layout help for CY8CMBR3002

[ChCa_995236]

Hello, I need experts assistance for Pcap button layout.

Because of the limited board area, we plan to use 4-layer board for a 8 buttons, 5 LEDs and 2 connectors (have other PWM signal,  LED signals......). see attached mechanical drawing.

Follow AN64846 <Getting Started with CapSense>, the top layer is CapSense; layer2 is CapSense traces; layer3 is hatching Ground; bottom layer is other components and traces.

Our mechanical design has 2 connectors exactly underneath 3 buttons. According to <Getting Started with CapSense>, it says:

.........

* Isolate switching signals, such as PWM, I2C communication lines, and LEDs, from the sensor and the sensor PCB traces. Do this by placing them at least 4 mm apart and fill a hatched ground between CapSense traces and nonCapSense traces to avoid crosstalk.

* Avoid connectors between the sensor and the controller pins because connectors increase CP and decrease noise immunity.

.......

With a hatching Ground plane layer3, Can the connector directly under the button? And should all other traces (on bottom layer) at least 4mm apart from CapSense (on top layer) and CapSense traces (on layer2)? 

Thanks.

Chang Cao

[Hari · Accepted solution]

Hi ChCa_995236​

This recommendation is a best practice. However, if your design requires the connectors directly below the sensor, it can be implemented since the ground hatch in third layer will provide some isolation.

Regarding traces, it is recommended to not have any traces running parallel to sensitive analog traces. You can route the traces orthogonal to CapSense traces. But if they run parallel, there will be noise coupled from the switching digital traces to the sensitive analog traces.

Thanks,

Hari