The contact resistance specification of module of DAQ?
The contact resistance specification of module of DAQ?
Troubleshooting (FAQ)
Summary
The impact of contact resitanceQuestion
The contact resistance specification of module of DAQ?Answer
The specification note that you have quoted is a caveat based on the relay manufacturer’s reluctance to guaranty the contact resistance. Since having the problem that resulted in changing contact resistance from a specification to a characteristic and adding this caveat, the relay manufacturer has substantially improved his process and we are now seeing much better contact resistance performance. Note that the 34901A multiplexer for the 34970A/72A uses the same relays and do not list this limitation in the data sheet, because it is not necessary. Based on engineer experience with the new relays, they would estimate that there is very little chance that you will see any problem.
The contact resistance is only a concern for 2-wire resistance measurements. For voltage measurements, it has negligible effect due to the 10 Mohm input impedance of the DMM, which can be increased to greater than 10 Gohm if needed for very sensitive measurement. If the customer can do 4-wire resistance measurements, the contact resistance is eliminated from the measurement.
If the contact resistance is a serious concern, the best thing to do is to buy the Y1131A Test and Verification software along with the option 001 test fixture and use this to test the relay contact resistance periodically. Switching relays for 2K cycles prior to use may reduce the variation in channel resistance. It also may not, particularly if there is a damaged relay, but this is not likely. Engineer would recommend running the 2K cycle exercise once a week if they are making low resistance measurements where more than 2 ohms error is critical.
The initial closed channel resistance is specified for the 34937A, 34938A, and 34939A.
