This article studies the influence of IC output drive strength on electromagnetic interference (EMI) by performing post-layout simulation of signal integrity and EMI using Mentor Hyperlynx.
It is crucial to understand the influence of signal integrity, in terms of output signal drive strength of an IC’s transmitter on electromagnetic interference (EMI). A PCB with poor signal quality, e.g., signal with overshoot, leads to the problem of EMI. The subsequent section of this article demonstrates the experimental work that shows the relationship between transmitted signal drive strength and EMI on PCB layout design.
Post-layout simulations and results of signal integrity and EMI
The post-layout simulation of signal integrity using Mentor Hyperlynx is carried out on the signal with the black color highlight depicted in Fig 1. The square wave signal, with 3.3 Vpp amplitude and 100 MHz fundamental frequency, is transmitted from an IC on the left and received by an IC on the right. Two test cases listed in Table I are carried out to investigate the impact of IC output drive strength on signal integrity and EMI. In test case 1, the IC drives the signal at 16 mA with fast slew. Meanwhile in test case 2, the IC drives the signal at 8 mA with fast slew (i.e., reduced drive strength).
Table I. Test cases to study the influence of IC output drive strength on signal integrity and EMI
With reference to signal waveform probed at the receiver IC in time domain depicted in Fig. 2a (i.e., voltage in V versus time in ns), the overshoot at rising/falling edges is trimmed after the drive strength is reduced from 16 mA to 8 mA. With 8 mA drive strength, the signal at the receiver IC still meets the required specifications, i.e., logic low threshold at 0.6 V, logic high threshold at 2.5 V, monotonic edges, minimum pulse width, and absolute maximum and minimum voltage levels.
Subsequently, the signal waveforms shown in Fig. 2a are imported to LTspice to carry out fast Fourier transform (FFT) and the plots in frequency domain (i.e., power spectrum in dB at y-axis versus frequency in Hz at x-axis) are depicted in Fig. 2b. With reference to Fig. 2b, the 2nd, 3rd, 5th and other higher harmonics (i.e., 200 MHz, 300 MHz, 500 MHz, …) have lower power spectrum (i.e., ~ 5 dB lower) after drive strength is reduced from 16 mA to 8 mA.
Next, the simulated EMI at 3-meter distance using Mentor Hyperlynx is conducted to compare the radiated noise level of the signal transmission with drive strength of 16 mA versus 8 mA. The simulated EMI profiles in Fig. 3a (i.e., with 16 mA drive) and 3b (i.e., with 8 mA drive) correlates with the FFT plots in Fig. 2b. The signal transmission with drive strength of 8 mA exhibits lower magnitude of EMI for all the harmonics versus the drive strength of 16 mA, by at least 3 dB. Theoretically, a PCB trace connecting the transmitting IC and receiving IC acts as an antenna. Once the signal that propagates from output buffer of transmitter reaches the receiving end, some energy of the signal is radiated to air. This is due to the fact that the input buffer of receiving IC has very large resistance (i.e., kilo-ohm range and beyond). This makes the unterminated end of PCB trace an antenna tip. Hence, it is crucial to adjust the signal drive strength not only to meet the specification in the aspect of signal integrity, but also to minimize the level of EMI.
The impact of signal drive strength on signal integrity and EMI is proven in this experimental work. It is critical that the IC output buffer be set optimally to ensure the quality, robustness and functionality of the PCB layout design.