Consider the guard amplifier for making more accurate sub-µA current measurements with your DC source

As is the case with many sourcing and measurement challenges, when attempting to measure extreme values of most anything, factors that you can be blissfully unaware of, because they normally have an inconsequential impact on results, can become a dominant error to deal with. One example of this is when trying to make good low level leakage current measurements on devices and components and “phantom” leakages exceed that of the device you are attempting to test.

When measuring leakage currents of around a µA and lower, it is important to pay attention to your test set up as it is fairly easy to have leakage currents paths in the set up itself that range from adding error to totally obscuring the leakage current of the DUT itself you are trying to test. These leakage current paths can be modeled as a high value resistor in parallel to the DUT, as shown in Figure 1.

Figure 1: Leakage current path in DUT test fixture

• Many things can cause leakage currents on the fixture contributing to leakage current measurement error of the DUT:
• Is the PC fixture board made from appropriate high impedance material?
• Is the PC board truly clean?
• Was de-ionized water used to clean the PC board?
• If already in service for quite some time, have contaminants slowly built up over time?
• Any components associated with the connection path to the DUT are, or have become, unexpectedly leaky?
• Any standoffs and insulators associated with the connection path to the DUT are, or have become, unexpectedly leaky?

Even with all the above items in check there are still times when more needs to be done to further reduce leakage current inherent in the test set up. To help in this regard a guard amplifier is often added on high performance source-measure units (SMUs) to mitigate errors introduced from leakage current paths in the test set up. The Agilent N678xA and the B2900 series are examples of SMUs that include guard amplifiers. Application of a guard amplifier is illustrated in Figure 2.

Figure 2: Guard amplifier in a leakage current test set up

The guard amplifier is a unity gain buffer connected to the output of the SMU to provide a voltage that matches the SMU voltage. The guard amplifier can typically furnish 100’s of µA or more to offset any leakage currents. The test set up needs to be designed to incorporate a guard, which is a conductive path that surrounds, but is not connected to, the SMU’s output path. The guard and guard amplifier do not eliminate any leakage paths. Rather they “intercept” and furnish the leakage current. Because the guard surrounding the SMU output path maintains its potential at that of the SMU’s output potential, the net difference is zero. Because the potential difference is zero no current “leaks” from the SMU output to the guard. The only current now flowing from the SMU output is that which is flowing into the DUT itself. This is just one more tool to get accurate results when making measurements at an extreme value; in this case when making extremely low leakage currents!

Establishing Measurement Integration Time for Leakage Currents

The proliferation of mobile wireless devices drives a corresponding demand for components going into these devices. A key attribute of these components is the need to have low levels of leakage current during off and standby mode operation, to extend the battery run-time of the host device. I brought up the importance of making accurate leakage currents quickly in an earlier posting “Pay Attention to the Impact of the Bypass Capacitor on Leakage Current Value and Test Time”(click here to review). Another key aspect about making accurate leakage currents quickly is establishing the proper minimum required measurement integration time. I will go into factors that govern establishing this time here.

Assuming the leakage current being drawn by the DUT, as well as any bypass capacitors on the fixture, have fully stabilized, the key thing with selecting the correct measurement integration time is getting an acceptable level of measurement repeatability. Some experimentation is useful in determining the minimum required amount of time. The primary problem with leakage current measurement is one of AC noise sources present in the test set up. With DC leakage current being just a few micro amps or less these noises are significant. Higher level currents can be usually measured much more quickly as the AC noises are relatively negligible in comparison. There are a variety of potential noise sources, including radiated and conducted from external sources, including the AC line, and internal noise sources, such as the AC ripple voltage from the DC source’s output. This is illustrated in Figure 1 below. Noise currents directly add to the DC leakage current while noise voltages become corresponding noise currents related by the DUT and test fixture load impedance.

Figure 1: Some noise sources affecting DUT current measurement time

Using a longer measurement time integrates out the peak-to-peak random deviations in the DC leakage current to provide a consistently more repeatable DC measurement result, but at the expense of increasing overall device test time. Measurement repeatability should be based on a statistical confidence level, which I will do into more detail further on. Using a measurement integration time of exactly one power line cycle (1 PLC) of 20 milliseconds (for 50 Hz) or 16.7 milliseconds (for 60 Hz) cancels out AC line frequency noises. Many times a default time of 100 milliseconds is used as it is an integer multiple of both 20 and 16.7 milliseconds. This is fine if overall DUT test time is relatively long but generally not acceptable when total test time is just a couple of seconds, as is the case with most components. As a minimum, setting the measurement integration time to 1 PLC is usually the prudent thing to do when short overall DUT test time is paramount.

Reducing leakage current test time below 1 PLC means reducing any AC line frequency noises to a sufficiently low level such that they are relatively negligible compared to higher frequency noises, like possibly the DC source’s wideband output ripple noise voltage and current. Proper grounding, shielding, and cancellation techniques can greatly reduce noise pickup. Paying attention to the choice and size of bypass capacitors used on the test fixture is also important. A larger-than-necessary bypass capacitor can increase measured noise current when the measuring is taking place before the capacitor, which is many times the case. Establishing the requirement minimum integration time is done by setting a setting an acceptable statistical confidence level and then running a trial with a large number of measurements plotted in a histogram to assure that they fall within this confidence level for a given measurement integration time. If they did not then the measurement integration time would need to be increased. As an example I ran a series of trials to determine what the acceptable minimum required integration time was for achieving 10% repeatability with 95% confidence for a 2 micro amp leakage current. AC line noises were relatively negligible. As shown in Figure 2, when a large series of measurements were taken and plotted in a histogram, 95% of the values fell within +/- 9.5% of the mean for a measurement integration time of 1.06 milliseconds.

Figure 2: 2 Leakage current measurement repeatability histogram example

Leakage current measurements by nature take longer to measure due to their extremely low levels. Careful attention to minimizing noise and establishing the minimum required measurement integration time contributes toward improving the test throughput of components that take just seconds to test.

Pay Attention to the Impact of the Bypass Capacitor on Leakage Current Value and Test Time

It is no secret there is big demand for all kinds of wireless battery powered devices and, likewise, the components that go into these devices. These devices and their components need to be very efficient in order to get the most operating and standby time out of the limited amount of power they have available from the battery. Off-mode and leakage currents of these devices and components need to be kept to a minimum as an important part of maximizing battery run and standby time. Levels are typically in the range of tens of microamps for devices and just a microamp or less for a component. Off-mode and leakage currents are routinely tested in production to assure they meet specified requirements. The markets for wireless battery powered devices and their components are intensely competitive. Test times need to be kept to a minimum, especially for the components. It turns out the choice of the input power bypass capacitor being used, either within the DUT on the DUT’s test fixture, can have a large impact on the leakage current value and especially the test time for making an accurate leakage current measurement.

Good things come in small packages?
A lot has been done to provide greater capacitance in smaller packages for ceramic and electrolytic capacitors, for use in bypass applications. It is worth noting that electrolytic and ceramic capacitors exhibit appreciable dielectric absorption, or DA. This is a non-linear behavior causing the capacitor to have a large time-dependent charge or discharge factor, when a voltage or short is applied. It is usually modeled as a number of different value series R-C pairs connected in parallel with the main capacitor. This causes the capacitor to take considerable time to reach its final steady state near-zero current when a voltage is applied or changed. When trying to test the true leakage current on a DUT it may be necessary to wait until the current on any bypass capacitors has reached steady state before a current measurement is taken. Depending on the test time and capacitor being used this could result in an unacceptably long wait time.

So how do they compare?
In Figure 1 I captured the time-dependent current response waveform for a 5.1 megohm resistor, a 5.1 megohm resistor in parallel with 100 microfarad electrolytic capacitor, and finally a 5.1 megohm resistor in parallel with 100 microfarad film capacitor, when a 5 volt step stimulus was applied.

Figure 1: Current response of different R-C loads to 5 volt step

The 5.1 megohm resistor (i.e. “no capacitor”) serves as a base line to compare the affect the two different bypass capacitors have on leakage current measurement. The film capacitor has relatively ideal electrical characteristics in comparison to an equivalent electrolytic or ceramic capacitor. It settles down to near steady state conditions within 0.5 to 1 second. At 3 to 3.5 seconds out (marker placement in Figure 1) the film capacitor is contributing a fairly negligible 42 nanoamps of additional leakage. In comparison the electrolytic capacitor current is still four times as great as the resistor current and nowhere near being settled out. If you ever wondered why audio equipment producers insist on high performance film capacitors in critical applications, DA is one of those reasons!

So how long did it take for the electrolytic capacitor to reach steady state? I set up a longer term capture in Figure 2 for the electrolytic capacitor. After about a whopping 40 seconds later it seemed to be fully settled out, but still contributing a substantial 893 nanoamps of additional steady state leakage current.

Figure 2: 100 microfarad electrolytic capacitor settling time

Where do I go from here?
So what should one do when needing to test leakage current? When testing a wireless device be aware of what kind and value of bypass capacitor has been incorporated into it. Most likely it is a ceramic capacitor nowadays. Film capacitors are too large and cost prohibitive here. Find out how long it takes to settle to its steady state value. Also, off-state current measurements are generally left until the end of the testing to not waste time waiting for the capacitor to reach steady state. If testing a component, if a bypass capacitor is being used on the test fixture, consider using a film capacitor. With test times of just seconds and microamp level leakage currents the wrong bypass capacitor can be a huge problem!