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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
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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 relay's insulation resistance in the open state.
In general, the higher the resistance in the open state, the lower the leakage between
contacts, and the less effect on signal integrity. Most relays have open-state resistance
specifications between 1MW and 1GW, which is sufficient for most applications, especially
DC measurements. For example, switching a power supply signal of 5V through a switch relay
contact has little to no effect due to open-state resistance. This is because a power supply
normally has low internal impedance that the high switch impedance does not affect. Table 1
provides examples of different relay types with their open contact isolation resistance and
other characteristics.
Table 1. Characteristics of different relay types.
Relay Type Isolation Speed Power Life at Rated Load
Electromechanical 10