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DACs: Protecting 3-wire analog outputs

Industrial DACs: Protecting 3-wire analog outputs

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Industrial DACs: Protecting 3-wire analog outputs

Kevin Duke

Aug 12 2014 10:38 AM

Comments 0

In my last post, I talked about how integration has simplified 3-wire analog output design.
In this post, I’ll show you one way to protect these designs against dangerous
industrial transients that can cause electrical overstress.

Let’s start with a few examples of what we’re trying to protect the system
against:

Some systems are installed or calibrated in environments that are not
ESD-safe, which can lead to ESD damage.

Industrial control systems are often large systems that span great
distances and may be exposed to hazards from nature, like lightning
strikes.

Switching transients coupled with environmental parasitics can create
high-frequency radiated and coupled emissions.

The transients you need to protect the analog output from are very dissimilar
from the low voltage (< 24V) and low frequency (<10kHz) signals it
generates. Industrial transients are high voltage, up to 15kV, and high
frequency, typically with a period of less than 100ns. Your circuit should take
advantage of these differences to provide protection, while not disturbing the
signal integrity of the analog output.

Figure 1. Attenuation and diversion
overview

Attenuation and diversion can be used to capitalize on the high-frequency and
high-voltage components of the industrial transients. Figure 2 shows a
protection circuit that uses these strategies to protect the single-channel,
16-bit DAC8760 for 4-20mA current loop applications.

Figure 2. Example protection
circuit

Attenuation leverages passive components with frequency-based responses, like
ferrite beads and capacitors, to attenuate high-frequency signals. In Figure 2,
the 100nF capacitor on each output terminal interacts with the source impedance
of the transient generator to attenuate high-frequency signals.

I included series pass elements between each stage of the circuit to limit
current flow between nodes that are clamped to different voltage potentials. I
used resistors as the series pass elements for the current output and nodes that
are inside the voltage output. Ferrite beads act as the series pass element in
the voltage output path outside of the feedback loop to maintain DC accuracy
while still limiting high frequency current.

Diversion uses diodes to redirect the high-voltage signals away from the
analog signal chain. You can then either steer the energy to ground by using
transient voltage suppressor (TVS) diodes or to a supply rail by using Schottky
clamp-to-rail diodes.

If you’d like to learn more about TVS diodes, I encourage you to check out
this blog
series from my colleague Art Kay, in which he explains several key
parameters and provides selection tips.

In short, you should select TVS diodes based on the following;

Working voltage: The maximum voltage that the diode can be exposed
to without conducting significant current. This should be high enough to
ensure the diode does not impact normal circuit operation.

Breakdown voltage: The voltage that causes the TVS diode to begin to
conduct. This should be low enough to keep transient voltages within the
supply rails.

Power rating: When the diode does breakdown, it will pass significant
power and needs to be rated accordingly.

Figure 2 also includes a clamp-to-rail stage using Schottky diodes, which
helps keep transients within the supply rails for two reasons:

TVS diode breakdown voltages rarely match supply configurations.

The TVS diode breakdown voltage will increase as it passes more
current.

While all Schottky diodes should feature low forward voltage, diodes used in
protection circuits need to maintain low forward voltage even when conducting
large current.

The circuit in Figure 2 was developed for TIPD153, a CerTIfied TI Precision Design for protecting the
DAC8760
against the IEC6100-4 test suite. For more information on component selection,
layout guidelines and the IEC6100-4 test results of the design, download the reference design guide.

If you have any questions on how to protect 3-wire analog outputs, let me
know in the comments section below. And be sure to check back next month, when
I’ll explain how to design a robust 2-wire, 4-20mA transmitter.

Related materials:

Watch my video on transient immunity for analog I/O modules that explains
the IEC61000-4 tests.

Watch my video on how to design protection circuits for analog I/O modules.

Read more on how to protect inputs from EOS damage in a blog post from
amplifier expert Bruce Trump.

Check out more blog posts in my Industrial DACs blog series.

$core_v2_language.FormatString($ti.GetResource('Blog_PostQuestionAnswerView_CommentsCountFormatString'),
$post.CommentCount)

Precision
DAC (=<10MSPS), TIPD153,
Industrial
DACs, DAC8760,
3-wire
analog output, attenuation,
diversion,
ESD
damage, Schottky,
TVS
diodes

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