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Optimizing a Switch System for Mixed Signal Testing Dale Cigoy Keithley Instruments, Inc. Switching systems are often needed to automate and speed up the testing of multiple devices in a production environment, and when making mixed signal measurements during R&D and production. Mixed signal measurements on multiple devices increases the importance of switching systems as a means of achieving high test system throughput. Still, there are a number of potential pitfalls in choosing and configuring the switch hardware and software for such a test system. These can lead to less than optimal speed, measurement errors, shortened switch life, and excessive system cost. Therefore, the test system developer needs to understand common sources of errors affecting the integrity of signals to be measured, switch configuration and cabling errors affecting throughput, and switch selection issues that can increase test system cost. Common Sources of Error For developers of new test systems and users having problems with an existing test system that uses a switch assembly it is a good idea to review potential sources of error. Relay contacts are a good place to start. Open State Contact-to-Contact Resistance. In the ideal open relay or switch, the resistance between contacts is infinite. In reality, there is always some finite resistance value that has to be taken into consideration. See Figure 1. The key is to find the magnitude of the open resistance and to determine if it is going to affect the signal passing through the system. There are many different types of Keithley Instruments, Inc. 28775 Aurora Road Cleveland, Ohio 44139 (440) 248-0400 Fax: (440) 248-6168 www.keithley.com 1 switches, and each of them has a specification for insulation/isolation resistance. Review the manufacturer's specification for contact-to-contact resistance in the open state. Contact Contact to contact insulation resistance Contact Figure 1. Representation of a switch re