


2. Electrical Characteristics: The Numbers That Matter

2.1 Power Supply & Consumption Profile

The AD693 is strictly a loop-powered device. It derives its operating power directly from the 4-20 mA loop, meaning it must operate on less than 4 mA of quiescent current to avoid offsetting the zero-scale output. * So What? This eliminates the need for local power supplies at the sensor head, but it limits the total current available for sensor excitation to 3.5 mA.

2.2 Performance Specs

  • Nonlinearity (< 0.05%): This is the standout spec for the AD693.
  • So What? High linearity ensures that the 4-20 mA output remains a faithful representation of the physical parameter (temperature or pressure) without requiring complex software linearization in the PLC.
  • Precalibrated Spans: 30 mV and 60 mV are standard.
  • So What? If your sensor output falls within these ranges, you can achieve high accuracy without adding external precision resistors, which are often prone to thermal drift.

2.3 Absolute Maximum Ratings — What Will Kill It

  • Loop Voltage: Exceeding the rated loop voltage (refer to datasheet for specific package limits) will lead to immediate thermal runaway.
  • Reference Current (> 3.5 mA): Attempting to draw more than 3.5 mA from the 6.2V reference pin will cause the internal regulator to drop out, leading to massive output errors.
  • Reverse Polarity: The AD693 does not have built-in high-voltage reverse polarity protection. Connecting the loop backwards often results in permanent silicon damage.

3. Pinout & Package Guide

AD693 pinout diagram with labeled pins for InAmp and Loop

3.1 Pin-by-Pin Functional Groups

Pin Group Pins Function
Power (Loop) Boost, Loop +, Loop - Connection to the 2-wire current loop
Input Signal In (+), In (-) Low-level signal from the transducer
Reference 6.2V Ref, Ref Com Excitation voltage for sensors/bridges
Adjustment Zero, Span Pins for fine-tuning the 4mA and 20mA points
Configuration COM, NC Common returns and no-connects

3.2 Package Variants & Soldering Notes

Package Pitch Thermal Pad? Soldering Method
PDIP-20 2.54 mm No Wave / Hand Solder
SOIC-20 1.27 mm No Reflow
  • Note: The CERDIP (Ceramic) version is highly sought after for high-reliability or high-temperature environments but is increasingly difficult to source.

3.3 Part Number Decoder

A typical part number follows this format: AD693 [A] [N] * AD693: Base part number. * A/B/S: Grade (defines accuracy and temperature range). "S" is usually the military grade. * N/R/Q: Package code (N = Plastic DIP, R = SOIC, Q = CERDIP).


4. Known Issues, Errata & Real-World Pain Points

4.1 Self-Heating Errors

Problem: Because the AD693 is monolithic, the heat generated by the output pass transistor (especially at 20 mA) can warm the input stage, causing an offset drift. Root Cause: Internal power dissipation at high loop voltages. Recommended Fix: Use an external NPN pass transistor connected to the "Boost" pin. This offloads the heat-generating power dissipation to a component physically separated from the sensitive input stage.

4.2 Limited Precalibrated Input Spans

Problem: Transducers often output signals like 10 mV or 100 mV, which don't match the internal 30/60 mV spans. Root Cause: Fixed internal gain resistors. Recommended Fix: Add external resistors to the span adjustment pins to program custom spans from 1 mV up to 100 mV.

4.3 High Cost and Aging Lifecycle

Problem: The AD693 is a legacy part with a unit cost often exceeding $80. Root Cause: Older manufacturing processes and declining volume. Recommended Fix: For new designs, evaluate modern alternatives like the XTR115, which offers similar functionality at a fraction of the price.


5. Application Circuits & Integration Examples

5.1 Typical Application: 100 Ohm RTD Interface

The AD693 is precalibrated for a 100 Ohm Platinum RTD. In this setup, the 6.2V reference provides the excitation current. * Layout Tip: Keep the RTD leads as short as possible, or use a 3-wire compensation technique to minimize lead resistance errors. * Expected Behavior: A temperature range (e.g., 0°C to 100°C) is mapped linearly to the 4-20 mA output.


6. Alternatives, Replacements & Cross-Reference

6.1 Pin-Compatible Drop-In Replacements

There are no 1:1 pin-compatible drop-in replacements for the AD693 due to its unique integrated InAmp and reference structure. Any change will require a PCB spin.

6.2 Upgrade Path & Alternatives

Part Number Manufacturer Key Difference Compatible?
AD694 Analog Devices Voltage-to-current only (no InAmp) ?? (Requires preamp)
XTR105 Texas Instruments Specifically for RTDs, much cheaper ? (Different Pinout)
XTR115 Texas Instruments Basic transmitter, requires external InAmp ? (Different Pinout)

7. Procurement & Supply Chain Intelligence

  • Lifecycle Status: NRND / Legacy. While still available, it is not recommended for new commercial designs.
  • Typical MOQ & Lead Time: Can be high (25+ weeks) if not in stock at major distributors like Mouser or Digi-Key.
  • BOM Risk Factors: High. The AD693 is a single-source part. If Analog Devices issues a Product Discontinuation Notice (PDN), finding a replacement will require a full redesign.
  • Authorized Distributors: Only buy from authorized sources. Due to the high unit price, the AD693 is a common target for counterfeiters in the secondary market.

8. Frequently Asked Questions

Q: What is the AD693 used for? The AD693 is primarily used in industrial process control to convert low-level sensor signals (RTDs, thermocouples) into a 4-20 mA current loop for long-distance transmission.

Q: What are the best alternatives to the AD693? The Texas Instruments XTR105 is the most common alternative for RTD applications, while the XTR115/117 are preferred for general-purpose 4-20mA transmission.

Q: Is the AD693 still in production? Yes, but it is in a legacy phase. It is expensive and primarily maintained for existing aerospace and industrial systems.

Q: How do I reduce self-heating in the AD693? Connect an external NPN transistor to the "Boost" pin. This shifts the power dissipation away from the IC, maintaining the accuracy of the input stage.


9. Resources & Tools

  • Official Datasheet: [Analog Devices AD693 Product Page]
  • Reference Designs: See ADI Application Note AN-345 for bridge circuit details.
  • SPICE Model: Available in the LTspice library under "Signal Conditioners."

AD693AE Documents & Media

Download datasheets and manufacturer documentation for Analog Devices Inc. AD693AE.

AD693AE PCB Symbol, Footprint & 3D Model

Analog Devices Inc. AD693AE

Analog Devices Inc.

IC SGNL COND 4-20MA TX 20-CLCC

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