SPST IC Switch Protects Sensors Against Overvoltage, Power-Off Faults

Many times, it’s the small, tightly focused component that does just one thing to transform a possible headache in a design into a non-issue. Or it makes the difference between a “pretty good” design that works well enough, but it can’t handle anticipated upsets, into a really good, “rock solid” design.

A good example is Analog Devices’ ADG7421F, a low-voltage fault-protection and detection switch. This dual single-pole single-throw (SPST) switch features on-resistance (R_ON) of just 12 Ω and provides overvoltage protection for small signals such as resistance-temperature-device (RTD) and thermocouple inputs (Fig. 1). The wide range of applications includes analog input/output modules, process-control/distributed-control systems, avionics, automatic test equipment, and even relay replacement in some cases.

1. The ADG7421F dual SPST fault-protection and detection switch, developed by Analog Devices, is a good fit for exposed thermocouple and RTD front ends.

Although perhaps at first glance it seems to be “just a switch,” it protects against high voltages up to ±60 V. And due to its low-voltage supply, the ADG7421F enhances development of complete low-voltage input stages for industrial applications. It consists of two switch channels of N-channel diffused metal-oxide semiconductor (DMOS) transistors, a construction that provides excellent R_ON performance in a small area.

Operation is such that when no power supplies are present, the switch remains in the “off” state, and the switch inputs are high impedance. When powered, if the analog input-signal levels on input pins S1 and S2 exceed V_DD or V_SS by a threshold voltage V_T, the switch automatically turns off and the digital fault flag (FF) pin drops to a logic low to indicate a fault.

Input-signal levels up to +60 V or down to −60 V relative to ground are blocked in both the powered and unpowered condition. The switches turn on with a Logic 1 input and conduct equally well in both directions with an analog signal range of V_SS + 0.1 V to V_DD − 0.55 V for a 5-V single supply.

Other noteworthy attributes include ultra-flat on-resistance of 0.005 Ω (typical), 3-kV human-body-model (HBM) ESD rating, and full latch-up immunity under any circumstances. The digital input is compatible with 1.8-V logic inputs over the full operating supply range.

Despite its diminutive size (10-lead, 3- × 2-mm LFCSP) and deliberately limited functionality, the ADG7421F comes with a comprehensive 30-page datasheet that includes tables and graphs characterizing performance under different operating conditions (e.g., temperature).

Further, to support better understanding of the switch and its functions, Analog Devices offers the EVAL-ADG7421F Evaluation Kit (Fig. 2), which allows users to connect signals and observe performance in various situations (Fig. 3). The kit is supported by its own eight-page User Guide that includes setup, operating instructions, schematic diagram, PCB layout, and bill of materials (BOM).