The guard terminal of an electrometer will depend on the function. The guard terminal of the electrometer ammeter is the LO input terminal. The difference in potential between the HI terminal and the LO terminal is usually <1 mV. The guard terminal of the electrometer voltmeter is the preamplifier output terminal. The guard terminal of an SMU is the inside shield of the output triax connector. The SMU guard voltage is kept within 2 mV of the signal voltage on the center conductor.