Si8440/1/2

Rev. 0.3

17

4.4. RF Immunity and Common Mode Transient Immunity

The Si8440 family has very high common mode transient immunity while transmitting data. This is typically

measured by applying a square pulse with very fast rise/fall times between the isolated grounds. Measurements

show no failures up to 30 kV/µs. During a high surge event the output may glitch low for up to 20–30 ns, but the

output corrects immediately after the surge event.

The Si844x family passes the industrial requirements of CISPR24 for RF immunity of 3 V/m using an unshielded

evaluation board. As shown in Figure 15, the isolated ground planes form a parasitic dipole antenna, while

Figure 16 shows the RMS common mode voltage versus frequency above which the Si844x becomes susceptible

to data corruption. To avoid compromising data, care must be taken to keep RF common-mode voltage below the

envelope specified in Figure 16. The PCB should be laid-out to not act as an efficient antenna for the RF frequency

of interest. RF susceptibility is also significantly reduced when the end system is housed in a metal enclosure, or

otherwise shielded.

Figure 15. Dipole Antenna

Figure 16. RMS Common Mode Voltage vs. Frequency

Isolator

GND1

GND2

Dipole

Antenna

0

1

2

3

4

5

500

1000

1500

2000

Frequency (MHz)