The speed of data transmission has increased significantly over the last two decades. At the same time, signal levels have decreased. Distinguishing between a “1” and a “0” on single lines has become increasingly difficult. To overcome this detection problem, the concept of differential signaling was introduced.
Differential signaling uses two data lines to transmit the same information in the form of low/high signals, but with a 180° phase shift between them, as shown in Figure 1.
At the receiver side, the voltage difference (delta voltage) between D+ and D− is detected. This makes the system less sensitive to signal attenuation.
A typical setup for differential data transmission is shown in Figure 2.
Data transmission is a time-domain-based process. It involves feeding your device under test (DUT) with transient data, detecting the output signal in the time domain, and overlaying a 1-0 transition to evaluate signal quality using a so-called Eye Diagram (see ESD Handbook 6.5.2 – Physical Layer Test).
It is already a challenge to set up a compliant test environment for external protection devices for a single standard. There are, of course, multiple standards.
To simplify the measurement process, you can use Fourier transformation to characterize a wide range of standards by measuring in the frequency domain with a vector network analyzer (VNA). Fourier transformation can be applied to single-ended lines, but it is even more important for differential data lines due to their higher speeds and the greater number of differential standards.
To analyze a system’s differential capability, it is necessary to measure both lines of the differential pair simultaneously. This requires a 4-port S-parameter measurement. Such an S-parameter setup can be implemented in a standard single-ended configuration, resulting in a 4×4 matrix that includes insertion loss, return loss, and crosstalk information.
Figure 3 shows a 4-port simulation setup for differential channels (IO_1 for the D+ line and IO_2 for the D− line) used in a single-ended simulation.
From this single-ended S-parameter, you can easily calculate the mixed-mode S-parameter, which also includes differential mode insertion loss and differential mode return loss. Сrosstalk is not part of this mixed-mode s-parameter set, but mode conversions, as well as common mode insertion loss and common mode return loss, are additional pieces of information that can be derived from it. More details can be found in the ESD Application Handbook, Chapter 6.1.2.
For the benefits of the FCLGA (flip-chip land grid array) package, we will focus on:
- Differential mode behavior for transmission, which is the main parameter used to assess the feasibility of differential data transmission:
SDD21=0.5⋅(S2,1−S2,3−S4,1+S4,3)SDD21=0.5⋅(S2,1−S2,3−S4,1+S4,3)
- Mode conversion from differential mode to common mode during transmission. This value indicates how much of the injected differential signal is lost due to unwanted conversion into common mode, typically caused by asymmetry:
SCD21=0.5⋅(S2,1−S2,3+S4,1−S4,3)SCD21=0.5⋅(S2,1−S2,3+S4,1−S4,3)
- Differential mode reflection, which indicates losses due to reflection:
SDD11=0.5⋅(S1,1−S1,3−S3,1+S3,3)SDD11=0.5⋅(S1,1−S1,3−S3,1+S3,3)
Comparison of Multi-Pin Packages
As a traditional high-speed capable package, we selected the DFN2510D-10 (SOT1176) and compared it to the latest multi-pin package in FCLGA, the DFN1006LD-3. The DFN2510D-10 is a 10-pin package typically used for ESD protection devices, utilizing 6 pins. Two of these are GND connections, and the remaining four pins contain protection devices for two differential channels.
In contrast, the DFN1006LD-3 protects only one differential channel. Therefore, we compare one differential channel of the DFN2510D-10 with the available channel of the DFN1006LD-3.
Differential Mode - transmission
The differential bandwidth of the PESD5VH2BFG-Q, which is built using the new FCLGA technology, is 17.4 GHz, whereas the classical product offers only 8.2 GHz. This means the bandwidth has increased by more than 100%.
In the USB 3.2 standard, the data rate is 10 Gbit/s, which corresponds to a fundamental frequency of 5 GHz and a third harmonic of 15 GHz. The use of the FCLGA-based product provides significantly more bandwidth margin for other system elements within the application.
Differential Mode to Common Mode Conversion
In addition, the new package technology offers significantly lower loss due to differential mode to common mode conversion. At 1 GHz, the DFN package already shows higher conversion, with a magnitude of around −65 dB, which is 10 dB higher than that of the FCLGA package. At 4 GHz, the situation worsens for the DFN package, and by 10 GHz, the magnitude of the classical DFN package reaches −13 dB, whereas the FCLGA product maintains a much lower level of −53 dB.
In the USB 3.2 standard, the conversion at the second harmonic frequency is already four orders of magnitude higher.
Differential Mode Reflection
Between 100 MHz and 10 GHz, the classical lead frame-based type exhibits 4 dB higher reflection. This results in a maximum reflection value of −3.6 dB at 12 GHz for the lead frame-based type, compared to −4 dB at 21 GHz for the FCLGA package. This means the peak reflection occurs 9 GHz later which is highly beneficial for high-speed data transmission.
Data Summary
Conclusion
It has been demonstrated that the new FCLGA technology offers significant advantages for differential data transmission compared to classical DFN packages. These benefits have been shown in terms of:
- Differential Mode Bandwidth
- Differential Mode to Common Mode Conversion
- Differential Mode Reflection
