PXI Express–based SMUs from NI give you the flexibility to perform both high-speed and high-precision DC measurements by varying the measurement aperture time. A short measurement aperture gives you the ability to use the SMU as a digitizer and sample up to 1.8 MS/s, while an extended measurement aperture gives you the ability to take high-precision measurements with up to 10 fA of current resolution.
NI SMUs can perform high-precision measurements with current sensitivity down to 10 femptoamps (fA). High-precision measurements require an aperture time that gives the SMU analog-to-digital converter sufficient time to integrate the signal and apply noise-rejection filters. The SMU aperture time is often specified in number of power line cycles (NPLCs), and usually defaults to 1 NPLC to filter out power-line induced noise (60 Hz or 50 Hz).
NI SMUs combine high precision and high speed measurements
NI SMUs provide the benefit of low noise measurement performance over a range of aperture times, even at high speeds. The graph below compares the measurement performance between the NI PXIe-4139 system SMU and a traditional box SMU, with both instruments having a 100 fA current sensitivity specification. As the aperture time (NPLC) is increased, the SMU integrates the signal for a longer period of time and reduces the noise through techniques such as averaging and filtering.
Figure 2. Current Measurement Noise (amps) As a Function of Aperture Time (NPLC)
(Refer to the numbered list below for details.)
The data captures the RMS current noise of the two SMUs across various aperture times (0.001, 0.01, 0.1, 1, and 2 NPLCs), using the same 1 A current measurement range. Both instruments were calibrated at the same interval and maintained the same test parameters. The following two scenarios show how NI SMUs’ measurement quality and speed compare to a traditional box SMU.
- Comparing the noise performance at the same measurement aperture
The NI SMU provides between 0.1 to 0.2 μA of noise at 1 NPLC, compared to 10 to 20 μA of measurement noise on the box SMU. This represents about a 100X reduction of noise at the same speed, providing better measurement resolution within the same current range.
- Comparing the measurement speed at the same current noise
With traditional box SMUs, you may need an extended aperture time to meet the noise requirements of your measurement. However, the NI SMU achieves the same amount of noise at 0.005 NPLC as the traditional box SMU achieves at a full 1 NPLC, representing over a 100X improvement in speed. This gives you the ability to maintain the same measurement performance while dramatically reducing the overall test time.
High Speed Sampling and Streaming
NI SMUs have a much higher sampling rate than traditional box SMUs, giving you the ability to use the SMU as a high voltage or current digitizer. Additionally, with fast update rates and a customizable SMU response, NI SMUs give you the ability to step through large sequences very quickly or use the SMU to generate arbitrary waveforms. Direct DMA streaming between the host PC and SMU removes the data transfer bottlenecks associated with traditional bus interfaces such as GPIB and Ethernet, and ensures you can stream large waveforms at the full update rate of the SMU. The table below shows how specific NI SMUs compare to a traditional box SMU.
||NI PXIe-4141/3/5 SMU
||Traditional Box SMU
|Max Sampling Rate
|Max Update Rate
||Slow, Normal, Fast, Custom (SourceAdapt)
||Slow, Normal, Fast, Custom (SourceAdapt)
Table 4. Compare the sampling rate, update rate, and transient response settings for NI SMUs to a traditional box.
This digitizer functionality of NI SMUs is essential for capturing detailed transient characteristics of the SMU response or for characterizing device under test (DUT) behavior such as line and load transients. Without this functionality, you would need an external oscilloscope.
Figure 3. NI SMUs provide higher sampling rates than traditional box SMUs.
The figure above shows an SMU generating a current pulse and sampling the output at 20 kS/s and 1.8 MS/s. The 1.8 MS/s sampling rate of the NI SMU gives you the ability to see the detailed transient characteristics of the pulse, and ensure the response has no overshoots or oscillations.