Turning down the noise: When 'low' keeps getting lower

Wireless/portable electronic products have come to be considered necessities rather than luxuries among both business people and regular consumers. When was the last time you saw a teenager (or a pre-teen, for that matter) without a sophisticated mobile phone? Today’s increasingly tech-savvy consumers constantly demand new, more capable wireless products, forcing consumer products companies to churn out new smartphone and tablet computer designs quickly. All of these wireless products have one thing in common—they’re all designed to run on battery power.

The desire to extend a wireless product’s battery life by reducing power consumption is driving IC manufacturers to develop devices that operate at extremely low current levels. Characterizing the level of current these devices consume requires test engineers to make very low-level current measurements with high accuracy. This is a major challenge because the level of the current can approach the test setup’s noise threshold. Therefore, minimizing system noise is critical to characterizing low-level currents.

Test engineers have a variety of instrument options from which to choose for measuring low currents:

• Digital multimeters (DMMs). Although a low-cost, 3-1/2 digit handheld DMM isn’t an appropriate solution for a low-level measurement, high-precision laboratory DMMs are available that can measure currents as small as ten picoamps.
• Electrometers. An electrometer is a highly refined DC multimeter. As such, it can be used for many measurements performed by a conventional DC multimeter. Additionally, an electrometer’s high input impedance (often one tera-ohm or higher) and high sensitivity allow it to make voltage, current, resistance, and charge measurements far beyond the capabilities of a conventional DMM. Some electrometers, such as Keithley’s Model 6517B Electrometer/High Resistance Meter (pictured above right), can measure currents as low as a device’s input offset current, which can be as little as one femtoamp in some cases.
• Ammeters. Ammeters optimized for measuring very low currents are known as picoammeters. When compared with an electrometer, a picoammeter has a similar low voltage burden, similar or faster speed, less sensitivity, and a lower price. It may also have special characteristics, such as high speed logarithmic response or a built-in voltage source.
• Source Measurement Units (SMUs). Instrument-based SMUs like Keithley’s Series 2600A System SourceMeter instruments (pictured right) combine the functions of precision current and voltage sources with sensitive voltmeters and ammeters. An SMU can simultaneously source, measure, or sink current and voltage. An ultra-sensitive SMU can have current sensitivity down to an astounding low level of ten attoamps.

Minimizing noise
No matter which instrument a test engineer chooses to measure low currents, noise (both internal and external to the instrument) will limit its sensitivity. The device under test (DUT) plays a part in the level of current that a given instrument can detect because the DUT’s source resistance (RS) establishes the level of Johnson current noise (IJ), the low-level noise caused by temperature effects on electrons in a conductor. Johnson noise, which can be expressed in terms of either current or voltage, is essentially the voltage noise of a device divided by the device resistance:

 
where k = Boltzmann’s constant (1.38 × 10–23 J/K),
T = Absolute temperature of the source (in degrees Kelvin),
B = the noise bandwidth (in Hz), and
R_S = the resistance of the source (in ohms).

Both temperature and noise bandwidth affect the Johnson current noise. A reduction in either parameter will also reduce the Johnson current noise. Cryogenic cooling, for example, is often used to reduce noise from the device- or sample-under-test, amplifiers, and other circuits but adds cost and complexity. Reducing the noise bandwidth by filtering will also lower the Johnson current noise, but this will slow the measurement speed.

There are many potential sources of noise current, including the cables used to connect test instruments to each other or to the DUT. When typical coaxial test cables are flexed, they can generate as much as tens of nanoamps of current when the outer shield of a coaxial test cable rubs against the cable’s insulation. When this occurs, electrons are stripped from the insulation and added to the current total. In some applications, the current generated by this triboelectric effect may exceed the level of current to be measured from the DUT.

The noise due to the triboelectric effect can be minimized by using low-noise triaxial cable, with an inner insulator of polyethylene coated with graphite underneath the outer shield. In addition, keep tests cables as short as possible and isolate them from vibration.

Mechanical stress on a DUT can also lead to low-current measurement errors. This piezoelectric effect varies by material. Polytetrafluoroethylene (PTFE) dielectrics, for example, can produce relatively large currents for a given amount of stress and vibration, while ceramic materials produce lower current levels. To minimize the amount of current generated by this effect, minimize mechanical stress on insulators, and construct any low-current test system using insulating materials with minimal piezoelectric properties.

Dirty or contaminated insulators can contribute nanoamp-level noise currents. On an insulator, the contamination forms a low-current battery at a sensitive current node within the insulator. To minimize the potential for this type of error, operators should wear gloves when handling insulators or simply avoid touching them altogether. The use of solder should be minimized, and solder areas should be cleaned with an appropriate solvent, such as isopropyl alcohol.

Finally, stray magnetic fields may generate noise currents. Properly shielding instruments and test fixtures will help prevent false readings.

Obviously, making low-level current measurements can be quite complex. However, through close attention to test system and fixture design, it’s possible to minimize current noise to ensure more accurate measurements. For more tips and techniques on low-current characterization, download a free copy of Keithley’s white paper, Optimizing Low-Current Measurements and Instruments.

Feel free to share any tips or ask questions about your low-level measurement challenges in the comments.
 
About the author:
Robert Green is a Senior Market Development Manager focusing on low-level measurement applications at Keithley Instruments, which is part of the Tektronix test and measurement portfolio. During his 20+-year career at Keithley, Mr. Green has been involved in the definition and introduction of a wide range of products, including picoammeters, electrometers, digital multimeters, and temperature measurement products. He received a B.S. in Electrical Engineering from Cornell University and an M.S. in Electrical Engineering from Washington University, St. Louis, Missouri.

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