Settling Time and Switches
When using switches in an application, you need to consider switching and measurement timing issues. Switches take time to actuate and settle, so changing channels requires care to avoid compromising the measurement with a DMM.
Electromechanical switches use coil-induced magnetic fields to move an armature and connect two contacts, completing the circuit. The time it requires is called relay actuation time. The actuation time required to close a contact usually takes longer than the time it takes to break the connection.
FET switches use transistor devices to modulate path resistance. The actuation time is based on charging the gate of these transistors and semiconductor device physics, and it is much faster than actuating the armature of an electromechanical switch.
Solid–state relays (SSR) also use transistors to create the circuit. To provide high-voltage switching capability, the gate is driven by an optical isolator. The optical isolator takes longer to charge and actuate the switch than the non-isolated gate of a FET switch, but it is faster than an electromechanical switch.
In addition to actuation time, switches require a settling time. After an electromechanical switch actuates, the armature vibrates, or bounces, on the channel contact. Measurements cannot be made until this effect has subsided. Settling time can also be affected by the length and type of external cabling. The DMM should wait for the switch to actuate and for the system to settle before taking a measurement. The total settling time is dependent on the required accuracy. For information on settling time, refer to Settling Time.
If you are using National Instruments switches, you can exchange triggering signals between the switches and the DMM (handshaking), or the DMM can signal the switch to actuate (synchronous scanning). Refer to Scanning Switch Modules for information about how National Instruments DMMs and switch modules can exchange trigger signals.