NI switch modules are capable of switching 1, 2, and/or 4-wires. In 1-wire mode, you connect the positive leads to the relays and the negative leads to a common connection. All signals are referenced to this common connection.
Figure 7. Single-Ended Multiplexer
Sometimes more than one signal needs to be switched at the same time. In this situation a switch that employs 2-wire or 4-wire mode can be used. In 2-wire mode, you connect both positive and negative leads to the terminals of a channel. An advantage of 2-wire switching is great common-mode noise rejection. Some applications in which 2-wire is typically used are differential measurements, low voltage, high current, and resistance measurements in the 100 – 10 M Ohm range. 4-wire mode is usually used for 4-wire resistance measurements. Two leads are used for the current excitation and another two leads are used for measuring the voltage drop across the resistor.
Figure 8. 2-wire (differential) and 4-wire Multiplexers
National Instruments relay drivers are the ideal choice when the current and voltage requirements for relays exceed those found in existing relay modules (or for relays embedded in a test system). Like NI switches, relay drive modules are controlled with NI-SWITCH driver software, so engineers can program external relays connected to the relay driver and standard PXI and SCXI switch modules identically. For added safety against flyback voltages, a flyback diode has been added across the relay. The SCXI-1167 has a 5 V source and the PXI-2567 has a 5 V and a 12 V source available to drive relays. The 5 V source on the SCXI-1167 can provide up to 0.75 A of current. The 5 V source on the PXI-2567 can provide up to 1.25 A of current and the 12 V source can provide 0.50 A of current.
Figure 9. PXI-2567 External Relay Driver
The National Instruments RF (radio frequency) switch modules are ideal for expanding the channel count or increasing the flexibility of systems with signal bandwidths greater than 10 MHz. RF is not a topology but RF switches can be any topology. High-density multiplexers, dimensionally flexible sparse-matrices, and general-purpose relays are among the available configurations in PXI and SCXI switch modules. Each of these modules has been optimized for minimal insertion loss, reflection, cross-talk, and maximum isolation between channels. For more information on these parameters see the links below for the Complex RF Switching Architectures – Part I and Part II.
For available National Instruments switch modules designed for RF switching, please visit NI PXI Switch Product Selection Guide. For the design and operational concepts of RF switches, please visit Complex RF Switching Architecture -- Part I and Complex RF Switching Architecture -- Part II.