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Maxim > Design Support > Technical Documents > Application Notes > A/D and D/A Conversion/Sampling Circuits > APP 474
Keywords: uP supervisor, microprocessor supervisor, analog io, DAC, optocoupler, digital analog converter,
programmable logic controller, PLC, data acquisition
APPLICATION NOTE 474
Precision DAC and Watchdog Improve Analog Output
Safety
By: Damian Anzaldo, Communication Segment Manager, Maxim Integrated
Feb 15, 2012
Abstract:
Using a
DAC and a microprocessor supervisor, the system safety can be improved in industrial
controllers, programmable-logic controllers (PLC), and data-acquisition systems. The analog output is set to zero-
scale (or pin-programmable midscale) when a microprocessor failure, optocoupler failure, or undervoltage
condition occurs. A simple application is shown on how to implement this function.
A similar version of this article appeared on Hearst Electronic Products, December 18, 2012.
Introduction
The need for safe states in industrial applications is critical when system-level failures occur. A feature-rich
precision digital-to-analog converter (DAC) and microprocessor (µP) supervisory circuit with watchdog timer can
be used to assure that analog outputs default to a known midscale or zero-scale level.
Circuit Operation
Common analog-output control signals used in industrial controllers, programmable-logic controllers (PLCs), and
data-acquisition systems (DAS) include voltage outputs such as 0 to 5V, 0 to 10V, ±5V, ±10V, and 4–20mA
current loop. To ensure safety integrity, the analog-output control signal should default to a known midscale or
zero-scale safe state in response to a µP or system failure. For additional safety, analog output modules often
employ optocouplers to provide galvanic isolation between the µP and analog output signals.
Requirements for analog output safety and reliability are met by the Figure 1 circuit. When a µP failure,
optocoupler failure, or undervoltage condition occurs, the supervisory circuit (U3, the MAX6303) sets the 12-bit
DAC (U2, the MAX5120) analog output to mid-scale, which in turn sets V
OUT
to 0V safe-state output. This circuit
also improves mean time between failure (MTBF) by reducing the number of I/O pins at the µP and by eliminating
an optocoupler.
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