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 2 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.
Fig 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 1Mx 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.
Closed State Contact-to-Contact Resistance:
In the ideal closed relay or switch, there is no resistance between contacts. In the real world, closed switches have some small amount of contact resistance, typically on the order of a few milliohms. Depending on relay and contact design, most new relays have a closed-contact resistance specification of less than 100mW. This resistance usually increases with use. Most relays have an end of life specification of about 2W. Depending on relay type (see table 1), this typically occurs after millions of cycles of use. Even at such high resistance, a relay can still function, although it may begin to have a greater impact on the signal passing through the switch.
This is a voltage produced between contact terminals due to dissimilar metals and a temperature gradient across relay contacts and contact-to-terminal junctions. The temperature gradient typically arises due to power dissipated by the energized relay coil. Contact potential can be significant when conducting low-level voltage and resistance measurements. It can range from several nanovolts to as much as 1mv, depending on contact design. For best measurement results, the contact resistance should be substantially lower than the smallest signal being measured.