When do you need a multichannel picoammeter?

Steven Burns -April 03, 2013

For many engineers, a picoammeter is the “go-to” instrument for a broad range of low current measurement applications when a digital multimeter’s (DMM) sensitivity is not enough. Measuring low DC currents often demands a lot more sensitivity than a DMM (that other instrument in every EE’s toolbox) can deliver. Generally, DMMs can’t measure currents lower than 100nA accurately.

Even at higher currents, a DMM’s input voltage drop (voltage burden) of hundreds of millivolts can complicate making accurate current measurements. In these cases, engineers typically use electrometers or picoammeters. Electrometers can measure low currents with high accuracy, but the cost of the circuitry needed to measure extremely low currents, combined with functions like voltage, resistance, and charge measurement, often means the price of an electrometer exceeds the project’s hardware budget. Picoammeters, in contrast, combine the economy and ease of use of a DMM with low current sensitivity near that of an electrometer.

Picoammeters are widely used in applications like materials and components characterization, photodiode current measurements, dark current measurements, fiber alignment, SEM beam current measurements, particle and beam monitoring, fluorometer and spectrometer measurements, and general-purpose circuit test and analysis. Picoammeter/voltage source instruments incorporate a voltage source into a picoammeter design to simplify measuring high resistances directly or to simplify applying bias voltages to DUTs. These combination instruments can simplify measuring resistivity, insulation resistance, leakage currents, and photomultiplier currents, as well as device I-V and sensor characterizations.

Dual-channel picoammeters also offer significant advantages for many test setups. Obviously, however, not every application justifies the somewhat higher cost of a dual-channel instrument. Answering a few questions might help determine if a dual-channel picoammeter makes sense for a specific job:
  • Is your test rack or system enclosure already too crowded? By putting the functionality of two picoammeters and two voltage sources in one box, the newest dual-channel picoammeters can not only help reduce the number of instruments involved but simplify system integration. For example, manufacturers of scanning electron microscope (SEM) systems often need to incorporate multiple picoammeters into their systems for beam monitoring. In these cases, a higher density solution not only helps conserve limited space but reduces equipment costs and integration complexity as well.
  • Would it be helpful to be able to compare a reference channel with a sensor channel in the same box? The most common reason users want two measurement channels is that they have a reference or control device (such as a photodiode or some other source of current) and a variable and they want to understand how the two measurements are related. The newest dual-channel picoammeters offer built-in ratio and delta functions, which can be used to calculate the ratio or difference between measurement results from channel 1 and channel 2.
  • Are you testing a multi-pin component, monitoring current levels in multiple locations, analyzing multichannel devices, or recording data from multiple sensors simultaneously? A dual-channel picoammeter will allow for easier control and data aggregation, especially if it includes a separate memory buffer for each channel.
  • Do you need dual-channel true analog outputs? Several of the newest dual-channel picoammeter/source combinations, including Keithley’s Model 6482 (see Figure 1), provide dual 0-10V true analog outputs (i.e., the signal is not reconstituted from a digitizer), which is helpful for applications that require monitoring the analog signal directly in real time, such as from an oscilloscope, where digitization is undesirable. 

Figure 1. The Model 6482 dual-channel picoammeter/voltage source 

Although not every low-current measurement application will require the use of dual-channel picoammeters, their increasing affordability and the growing complexity of today’s test applications can make them a good addition to many engineers’ toolboxes.

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About the Author
Since 2011, Steven Burns has been a product marketer with Keithley Instruments, Inc., headquartered in Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. Prior to joining Keithley, he was a strategic intelligence associate at The Lubrizol Corporation. He holds a Master of Engineering and Management degree from Case Western Reserve University’s Weatherhead School of Management, as well as a Bachelor of Engineering in Biomedical Engineering from the same institution.

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