Signal Path Stray Capacitance Variation in Matrix Switches

Updated Oct 15, 2018

Reported In

Hardware

  • PXIe-2738

Issue Details

I would like to use a PXIe-2738 Matrix Switch Module to connect multiple impedances like capacitive, resistive or inductive elements to an LCR-meter, so I can measure their impedance parameters consecutively. I observe significant variation in stray capacitance depending on the actual switch channel connecting the measured element to the LCR-instrument. This causes problem in the offset impedance compensation of the LCR-meter.

Solution

The PXIe-2738 is a 2-wire 8 x 32 relay matrix switch. The relays, i.e. the crosspoints are positioned in a matrix arrangement on the PCB card. When a signal route is established by switching a relay on, actually two pairs of traces are connected to each other at the crosspoint. Each pair of traces has different layout. Consequently each has different stray capacitance between the two traces, and relative to the ambience. When two such trace pairs are connected by their common relay, their respective stray capacitances and their mutual stray capacitance together result in a channel specific stray capacitance up to several hundreds of pF-s. In summary, the observed phenomenon is physically well understood and doesn't violate device specifications. The LCR-meter needs to be recalibrated each time the channel is changed, with the exception of the case, when the stray capacitance variations are negligible.

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