How Measurement Setup Affects Accuracy in Femtofarad-Range Devices
ESD protection devices designed to safeguard sensitive high-speed interfaces must meet stringent signal integrity requirements. As bandwidth demand increases, these devices require increasingly lower capacitance, as shown in Figure 1. However, with the rise of high-speed applications like HDMI 2.2 and USB 4, ESD solutions should not only have smaller capacitance but must also be designed with robust signal integrity practices to ensure compliance with stringent performance standards.
Accurate and reliable measurements are vital to verify the compliance of ESD devices with interface standards, and it is critical that the measurements are not distorted by the measurement setup. In this article, we will focus on S-parameter measurements, which are widely used to characterize the high-frequency behaviour of ESD devices. As the capacitance of ESD protection devices continues to shrink into the femto-farad range, the influence of the measurement PCB and its layout on S-parameter results becomes increasingly significant.
Currently, most manufacturers of discrete ESD protection devices measure S-parameters of their devices using test PCBs compatible with 4-port Z-probes, or directly on the device using ACP probes for 2-pin devices. It is well understood that the design and quality of the measurement of a PCB can impact the signal integrity performance of the device under test (DUT). However, in many cases, these measurement PCBs are not specifically optimized for high frequency or RF performance, which can compromise the accuracy of the results.
In this article, we explore the impact of the measurement PCB on S-parameter characterization by examining ESD protection devices with two different capacitance values. We demonstrate how the PCB can affect the accuracy of the high-frequency measurements, particularly as device capacitance enters low femto-farad range. To better predict these effects, we utilize 3D electromagnetic (EM) simulations to model the device along with the measurement PCB. The simulations are validated through direct comparison to measurements, highlighting the importance of simulation-driven design and verification in ensuring accurate characterization of ESD devices.
Understanding Integrated Common Mode Filters
Integrated common mode filters combine an ESD protection device and a common mode filter in a single chip. In the event of an ESD strike, the integrated ESD protection device safely diverts the high-voltage pulse from the signal line to ground, thereby protecting sensitive downstream components. During normal data transfer, the common mode filter attenuates unwanted common mode noise that can degrade signal integrity in differential signalling systems. A simple representation of an integrated common mode filter is shown in Figure 2.
The advantage of using an integrated common mode filter instead of a separate common-mode filter and an ESD protection diode lies in its enhanced performance and compact design. By combining both functionalities into a single chip, these integrated solutions (referred to in this article as protection common-mode filters, or PCMFs) offer superior ESD suppression by reducing the peak voltage of an ESD pulse and provide good suppression against electromagnetic interference (EMI).
Furthermore, integrating both functions into a single package significantly reduces the board space and minimizes package parasitics. This reduction in package parasitics helps lower the inductance and capacitance of the overall device, thereby increasing its differential bandwidth.
PCMF devices are offered in package variants that support one, two, or three differential channels, as illustrated in Figure 3. This range of configurations provides designers with flexible protection options tailored to a wide variety of high-speed interface applications. An example of a PCB used to measure the S-parameters of the PCMFs is shown in Figure 4. This PCB is most ideal to measure the three-channel protection PCMF. But it can also be used for measuring one- and two-channel protection devices.
The measurement PCB is a two-layer board with a top metal layer of 18um and a dielectric layer with a dielectric constant of 9.8 and a dielectric loss factor of 0.01 at 1GHz. When testing a one-channel PCMF device, it can be mounted in either of two positions on the PCB: A1–A2/C1–C2 (referred to as Position 1) or A2–A3/C2–C3 (referred to as Position 2), as illustrated in Figure 5.
When the PCMF is placed in position 1, the differential traces used for measurement are asymmetric, with unequal lengths and inconsistent spacing between them. This asymmetry may cause signal integrity issues that are not immediately apparent through basic analysis.
When we use a standard impedance trace calculator to estimate the impedance of the PCB traces, it may show that the characteristic impedance of the PCB lines is around 50 Ohms, as they typically assume uniform trace lengths and consistent spacing to the ground. But they do not count for the impedance discontinuity when the spacing to the ground plane is varied, which can lead to signal reflections and increased insertion loss, effects that are difficult to estimate from traditional calculations or simple equivalent circuit simulations.
To address this, 3D electromagnetic (EM) simulations offer a powerful and efficient way to model the complete PCB structure, including all geometric and material complexities.
Using 3D EM Simulations to Estimate Device S-Parameters
3D EM models of the PCMFs are created by incorporating the metal structures and dielectric layers of the device, along with the package details accurately modelled with material specifications required for EM simulations. The active region of the ESD diode within the PCMF is modelled as a discrete capacitor. The ESD diode junction capacitance is initially roughly estimated based on device design, then fine-tuned by comparing simulation results to measured data. Once the discrete capacitance value of the ESD diode model is validated, the same ESD diode model can be used for predicting the bandwidth performance of the device across different package configurations.
For these simulations, we utilize a powerful tool for high-frequency 3D EM modelling.[1] To ensure accurate correlation between simulation and measurement, the PCB used for measurement is also included in the simulation environment. This allows for a realistic representation of the measurement setup, including all parasitic effects introduced by the board.
The differential insertion loss of a PCMF device (referred to as PCMFa) is simulated in position 1 and position 2 of the measurement PCB, which are shown in Figure 5. The simulation results are compared to measurements as shown in Figure 6. The correlation between simulation and measurement is quite good up to 30GHz, demonstrating the accuracy and reliability of the modelling approach.
Impact of Measurement PCB on S-Parameter Characterization
The differential S-parameters of a state-of-the-art PCMF, PCMFa used for HDMI and USB standards are measured using the measurement PCB in both position 1 and position 2. The resulting differential insertion loss Sdd21 is presented in Figure 7. From the graph, we can clearly see that the differential insertion loss and the differential bandwidth of the device are highly influenced by the measurement position on the PCB. We can see that the PCB influences the device at high frequencies above 10GHz. When the device is measured in position 2, there is a gain of nearly 7GHz in differential cutoff frequency.
This strong dependence on PCB layout is primarily due to the extremely low capacitance of the ESD diode (on the order of femtofarads) and the small inductance values of the common-mode filter, typically in the nanohenry range. In such cases, the impedance discontinuity introduced in the PCB due to asymmetric traces and varying spacing to ground can have a pronounced impact on the measured performance.
When we measure a different PCMF, PCMFb, which has nearly two times higher inductances of the common mode filters and capacitance of the ESD diodes, the influence of the PCB becomes less significant. As shown in Figure 8, the differential insertion loss for PCMFb exhibits much smaller variation between the two measurement positions. This indicates that the device’s intrinsic electrical characteristics dominate its performance, making it less sensitive to the measurement PCB.
Conclusion
As high-speed digital interfaces continue to evolve, the demand for ESD protection devices that offer both ultra-low capacitance and excellent signal integrity performance has never been greater. This article has highlighted the critical role that accurate S-parameter measurements play in verifying the compliance of these devices with modern interface standards such as HDMI 2.2 and USB4. However, as demonstrated, the accuracy of these measurements is highly dependent on the design and quality of the measurement setup, particularly the PCB used during testing.
Through measurements and 3D EM simulations, we have shown that variations in PCB layout, such as trace asymmetry and inconsistent spacing to the ground plane, can significantly affect the performance of ESD protection devices, especially those with capacitance in the femtofarad range. The use of advanced simulation tools enables designers to model these effects with more accuracy, allowing for better prediction and optimization of device behavior in real-world conditions.
Ultimately, this article underscores the importance of co-designing ESD protection devices and their measurement environments. By combining accurate modelling, careful PCB design, and robust measurement practices, engineers can ensure that ESD protection solutions meet the stringent demands of today’s high-speed digital systems.
EndNotes
- CST Studio Suite by Dassault Systèmes, 05 June 2025.
