Switches and Multiplexers Break BeforeMake Considerations Joe Creech

Contents ► Background – Break-Before-Make Timing ► Load conditions to ensure Break Before Make

Background – Break-Before-Make Timing ► Break-before-make (BBM) switching is to ensure that inputs do not get shorted together ► BBM ensures that one switch is off before the next switch turns on ► BBM is usually measured between the 80% (relative to ground) point of both switches when switching from one address state to another 3 // 06 March 2021 © 2019 Analog Devices, Inc. All rights reserved.

Load Conditions to Ensure BBM ► BBM switching is heavily dependant on the load conditions on the switch or multiplexer output ► While the switch channel is off and there is no internal connection between multiplexer channels, the load may still be charged, which may cause issues when the next switch channel is engaged ► BBM is typically specified using a 300Ω resistor and a 35 p. F capacitor to ground (see diagram on previous page) ► Changing these values will affect the BBM performance § Reducing the resistance value increases the BBM time § Increasing the resistance value decreases the BBM time ► Fully capacitive loads may not exhibit a BBM characteristic as the charge on the capacitor has no path to ground § Relies on leakage current to discharge the capacitor voltage § May take a long time depending on setup 4 // 06 March 2021 © 2019 Analog Devices, Inc. All rights reserved.

Effect of BBM Timing ► The longer the BBM time of a multiplexer the longer the wait time before another channel can be switched on without negative effects ► If a channel is engaged before the load is discharged, shorts can occur which can create inaccuracies on the load voltage ► If no load resistor is present a long wait time must be used to ensure leakage has discharged the load capacitor or pin capacitance of the next component – This is not recommended 5 // 06 March 2021 © 2019 Analog Devices, Inc. All rights reserved.

Conclusions ► Multiplexer load conditions effect the BBM timing ► To ensure predictable BBM times the signal must be provided a path to ground (or another reference) ► Relying on parasitic effects such as leakage to discharge the load node is not recommended ► BBM times should be considered when switching between multiplexer channels to ensure signal integrity 6 // 06 March 2021 © 2019 Analog Devices, Inc. All rights reserved.