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Addressing satellite mission challenges

30 Sep 2015  | Kiran Bernard, Joshua Broline

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Analogue multiplexers are a staple for most signal processing applications, especially satellite systems. Microprocessors have a limited number of I/O (input/output) channels and thus use a multiplexer's multiple switching inputs to sense voltage supplies and telemetry signals from a variety of sensors. As simple as they may seem in concept, there is a lot that goes on inside a multiplexer to make them reliable for space flight applications. Any integrated circuit (IC) aboard a satellite will be exposed to total ionizing dose (TID) and single-event effects (SEE). Both of these phenomena can have adverse effects on semiconductors, if not properly handled.

This article analyses the effects of radiation on analogue multiplexers by breaking down its three different sections—level shifters for the digital inputs, and decoder and switches for each I/O channel. We also examine how SEE can affect the behaviour of an analogue multiplexer, and present the options for overcoming the variety of radiation effects encountered during satellite missions.

Radiation effects
The level shifter converts the logic-level voltages externally applied to the address pins/enable pin into the internal logic level voltages that drive the decoder, which in turn controls which switch to turn on (figure 1). If any of these sections are affected by radiation, the multiplexer's performance will be degraded. SEE and TID affect the multiplexer in very different ways, so let's look at SEE first. Single-event effects is a collective term for the various ways high-energy particles can interact with an IC.

Figure 1: ISL71840SEH 16-channel Multiplexer Block Diagram.

SEE can be divided into destructive and nondestructive phenomena. Destructive effects include latchup, burnout and MOS gate oxide rupture, which can lead to permanent damage, nonfunctional parts, and possible mission failure. Single-event latchup (SEL) is usually reversed with a power cycle, but it may lead to immediate or latent damage. Single-event burnout is a form of SEL and causes permanent damage to the part. Nondestructive effects include bit flips and transients on the outputs of analogue functions.

For logic devices, a logic state change due to a high-energy particle is called a single-event upset (SEU). A temporary glitch from which the multiplexer is able to recover with time is termed a single event transient (SET). There are many more single-event effects but from an application standpoint, the phenomena described here are the primary concerns for analogue multiplexers. Figure 2 shows an ISL71840SEH 16-channel multiplexer test plot of an SET for linear energy transfer (LET).

Figure 2: Composite plot of SET for LET = 43MeV•cm2/mg in Test with ±10.8V supplies.

Behaviour of multiplexers exposed to radiation
Now let's look at how each of these effects can affect the behaviour of an analogue multiplexer. If a high-energy particle were to strike the level shifter portion of the multiplexer, it could cause shoot-through current, which may lead to a single event burnout (SEB) or single event latch-up (SEL) if it's not mitigated correctly during the IC design phase. A strike in the decoder section may cause a momentary change to the selected switch channel being monitored. For example, even when channel 5 is addressed, channel 6 might turn on for a brief moment before recovering to normal operation.

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