Common pulsed measurement challenges are defined.
In case you missed it, Part 1 is available here.
BY DAVID WYBAN, Keithley Instruments, a Tektronix Company, Solon, Ohio
For SMU and PMU users, an issue that sometimes arises when making transient pulse measurements is the presence of “humps” (FIGURE 1) in the captured current waveform at the rising and falling edges of the voltage pulse. These humps are caused by capacitances in the system originating from the cabling, the test fixture, the instrument, and even the device itself. When the voltage being output is changed, the stray capacitances in the system must be either charged or discharged and the charge current for this either flows out of or back into the instrument. SMUs and PMUs measure current at the instrument, not at the DUT, so the instrument measures these current flows while a scope probe at the device does not.
This phenomenon is seen most often when the change in voltage is large or happens rapidly and the current through the device itself is low. The higher the voltage of the pulse or the faster the rising and falling edges, the larger the current humps will be. For SMUs with rise times in the tens of microseconds, these humps are usually only seen when the voltages are hundreds or even thousands of volts and the current through the device is only tens of microamps or less. However, for PMUs where the rise times are often less than 1μs, these humps can become noticeable on pulses of only a couple of volts, even when the current through the device is as high as several milliamps.
Although these humps in the current waveform may seem like a big problem, they are easy to eliminate. The humps are the result of the current being measured at the high side of the device where the voltage is changing. Adding a second SMU or PMU at the low side of the device to measure current will make these humps go away because at the low side of the device the voltage does not change so there’s no charge or discharge currents flowing and the current measured at the instrument will match the current at the device. If this isn’t an option, this problem can be minimized by reducing the stray capacitance in the system by reducing the length of the cables. Shorter cables equal less stray capacitance, which reduces the size of the humps in the current waveform.
The next common pulse measurement issue is test lead resistance. As test currents get higher, the impact of this resistance becomes increasingly significant. FIGURE 2 shows an SMU that is performing a pulse I-V measurement at 2V across a 50mΩ load. Based on Ohm’s Law, one might expect to measure a current through the device of 40A, but when the test is actually performed, the level of current measured is only 20A. That “missing” 20A is the result of test lead resis- tance. In fact, we were not pulsing 2V into 50mΩ but into 100mΩ instead, with 25mΩper test lead. With 50mΩ of lead resistance, half of the output voltage sourced was dropped in the test leads and only half of it ever reached the device.
To characterize the device correctly, it’s essential to know not only the current through the device but the actual voltage at the device. On SMUs this is done by using remote voltage sensing. Using a second set of test leads allows the instrument to sense the voltage directly at the device; because almost no current flows through these leads, the voltage fed back to the instrument will match the voltage at the device. Also, because these leads feed the voltage at the device directly back into the SMU’s feedback loop, the SMU can compensate for the voltage drop across the test leads by outputting a higher voltage at its output terminals.
Although SMUs can use remote sensing to compensate for voltage drops in the test leads, there is a limit to how much drop it can compensate for. For most SMUs, this maximum drop is about 3V/lead. If the voltage drop per lead reaches or exceeds this limit, strange things can start happening. The first thing is that the rise and fall times of the voltage pulse slow down, significantly increasing the time required to make a settled measurement. Given enough time for the pulse to settle, the voltage measurements may come back as the expected value, but the measured current will be lower than expected because the SMU is actually sourcing a lower voltage at the DUT than the level that it is programmed to source.
If you exceed the source-sense lead drop while sourcing current, a slightly different set of strange behaviors may occur. The current measurement will come back as the expected value and will be correct because current is measured internally and this measurement is not affected by lead drop, but the voltage reading will be higher than expected. In transient pulse measurements, you may even see the point at which the source-sense lead drop limit was exceeded as the measured voltage suddenly starts increasing again after it appeared to be settling.
These strange behaviors can be difficult to detect in the measured data if you do not know what voltage to expect from your device. Therefore, inspecting your pulse waveforms fully when validating your test system is essential.
Minimizing test lead resistance is essential to ensuring quality pulse measurements. There are two ways to do this:
Minimize the length of the test leads. Wire resistance increases at a rate that’s directly proportional to the length of the wire. Doubling the wire’s length doubles the resis- tance. Keeping leads lengths no greater than 3 meters is highly recommended for high current pulse applications.
Use wire of the appropriate diameter or gauge for the current being delivered. The resistance of a wire is also directly proportional to the cross sectional area of the wire. Increasing the diameter, or reducing the gauge, of the wire increases this area and reduces the resistance. For pulse applications up to 50A, a wire gauge of no greater than 12 AWG is recommended; for applications up to 100A, it’s best to use no greater than 10 gauge.
Excessive test lead inductance is another common issue. In DC measurements, test lead inductance is rarely considered because it has little effect on the measurements. However, in pulse measurements, lead inductance has a huge effect and can play havoc with a system’s ability to take quality measurements.
Humps in the voltage waveform of transient pulse measurements (FIGURE 3) are a common problem when generating current pulses. Just as with humps in the current waveforms, these humps can be seen in the data from the instrument but are nowhere to be seen when measured at the device with an oscilloscope. These humps are the result of the additional voltage seen at the instrument due to inductance in the cabling between the instrument and th
Equation 1 describes the relation between inductance and voltage. With this equation, we can see that for a given change in current over change in time (di over dt), the larger the inductance L is, the larger the resulting voltage will be. This equation also tells us that for a fixed inductance L, the larger the change in current or the smaller the change in time, the larger the resulting voltage will be. This means that the larger the pulse and or the faster the rise and falls times, the bigger the voltage humps will be.
To remedy this problem, instruments like SMUs offer remote voltage sensing, allowing them to measure around this lead inductance and measure the voltage directly at the device. However, as with excessive lead resistance, excessive lead inductance can also cause a problem for SMUs. If the inductance is large enough and causes the source-sense lead drop to exceed the SMU’s limit, transient pulse measurement data will have voltage measurement errors on the rising and falling edges similar to the ones seen when lead resistance is too large. Pulse I-V measurements are generally unaffected by lead inductance because the measurements are taken during the flat portion of the pulse where the current is not changing. However, excessive lead inductance will slow the rising and falling edges of voltage pulses and may cause ringing on current pulses, thereby requiring larger pulse widths to make a good settled pulse I-V measurement.
Although SMUs are able to compensate for some lead inductance, PMUs have no compensation features, so the effects of inductance must be dealt with directly, such as by:
- Reducing the size of the change in current by reducing the magnitude of the pulse.
- Increasing the length of the transition times by increasing the rise and fall times.
- Reducing the inductance in the test leads
Depending on the application or even the instrument, the first two measures are usually infeasible, which leaves reducing the inductance in the test leads. The amount of inductance in a set of test leads is proportionate to the loop area between the HI and LO leads. So, in order to reduce the inductance in the leads and therefore reduce the size of the humps, we must reduce the loop area, which is easily done by simply twisting the leads together to create a twisted pair or by using coaxial cable. Loop area can be reduced further by simply reducing the length of the cable.