Considerations When Switching Low Capacitance Measurements

Überblick

An automated test system (ATS) developed to measure capacitances in the pF range requires planning to address several capacitance measurement challenges and to choose a suitable system configuration. This tutorial will describe those challenges and the methods and equipment that should be used to make accurate low-level capacitance measurements. 

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Contents

Introduction

An automated test system (ATS) used to perform low capacitance measurements will likely consist of the following equipment:

  • DMM or capacitance measurement device
  • Switches to route signals from a test fixture to the measurement device
  • Test fixture for your Devices Under Test (DUTs)
  • Cabling to connect each of the above components

Measurement system requirements such as accuracy, speed, and number of DUTs, dictate the equipment and cabling that can be used within the ATS. As requirements become more stringent, general use-case measurement techniques and switching configurations may no longer deliver adequate performance. The recommended techniques and specialized configurations in this tutorial can then be employed to achieve the desired results.

Basic Considerations

There are many considerations to take into account when planning for a low level capacitance measurement. The following information addresses the most common considerations.

Selecting Hardware

One option for the capacitance measurement device is the NI PXI-4072 (NI 4072). The NI 4072 is a 6½ digit digital multimeter (DMM) that has a built-in LCR meter to measure inductance and capacitance. It has capacitance ranges from 300 pF to 10,000 uF and can measure with a resolution as low as 0.05 pF. For more information on how the NI 4072 performs capacitance and inductance measurements, refer to Capacitance/Inductance Measurements at ni.com/zone.

After selecting a capacitance measurement device, decide how to route the test signals to this device. A cost-effective and standardized approach is to incorporate switching on the front end of your system.

Nearly every ATS application uses some type of switching. Switching allows for customizable configurations, fast connection speeds, and the ability to programmatically connect a high number of channels to a minimal number of test instruments, thereby improving system performance while reducing the cost of an automated test system. For example, with high-density NI PXI and SCXI switch modules, you can automate the connection of one NI 4072 to hundreds of DUTs. The connection speed can be altered to maximize throughput or accuracy, and the configuration can be changed using software to run subsequent tests.

National Instruments provides a variety of flexible, cost-effective switch modules. To see our complete switch offering, visit ni.com/switches.

Switch Topologies

A switch topology is an organizational representation of the channels and relays in a switch module. One or more switch modules configured in a variety of topologies is necessary to provide your system’s required connection points and functionality. For additional information on available topologies and their respective configurations, refer to Understanding Switch Topologies at ni.com/zone or the NI Switches Help.

There are three common topologies used for switching capacitance measurements: MUX, Single-Pole Double-Throw (SPDT), and Matrix. 

The MUX topology is often selected for multiple channel measurement applications, where numerous channels connect one at a time to a measurement device. This topology can be used for larger capacitance measurements. However, with mid- and low-level capacitance measurements, parasitic offsets can significantly affect results, and the MUX topology may no longer be the best option. 

SPDT and Matrix topologies provide system configurations that assist in limiting parasitic offsets when performing mid to low-level capacitance measurements. These topologies and their useful configurations are discussed in the Advanced Considerations section of this tutorial.

Compensation

In most ATS applications the DMM will be connected to the DUT through cables, switches, terminal blocks, and fixtures. These additional components add parasitic capacitance which can cause undesirable error in the measurement. Figure 1 shows a model for the other residuals that may exist in your system. For capacitance measurements, we are mostly concerned with the parasitic capacitance, Cp, but the other residuals may need to be considered, depending on your test system. Cp can easily reach the uF range in a large switching system if certain precautions are not taken into account. 

Figure 1: Model of the Test Fixture Residuals

Compensation can be performed to reduce the error between the DMM and the DUT. Two commonly used methods of compensation are offset nulling and open/short compensation. 

Offset nulling, or Open compensation, is useful if you assume that Rs and Ls are negligible. Offset nulling involves taking two measurements and subtracting them to remove the effect of parallel capacitances (Cp in Figure 1) that exist in your test system. An initial “offset” measurement is taken with the DUT disconnected (open) and a second measurement is done with the DUT connected. Subtracting the initial offset from your DUT measurement produces the more accurate result.

Open/Short compensation is useful if you assume that your system error is a combination of many different test fixture residuals. Open/short compensation incorporates offset nulling, but also measures when the connections next to the DUT are shorted. This short circuit allows Rs and Ls to be measured. For more information on how to perform open and short compensation please refer to Capacitance/Inductance Measurements at ni.com/zone or the NI-DMM Help File.

Note: The NI-DMM driver that is used with the NI 4072 can automatically compensate for all subsequent measurements after taking the open and short measurements.

Noise and Averaging

When taking low capacitance measurements, noise and other environmental factors can result in measurement error. Therefore, every effort should be made to remove or protect your system from noise sources. The NI 4072 has a preset aperture time that it uses during capacitance measurements. If a greater effective resolution is desired, multiple samples can be averaged by modifying the “Number of LC Measurements to Average” property. By averaging more samples together in the driver, you can reduce the effect of noise on your signal.

Use Relays with Low Minimum Current and Low Path Resistance Specifications

The NI 4072 DMM sources a small test current to measure capacitance. Switch modules that have low minimum current specifications should be used to maintain consistent signal integrity. 

Switch modules with low path resistance specifications should be used to keep the impedance between the DMM and DUTs as low as possible. However, keep in mind that higher path resistance switches can be used as long as a short compensation is performed.

Consider Terminal Block Connectivity

Verify that the switches you choose can be connected cleanly to your test fixture. NI switches have several connectivity options, including front-mounting terminal blocks and cables. NI also provides mating connector part numbers so customers and third-party companies can build their own interconnects.

Isolation and cabling

Arrange switch channels in a manner that reduces the amount of capacitance that needs to be compensated by the DMM.  Positioning switch channels in parallel is a simple and often-used configuration, but it results in the summation of parasitic capacitances and a higher probability of measurement error. Instead of placing switch channels in parallel, another configuration option is to arrange the switches in a tree structure to prevent the parasitic capacitances from adding up. This is discussed further in the Advanced Considerations section of this tutorial. 

Isolating the switch channels into different sections also reduces the amount of capacitance that needs to be compensated by the DMM. Since the total capacitance is lower, it could allow you to use a lower range on the DMM, which will increase the resolution of the measurement. When using this technique, it is important to shield different sections of channels from one another.

Shielded cables can be used for all channels to reduce the effect of noise and channel-to-channel capacitance. However, shielded cables have some capacitance between the conductor and the shielding which would increase the amount of channel-to-ground capacitance. This increase in channel-to-ground capacitance creates a larger offset capacitance for the DMM. This trade-off between noise and offset capacitance should be taken into account before using shielded cables for every channel. Shielded cables are also typically larger, which can create a bulkier test fixture.

When cables are stressed, moved around, or rubbed against one another, piezoelectric and triboelectric effects can create potentials that can lead to erroneous readings. Make sure cables are securely positioned using tie-wraps so that they do not move around.

Note: If individual cable diameter is an issue, micro coax cables may be an option for interfacing between the instruments and the test fixture, as it would still provide shielding.

Advanced Considerations with Low-Level Capacitance Measurements

A low capacitance measurement system often requires the utilization of specialized configurations and compensation techniques to minimize the effect of parasitic channel-to-channel capacitance and neighboring DUT capacitances.  

Neighboring Capacitors and the Multiplexer Topology

When capacitive DUTs are connected to multiple channels in a MUX topology, performing Open compensation becomes a more involved procedure when low capacitance measurements are being taken. The MUX topology is not well suited to low capacitance measurements, as will be explained below.

When a single capacitive DUT is measured, Open compensation compensates for the effect of parallel capacitances in the system when the DUT is disconnected from the test fixture. However, when multiple DUTs are connected to a test fixture, Open compensation becomes more complicated because the other DUTs are coupled into the system by parasitic capacitances when their switch connections are open. Therefore, Open compensation needs to be performed by disconnecting only the DUT from the test fixture on the channel being measured (only channel where the switch relay is closed), while keeping all of the other DUTs connected to the test fixture. This can be a difficult process to perform since an Open compensation must be done for every channel.

Figure 2 depicts a system setup where the attempt is to measure one capacitive DUT in a system where multiple DUTs are connected to neighboring channels. Due to the series channel-to-channel parasitic capacitance, CP2 and CP3, every DUT that is not being measured, C2 and C3, is in parallel with the DUT being measured, C1.

Figure 2: Potential Issues with a MUX due to Parasitic Capacitances 

A proper Open compensation for C1 requires C2 and C3 to be connected. This would require the MUX to open channel 1 and close channels 2 and 3.  The MUX topology does not allow such a configuration state, and therefore the topology shouldn’t be used when accurate low-level Open compensation is required.

Although the MUX topology will not provide an accurate low-level Open compensation, SPDT and Matrix topologies will. Additionally, both of these topologies reduce the parallel capacitance caused by the neighboring DUTs. 

Neighboring Capacitors and the SPDT Topology

SPDT relay modules (shown in Figure 3) can be used to remove capacitance offsets caused by parasitic channel-to-channel capacitances. This is done by connecting the normally closed (NC) terminal to a common reference. Since all channels that are not being measured will be connected to NC, it forces 0 V across the capacitor. This removes DUTs that are not being measured from affecting the measurement since they are no longer in series with the channel-to-channel capacitance. The remaining parasitic capacitances can be easily removed from the measurement result by using compensation.

 

Figure 3: Using a SPDT Topology to Reduce the Effect of Parasitic Capacitances

Note: A requirement of using this technique is that the capacitive DUTs must be terminated to the reference potential.

For example, a DMM/switch setup made to measure six 10 pF capacitors using the NI PXI-2566 SPDT switch module was successful in achieving 0.1 pF noise-free resolution. This SPDT system can be expanded to employ multiple NI PXI-2566 modules, each configured to form a 16x1 MUX, and cascade them to create a 256 channel switching system. The more channels that are added the higher the offset capacitance can be. However, if the switch modules are cascaded using a tree structure it will limit the overall offset capacitance and will potentially allow a lower range on the measurement device to be used. The offset capacitance is limited because each branch of the tree is isolated from the other branches.

One disadvantage of using SPDT relays is that the modules are typically less dense so the system size could be larger.

Neighboring Capacitors and the Matrix Topology

Like the SPDT method, a matrix topology (shown in Figure 4) can be used to reduce the error caused by other DUTs connected to the system. This is done also done by connecting a common reference to both ends of DUTs that are not being measured. 

Figure 4: Using a Matrix Topology to Reduce the Effect of Parasitic Capacitances

Some Matrix modules do not allow every channel to be connected because they can only supply enough power to actuate a certain number of relays. If only a subsection of the relays in the matrix can be closed, then the parasitic channel-to-channel capacitances between the desired signal route and the unclosed relays will cause some error, and an inaccurate Open compensation will result. Therefore, consider only Matrix modules that allow every relay to be closed simultaneously (see the Recommendations section below).

Compensation

When compensating for a switching system, keep in mind that all of the channels in your system may not be equal. Therefore, taking one compensation value and using it for every channel may or may not be sufficient depending on your measurement requirements. It may be necessary to compensate each channel individually and store those compensation values in software. Compensation should be performed on a regular basis and as close to the DUT as possible for the best performance.

When measuring a small capacitance through a large system capacitance, compensation can still yield very good results. However, the system capacitance may cause the use of a larger measurement range with the DMM, which reduces the resolution of the final measurement. 

Always attempt to reduce the offset capacitance of the system, and this is especially true with low-level capacitance measurements. If the NI 4072 is used in an ATS to measure a 100 pF DUT, then ideally the 300 pF range should be used to achieve a resolution of 0.05 pF. However, if the capacitance of the switch, cabling, and fixture is 2 nF, then the 10 nF range of the DMM is required to measure and compensate for the offset, and this range allows for a resolution of only 1 pF. In short, reduce the offset capacitance, allow the DMM to measure in a tighter range, and achieve a better resolution.

Recommendations

When setting up a system for low capacitance measurements, follow the guidelines below for best performance. 

Switch Module Recommendation

Depending on the capacitance measurement accuracy requirements, use one of the following switch topologies

  • SPDT switches
  • Matrices that permit simultaneous closure of all relays. 

Multiplexer switch modules should only be used if measuring higher capacitances where the parasitic offsets will not affect the measurement.

SPDT switches can be used to create systems requiring the most stringent resolution requirements since the offset capacitance that needs to be compensated out is typically lower, especially if using a tree structure. A lower offset capacitance typically allows a lower measurement range to be used which has a better resolution. 

Matrix modules are cost-effective for large channel-count systems. Dense matrix modules such as the NI PXI-2529, NI SCXI-1129, or NI PXI-2535 are recommended as these modules permit simultaneous closure of all relays, and multiple modules can be expanded to the appropriate number of rows and columns.

Shielded Cable Recommendation

Shielded cables can create a bulky test fixture. If individual cable diameter is an issue, then consider micro coax cables for a shielded interface between the instruments and the test fixture. Shielding should be used if isolating the switch channels into different sections.

Conclusion

The NI 4072 provides very accurate capacitance measurements at a very affordable price. The flexibility of the PXI platform allows you to choose the switches that best meet your needs. For a low-level capacitance measurement system you should choose either SPDT or matrix switches, which allow a ground connection to the channels that are not being measured. This prevents the DUTs on these channels from causing additional error. 

Unwanted parasitic capacitances exist in every test system, but compensation can be performed to minimize their effects. Proper compensation requires a test fixture and switch topology that is capable of at least performing open compensation. To minimize sources of error due to the test fixture, design secure cables, minimize cable length, use shielding, and isolate different switching portions.

By following the recommendations outlined in this tutorial, you should be able to design and build a system capable of accurately measuring very low capacitances.