4 ms·
> When PCB reflections start getting in the way of digital signaling, the usual culprit is a low-impedance source driving a comparatively high-impedance load (e
by mordae 3y ago
> When PCB reflections start getting in the way of digital signaling, the usual culprit is a low-impedance source driving a comparatively high-impedance load (e.g., a MOSFET gate).
Depends on the MOSFET. Most discrete MOSFETs have pretty hefty capacitance at their gate, looking like a short to high-frequency inputs.
> The simplest remedy may be adding a “sink” resistor on the receiving end, connected to the signal’s return path. This is usually paired with a series resistor on the driving side, both to limit peak current and to at least roughly match the specific impedance of the trace.
The simplest remedy would be to use series resistor matched to the trace impedance to prevent the reflection bouncing multiple times, probe at the gate and only add parallel termination resistor if it is actually needed. Because then you need to work around the voltage drop.
- dfox 3y agoEven the significantly smaller capacitance of some kind of CMOS input will appear as short for some amount of time. In combination with the series termination this will low-pass filter the signal, which is (maybe somewhat counterintuitively) often desirable for high-speed logic (it reduces EMC issues and the effect of excessive power consumption due to reduced slew rate is mostly limited to the first input buffer on chip, which cleans the edges for subsequent circuitry. Within reason, obviously). When driving some kind of power MOSFET you want it to switch as fast as possible because of the power loss in the linear region.