Check Analog Switch Behavior When the Signal Arrives Before Power

A switch that disconnects correctly during normal operation may behave differently when its own supply is absent. This matters when an external sensor stays powered while the local board is off. The review must cover the switch’s specified unpowered behavior and the complete path beyond it.

Draw the off-state circuit

Mark every supply that can disappear independently, including the source signal, switch supply, digital controller and receiving circuit. List the voltage expected on each analog terminal and control pin in those states. A schematic annotated only for normal operation can hide the condition that creates the problem.

Analog Devices’ ADG4612/ADG4613 datasheet explicitly specifies power-off protection on both analog sides. That is an example of a documented device feature, not a property to assume for every CMOS switch. Read the permitted signal conditions and control behavior for the actual selected part.

Separate isolation from logic control

Setting a digital enable low is not sufficient evidence of isolation after the switch loses power. Check whether that input itself remains valid and whether external pull resistors connect to a live rail. Also examine signal paths through protection networks, test equipment and other connected channels.

For a bidirectional connection, evaluate both terminals as potential sources. A downstream capacitor or separately powered module can place voltage on the terminal normally called the output. Use functional names in the review, but retain the manufacturer’s pin names when checking restrictions.

Define a controlled verification

Prepare a state table with local power on, local power off, source absent and relevant supply transitions. For each state, specify expected terminal voltages, allowable leakage or current and the intended state of the receiving circuit. Use the device’s operating and absolute limits to constrain the test setup before applying signals.

Measure the supposedly unpowered rail as well as the signal node. A receiving circuit that partially powers through an unexpected route may make the analog output appear plausible while violating the intended shutdown behavior. Record the instruments and connected cables because they can supply additional current paths.

Close the system-level requirement

A protected switch may resolve one path while another channel, control line or fixture connection remains active. Repeat the path review after adding isolation rather than assuming that one component feature completes the whole board’s power-state design.

The final evidence should show the selected switch, allowed terminal conditions, verified states and remaining dependencies. Distinguish specified power-off protection from behavior observed on one sample. This gives the next engineer a reproducible shutdown requirement and prevents a later switch substitution from silently removing the feature that the design relied upon.