Data-acquisition system uses fault protection

-April 15, 2004

Sensitive systems, such as those in aircraft, must withstand fault conditions, thereby avoiding component and system damage, because a sensor failure could cause a catastrophic event to occur. A channel protector, comprising two n-channel MOSFETs connected in series with a p-channel MOSFET, can protect sensitive components from voltage transients in the signal path, whether or not the power supplies are present (Figure 1). The channel protector acts as series resistor during normal operation. If the input exceeds the power-supply voltages, one of the MOSFETs turns off, clamping the output within the supply rails, thus protecting the circuitry in the event of overvoltage or supply-loss conditions. Because channel protectors work regardless of the presence of the supplies, they are also ideal for applications in which correct power sequencing cannot be guaranteed and for hot-insertion rack systems. Figure 2 shows an ADG465 channel protector with an input signal that exceeds the power-supply voltage. The protector clamps the output signal, protecting the sensitive components that follow the channel protector.

When a fault condition occurs, the voltage on the input of the channel protector exceeds a voltage set by the supply-rail voltage minus the MOSFET's threshold voltage. For a positive overvoltage, this voltage is VDD–VTN, where VTN is the threshold voltage of the NMOS transistor (typically, 1.5V). In the case of a negative overvoltage, the voltage is VSS–VTP, where VTP is the threshold voltage of the PMOS device (typically, –2V). When the input of the channel protector exceeds either of these voltages, the protector clamps the output within them. These devices offer bidirectional fault and overvoltage protection, so you can use the inputs or outputs interchangeably. Figure 3 shows the voltages and MOSFET states for a positive-overvoltage event.

The output load limits the current during the fault condition to VCLAMP/RL (Figure 4). If the supplies are off, the protector limits the fault current to nanoamps. Figure 5 shows how you can use the ADG466 channel protector to protect the sensitive inputs of an instrumentation amp from a sensor fault. In applications that require a multiplexer in addition to channel protection, you can use the ADG439F fault-protected, four-channel analog multiplexer (Figure 6). These multiplexers use a series n-channel, p-channel, n-channel MOSFET connection. During fault conditions, the inputs or outputs appear as open circuits, protecting the sensor or signal source as well as the output circuitry.

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