Industrial DACs: Protecting 3-wire analog outputs


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: 

  1. TVS diode breakdown voltages rarely match supply configurations.
  2. 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. 

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