Understanding Key Differences between SMUs and PPMUs

Überblick

This white paper highlights the key differences between source measurement units and per pin parametric units to serve as a product selection guide.

Table of Contents

  1. Introduction
  2. Example 1: Testing a TI LM83 Digital Temperature Sensor
  3. Example 2: Controlled Limiting of Current
  4. Conclusion

Introduction

In its most basic form, source measurement units (SMU) and per pin parametric measurement units (PPMU) both have the ability to provide stimulus (or power) to a device under test (DUT). Also, each device is able to then check the response from the DUT with some form of measurement. However despite their similarities, each product is designed with certain features for specific applications in mind. For the sake of discussion, when referring to specifications for an SMU, we will reference the NI PXIe-4143, and when referring to a PPMU, we will reference the NI PXIe-6556. As shown below in Table 1, SMUs and PPMUs have many things in common. The goal of this article is to highlight important differences between SMUs and PPMUs and explain how those differences relate back to DUT capabilities and application requirements.

Table 1: SMU vs. PPMU Product Feature Compare

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Example 1: Testing a TI LM83 Digital Temperature Sensor

In order to highlight the main differences between SMUs and PPMUs, we will use a specific application of testing a chip. For this exercise, we will use a Texas Instruments LM83 Triple-Diode Input and Local Digital Temperature Sensor. Figure 1 below shows the LM83’s connection diagram. We will focus on pins 2, 6 and 10. Pin 2 is Vcc, which is the positive supply voltage input, and takes in a DC voltage between 3.0 and 3.6 V. Pins 10 and 6 are ADD0 and ADD1 respectively which take in the user-set SMBus address inputs in the format of I2C communication.

Figure 1: LM83 Connection Diagram

For pin 2 we would choose an SMU, such as the NI PXIe-4143, for several reasons. First, some power pins may require a very precise source. The NI PXIe-4143 has a large voltage range from -24 to +24 V with a 20 mV resolution; it can also output -150 to +150 mA of current with resolution to 10 pA. Comparatively, the NI PXIe-6556 has a -2 to +7 V voltage range with 122 mV resolution and a -32 to +32 mA current range with 60 pA resolution. In general, SMUs will be able to source a higher current with better resolution compared to a PPMU. For applications that require high precision sourcing, it is generally recommended to use a SMU over a PPMU.

Note: If high accuracy sourcing and measuring are not a concern, it may be possible to use a general purpose programmable power supply such as the NI PXI-4110 for a Vcc pin as well.

Vcc provides power to the entire chip and must be consistently powered and monitored in order to test the other functionality of the LM83. The NI PXIe-4143 has four different channels that are able to constantly source voltage or current and measure voltage or current independently on each channel. Meanwhile the NI PXIe-6556 can provide a steady source of voltage or current, but it has a multiplexed architecture for measuring. Although the main reason for choosing an SMU over a PPMU in this case is for more precise sourcing, the multiplexed measurements of the PPMU may result in increased settling time when measuring large voltage swings across channels.

Lastly, it is often important to understand a DUT’s limitations in how much voltage or current each pin can handle. If too much current is sourced to the DUT, it can result in damage to the device. To prevent this, SMUs and PPMUs will often limit the voltage or current sourced. However, SMUs and PPMUs vary in their ability to limit voltage and current, which will be discussed in detail in the next example.

For pins 6 and 10 (ADD1 and ADD0), we will want to use a PPMU rather than an SMU. The NI PXIe-6556 has 24 high speed channels capable of per pin programmable voltage levels for digital communication. Each line offers per cycle bidirectional control and supports hardware compare. Section 1.5 of the LM83 manual details how the chip operates as a slave on the SMBus and how ADD0 and ADD1 serve as address select pins to set the address bits passed out. Because SMUs do not have the ability to do digital communication, PPMUs are required to test digital pins.

Channel count is another distinguishing factor between SMUs and PPMUs. The NI PXIe-4143 has 4 channels while the NI PXIe-6556 has 24 channels. Because of this, higher channel count applications may benefit from using a single PPMU rather than filling a chassis with six SMUs. However, as mentioned before, some cases require the high accuracy and precision offered by an SMU.

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Example 2: Controlled Limiting of Current

When interfacing with a DUT, it is often important to limit the voltage or current that the device will provide or draw in order to prevent damage to the DUT. There are important differences between the way limit functions work on SMUs and PPMUs. For SMUs, limiting voltage and current is completely programmable. (Note- there are some exceptions to this. Namely, the NI PXI-4130 for SMUs; however, these will not be discussed in the scope of this paper.) On the other hand, PPMUs have the ability to programmatically limit voltage, but current is regulated by the selected current range of the device.

Figure 2 below shows a basic battery test example which we will use to demonstrate the main differences in limiting between a SMU and a PPMU. Everything inside the dotted line represents the device in question. Vsrc is the voltage source; Vmeter is the onboard voltage meter; and Ameter is the onboard current meter. Src Hi and Src Lo are connected to the DUT to supply voltage or current while Sense Hi and Sense Lo are used for 4 wire voltage measurements. Bear in mind that this is not an entirely accurate depiction of a device’s block diagram and is used only for example purposes.

In the simplest case, we can force 5 volts across a 1 kohm DUT which will result in a 5 mA current. We can set a current limit of 9 mA to protect our DUT, and this scenario works fine without any issues.

Figure 2: Limiting

However, if we halve the resistance of our DUT, then the current that flows into the DUT now doubles. Now, the 10 mA current is higher than the 9 mA limit we set to protect our DUT. Because we’ve exceeded our current limit, the SMU will realize something is not right. However, it is not possible to force the current down to 9 mA while simultaneously forcing 5 V across a .5 kohm resistance. Ohm’s Law dictates that if the resistance is set, we only have control of either the voltage or the current. Because of this, the SMU will actually switch to a current source and force 9 mA, which will give us a resulting 4.5 V potential across the DUT as shown below in Figure 3. This will protect our DUT from being damaged even if a higher than anticipated voltage difference forces a large current.

Figure 3: Limiting, SMU

For PPMU, we can set the desired current range we want the device to operate in, but this is not the same as setting a limit. This current range defines the accuracy at which the device will source, but it must be known beforehand as the PPMU will not automatically choose a range. The NI PXIe-6556 has selectable current ranges of ±2 mA, ±8 mA, ±32 mA, ±128 mA, ±512 mA, ±2 mA, ±8 mA and ±32 mA. Setting the current range to the smallest range guarantees a current resolution of 60 pA provided the actual current sourced falls in that range. If the current sourced is outside of the preselected range, then the accuracy is no longer guaranteed and a new range must be manually selected.

As previously mentioned, setting the current range is not the same as setting a limit, and if the DUT requires more current (again, Ohm’s Law dictates we can only control one aspect) then the PPMU will provide it. However, setting the current range does provide a general guideline for how much current the device will source or sink. An example of this is shown below in Figure 4. Note that the actual current provided exceeds the current limit set, but not by too much. The current range will regulate the current provided but will not provide a hard cap. Because of this, it is important to know how much current your DUT can source or sink and to not force a voltage that will result in a current higher than that. If need be, PPMUs do provide fully programmable voltage limiting.

Figure 4: Limiting, PPMU

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Conclusion

To recap, both SMUs and PPMUs can source and sink voltage and current to a DUT. PPMUs have 24 programmable, high speed lines for digital communication, and they have the ability to do parametric characterization of digital pins on an IC. Meanwhile, SMUs can also perform parametric characterization on a broad range of devices. However they are much more precise compared to PPMUs but do not have digital communication ability. A full list of sourcing and measuring specifications for the NI PXIe-4143 and NI PXIe-6556 can be seen below in Table 2 and 3, respectively.

Furthermore, SMUs can limit voltage and current completely programmatically while PPMUs have the ability to limit voltage but only regulate current based on selected ranges. Applications with sensitive DUTs that need to be closely monitored should consider using SMUs as they provide full limiting capabilities.

Table 2: SMU vs. PPMU Sourcing Compare

Table 3: SMU vs. PPMU Measuring Compare

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