Architecture Details of the NI PXI-4132 Precision SMU

Table of Contents

1. Precision Sources from National Instruments
2. Analog Architecture of the PXI-4132 Precision SMU
3. Programmatic Hardware Control

Precision Sources from National Instruments

The NI PXI-4132 offers the best sensitivity out of all the NI precision DC sources. Compared to the NI PXI-4110 programmable power supply and the NI PXI-4130 power SMU, the PXI-4132 offers increased voltage range and sensitivity, improved current measurement resolution and accuracy, and integrated hardware sequencing/triggering.

 

Table 1. NI Precision DC Source Comparison

>>See Also: What Is a Source Measure Unit (SMU)?

Regarding output capabilities, the PXI-4132 has a single, isolated SMU channel that provides a four-quadrant, ±100 V output incorporating remote (four-wire) sense as well as external guarding. This channel is capable of sourcing and sinking up to 2 W maximum, as shown in Table 1. It can operate at up to 100 V at up to 20 mA, up to 40 V at up to 50 mA, and up to 20 V at up to 100 mA (see Figure 1).

Figure 1. NI PXI-4132 Precision SMU Output Capabilities

Analog Architecture of the PXI-4132 Precision SMU

In a dual control loop where voltage and current work together through the power amplifier stage, the PXI-4132 can operate in either constant voltage mode or constant current mode. In constant voltage mode, the PXI-4132 acts as a precision voltage source, and, regardless of the load, the voltage across the output terminals is held constant at the programmed value up to the programmed current limit. In constant current mode, the PXI-4132 acts as a precision current source, and, regardless of the output voltage, the current through the load is held constant at the programmed value up to the programmed voltage limit.

Figure 2. Simplified Block Diagram – PXI-4132 Precision SMU Analog Front End

A measurement circuit on the PXI-4132 can simultaneously read the voltage and current values present at the output terminals. These measurements are performed by two sigma-delta analog-to-digital converters (ADCs) that are synchronized at all times. You can use an integrated auto zero feature on the PXI-4132 to improve the measurement quality. Additionally, you can use Sense terminals when remote sense is enabled (constant voltage mode only) to compensate for current-resistance loss drops due to cables and switches. The PXI-4132 also features Guard terminals on the output connector. You can use Guard terminals to implement guarding techniques in cabling and test fixtures. As a result, this output is ideal for leakage test and other precision current measurements. 

Optimizing Measurement Speed and Resolution

The PXI-4132 uses integrating ADCs for sampling voltage and current. The PXI-4132 measurement circuitry operates in one of the three active states:

• Signal conversion — During signal conversion, the PXI-4132 samples the input signal for the programmed aperture time of the device.
• Zero conversion — When auto zero is enabled, the PXI-4132 samples with the signal disconnected and uses that to compensate for internal offsets. Zero conversion has the same aperture as the signal conversion.
• Settling time — During settling time, the analog circuitry of the device settles before the next measurement state occurs.

Figure 3 illustrates the sequence of these states for one sample measurement.

Figure 3. PXI-4132 Precision SMU Measurement Cycle

You can configure measurement options such as aperture time and auto zero on the PXI-4132 to achieve a desired accuracy and/or speed. This precision SMU supports discrete aperture times that are based on the configured power line frequency of the device. You can configure the programmable aperture time to optimize both measurement speed and quality for your application. Figure 4 shows a graph of typical measurement noise as a function of aperture time for any given range. For instance, at 1 PLC, current measurements have a sensitivity of roughly 1 ppm of the range – for the 10 μA range, this corresponds to 10 pA. The PXI-4132 was designed to optimize sensitivity at a variety of measurement rates (PLC settings). For instance, on the 10 μA current measurement range, you can typically achieve 100 pA sensitivity at up to 1,000 readings per second.

Figure 4. NI PXI-4132 Precision SMU Measurement Noise as a Function of NPLC

Table 2 lists valid configurable aperture times for the PXI-4132 and the typical noise-free current measurement resolution based on the current range setting. Note that actual source measure rates may vary slightly from the values listed in the table based on the value of the settling time property.

Table 2. NI PXI-4132 Precision SMU Valid Aperture Times and Typical Resolutions

Transient Effects on Measurement Speed

In addition to choosing the right aperture time to achieve the desired speed and accuracy combination for a particular application, it is important to consider the effects of both settling time and transient response on the accuracy of measurements.

Settling time specifies the time required for an output channel to reach a stable mode of operation. The typical settling time of the PXI-4132 is 300 µS to be settled to within 0.1 percent of the voltage output based on a 1 V step and a load of 50 percent of the current range setting. In addition to settling time at a particular load current, the step response of the output is a key way to determine the speed of the output under various loads. Figure 5 shows a graph of the step response of the PXI-4132 on the 10 µA, 100 µA, and 1 mA ranges.

Figure 5. Typical Step Response for 10 µA, 100 µA, and 1 mA Ranges on the PXI-4132 Precision SMU

As shown in Figure 5, the PXI-4132 has a programmable aperture time that you can set according to the desired output accuracy in a given application. For high-speed production test environments, an output settling percentage of 1 percent is often more than acceptable, but for characterization applications, a settling percentage of 0.1 percent or less may be required.  

Programmatic Hardware Control

You can use the NI-DCPower software test panel to quickly troubleshoot or debug SMU operation interactively. For fast programming, you can take advantage of the DCPower Express VI for an intuitive, configuration-based method of controlling NI SMUs in the NI LabVIEW graphical development environment. For low-level control of the SMU hardware, the IVI-compliant NI-DCPower instrument driver provides a complete API that exposes the full functionality of the hardware in an intuitive hierarchy. NI-DCPower also includes prewritten example programs that demonstrate concepts ranging from simple configuration to advanced sweeping and monitoring.

 

Figure 6. NI-DCPower Soft Front Panel Showing Three Outputs on the PXI-4110 Programmable Power Supply

The new PXI-4132 features a high-speed sequencing engine that you can use to synchronize multiple PXI-4132 SMUs or to synchronize to a single PXI-4132 with other instruments such as switches and high-speed digital devices. As shown in Figure 7, you can send and receive events and triggers across the PXI backplane, simplifying both programming and system wiring. Using this capability, you can easily perform accurate current and voltage sweeps for high-speed I-V characterization. The sequence engine of the PXI-4132 enables precise control over the sourcing functionality of the device and helps you achieve fast, deterministic timing. Additionally, you can use sequencing with the measurement functionality of a device to achieve specific measurement timing.

Figure 7. LabVIEW Programming Flow for Executing a Triggered Voltage Sweep on the PXI-4132 Precision SMU

A sequence is a collection of setpoints that is executed one after another. A setpoint is a single output setting for the device. When running a sequence, you can apply output values in succession (one setpoint followed by its corresponding source delay, immediately proceeded by the next setpoint) or you can use a trigger to configure the output at a precise time.

Figure 8. PXI-4132 Precision SMU Sequence Engine Diagram

Additional options allow for multiple executions of the sequence, synchronizing the engine at various points within its execution, and other advanced configurations. As such, the timing and triggering circuitry of the PXI-4132 enables precise control of the sourcing and measuring operations and synchronization with other devices.  Applications that can benefit from this sequence engine include high-speed discrete component test and transistor testing. 

Conclusion

With measurement resolution down to 10 pA and integrated guarding, the PXI-4132 precision source is ideal for high-accuracy leakage measurements on integrated circuits, discrete components, PCBs, and cables. You can also perform high-speed I-V measurements on a variety of components including diodes and organic LEDs using the onboard hardware sequencing engine. In addition, you can synchronize multiple PXI-4132 SMUs via the PXI backplane to provide high-speed I-V measurements on transistors and more complex devices. For parallel test applications, you can use up to 17 PXI-4132 modules in a single PXI chassis for 17 high-precision SMU channels in a 19 in., 4U space.

Resources

PXI-4132 Precision SMU Model Page

Transistor Test with the PXI-4130 Power SMU

NI Precision DC Sources Home