Using Switches

Refer to Triggering for more information.

In applications where you are connecting many test points to the DMM, it may be appropriate to incorporate a switch. Switches are used to route signals between test points and instruments.

Switch selection should be based on the signal types you are routing and the topology required.

For current and resistance measurements, the switch should have very low path resistance (for example, under 1–2 Ω path resistance). Refer to Offset Compensated Ohms for optimizing resistance measurements in the presence of relay thermal offsets. For capacitance and inductance measurements, it is necessary to have 1–2 Ω path resistance as well. Electromechanical switches are a good match for these applications.

For high–speed scanning through different channels, solid–state relays (SSRs) or FET switches work well. They have high path resistances, so they work best for high-impedance voltage measurements. FET switches operate at logic levels and are usually very fast (25 to 100 kHz), at the expense of low maximum voltage limits (<10 V). SSRs provide internal isolation that allow them to switch much higher voltages (300 V), but they are not as fast as their FET counterparts (1 to 2 kHz).

For low-voltage measurements, keep in mind that the relay section determines the ultimate measurement error due to contact thermal voltages. Refer to Optimizing Low-Voltage Measurements for more information.