Schematic hints

AN4184

10/19

DocID023777 Rev 2

Method 4

Series capacitor to obtain

= R

Figure 12. Matching the PCB line near the microphone in a frequency range

Basically this network works in the same way as that shown in Method 2, but only above a

fixed frequency. This frequency depends on the time constant fixed by the resistor and

capacitor and it must be chosen greater than twice the propagation time delay. This is a very

useful solution, but it impacts the rising and falling edge of the clock and the transmission

line delay.

Method 5

Figure 13. Matching the clock driver output impedance - driver side (with capacitor)

the driver output and that shown by the transmission line. The advantage of this solution is

that its implementation is very easy. It introduces negligible extra current consumption and,

as in Method 4, impacts the rising and falling edge of the clock.

Any of these solutions can be used to avoid the reflections on the transmission line, the

designer must select the method which is best suited to his application. The STEVAL-

MKI129Vx coupon boards do not implement these techniques since their use is for

promotional and debugging proposes only.

input of the MEMS microphones offered by STMicroelectronics. It is not included in the

previous equations because the methods listed are provided in order to properly terminate

the codec or the microphone with the transmission line; this capacitor simply impacts the

propagation delay.