Electronic Design

Low-Level Measurement Gets High-Level Treatment

A nanovoltmeter, an ac-dc current source, and a dc current source set new standards for low-level voltage, current, and noise measurements.

Low-level measurements in research, metrology, nanotechnology, superconductivity, and other low-voltage and low-resistance applications require a special breed of test equipment. Keithley Instruments accepts this challenge with three new instruments with a powerful set of capabilities.

When used together, the model 2182A nanovoltmeter, the model 6221 ac-dc current source, and the model 6220 dc current source raise the bar for low-level measurement accuracies, consistency, reliability, robustness, and cost-effectiveness. Improved delta-mode and pulsed-mode methods help users make faster and more accurate resistance, pulsed I-V characterization, and differential conductance measurements.

The 2182A's improved delta-mode method eliminates unwanted pump-out current, minimizing dc drift and noise common to low-level measurements. It also pumps up measurement speed.

"Not only can the improved delta-mode technique reduce the effects from thermal voltages 1000 times more than previously possible, but it also eliminates the effects of thermal voltage drift," says Ted Thorbjornsen, director of Keithley's Research and Education Business Unit. "It's three times faster than the predecessor delta-mode technique and 10 times better at rejecting drifting offsets. Compared to the alternative method of using lock-in amplifiers, it is less susceptible to phase-shift errors and has two times less noise."

The unit can make 15-nV p-p measurements at 1-s response times and 40- to 50-nV measurements at 60-ms response times. It measures noise levels down to 3 nV rms/√(Hz) and spans a voltage range of ±10 nV to ±100 V. Dual-input channels allow for measurement of temperature and voltage simultaneously, or the ratio of two voltages. Built-in thermocouple calculations are included for type J, K, N, T, E, R, S, and B thermocouples.

A hardware-trigger feature with sub-microsecond response latency enables pulsed-mode measurements when used with the Ethernet-enabled model 6221 ac-dc current source. The 6221 uses line sync and difference techniques to eliminate power-line noise and dc offsets. Up to 1000-point/s dc measurements are possible. Also, line-synchronization enables 110-dB rejection of the ac-line frequency interference.

When linked with the Ethernet-controlled model 6221 ac-dc current source, the combined system effectively operates as an Internet-controlled nanovoltmeter, giving users control of both sourcing and measuring through a single Internet IP address. The combination also allows the measurement of 10-nΩ to 1-GΩ resistances. And, it provides differential conductance measurements (tunneling spectroscopy and electron tunneling spectroscopy) 10 times faster and with lower noise than existing solutions, like lock-in amplifiers and ac resistance bridges. Pulsed resistance and I-V characterization measurements via pulses of as short as 50 µs are possible, even at low currents, minimizing the current placed into the device under test (DUT).

The 6221 ac-dc current source possesses a range of 1 mHz to 100 kHz. Features include an arbitrary-waveform-generator software interface, a GPIB interface, and pulse current rise and fall times between 2 and 10 µs.

The 6221 and the 6220 offer currents from ±2.0000 nA to ±100.00 mA at speeds of 1000 points/s. Each features low noise of 80 fA rms and a compliance voltage range of 0.1 to 100 V in 0.01-V steps. Their output impedance tops out at 1014 Ω, and the currents they provide are independent of the DUT's changing impedance.

Software support is available for all measurement functions using each instrument either alone or in combination. The menu-driven software guides users through setups and connections according to their choice of configuration and collects and displays data in intuitive formats.

The 2182A nanovoltmeter is priced at $2803. The 6221 ac-dc source and 6220 dc current source cost $3995 and $2995, respectively. All are available in eight weeks.

Keithley Instruments Inc.
(888) 534-8453
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