Used Electrometers & Picoammeters for Sale

An electrometer is a precision instrument designed to measure extremely small currents (femtoamperes to nanoamperes), very high resistances (up to 10¹⁶ Ω), and charge (picocoulombs). A standard DMM has an input impedance of about 10 MΩ, which loads down high-impedance sources and introduces leakage currents that swamp femtoampere measurements. An electrometer has an input impedance of >200 TΩ and input bias current below 1 fA — three to six orders of magnitude better than a DMM. Electrometers are used in semiconductor research (leakage current), nuclear radiation detection, electrochemistry, and any application where source impedances exceed 1 GΩ or currents are below 1 nA.

A picoammeter measures small currents (picoamperes to microamperes) but does not have the voltage measurement capability of a full electrometer. It connects in series with the current path and is optimized for fast, accurate current readings at low signal levels. An electrometer typically measures current, voltage, resistance, and charge — it is a more complete instrument. The Keithley 6485 and 6487 are picoammeters; the Keithley 6514 and 6517A/B are full electrometers. For current-only measurements where you need high throughput, a picoammeter is faster and simpler. For high-resistance and charge measurements alongside current, the electrometer is required.

A nanovoltmeter (Keithley 2182A, Keithley 181) measures DC voltages with resolution in the nanovolt range — typically 1 nV to 10 nV. These instruments are used in four-wire resistance measurements on superconductors, thermoelectric materials, and milliohm-range resistors where the DUT generates a voltage in the microvolt to millivolt range across a known current. The Keithley 2182A has 1 nV resolution and is specifically designed for use with the Keithley 6221 current source for delta-mode resistance measurements. Thermocouple and thermopile measurements in precision temperature measurement also require nanovolt sensitivity.

The Keithley 6517B is a combined electrometer and high-resistance meter. It measures current from 10 fA to 20 mA, voltage from 1 µV to 200 V, resistance from 10 Ω to 10¹⁶ Ω, and charge from 10 fC to 2 µC. It also includes a built-in 1 V to 1 kV voltage source for applying bias to high-resistance samples. This combination makes the 6517B the standard instrument for insulation resistance testing, MOSFET gate leakage measurement, and dielectric characterization. The built-in source eliminates the need for a separate bias supply, which is important for very high-resistance measurements where cable leakage from a second instrument would corrupt the reading.

The main error sources in low-current and high-impedance measurements are triboelectric (cable motion-induced) currents, dielectric absorption in cable insulation, and leakage through connectors and circuit boards. Use low-noise triaxial cable (Keithley triax or BNC-to-triax adapters) and guard the cable shield actively with the driven guard output of the electrometer. Keep cables short and stationary. Clean connectors and PCB surfaces with isopropyl alcohol — surface contamination creates parallel resistance paths. Use Teflon or polypropylene standoffs for the device under test. For measurements below 10 fA, the measurement chamber should be shielded from EMI and vibration.