Correct design of capsense controls

[HuEl_264296]

Hi,

I am trying to design a PCB with some capacitive controls. My first iteration did not seem to work well:

This was a 2-layer 1.6mm board with one circular control, and 5 buttons. The board is 48mm wide. The controller is a CY8C5468LTI-LP026. The controls were designed using the Cypress Touch Controls in Altium Designer.

The response from these sensors was very poor. One or two of them seemed to work OK, but there was a lot of interference between sensors, and a couple of them barely worked at all (even when looking at the raw signal).

My questions:

1. Is there something wrong with the PCB design?
2. Should I use the Cypress Touch Controls Altium Library, or should I do something else?
3. Should I have used a hatched ground plane around the controls on the top layer?

Many thanks

Hugo

[Hari]

Hi @HuEl_264296 

1. Can you please let me know the components in layer 2? I see some solid fills underneath the sensor, is that solder mask or is it solid ground? Please note that it is not recommended to have a solid ground layer underneath the sensors as this will lead to very large parasitic capacitance. 

2. You can use the Altium library. 

3. The hatched ground plane is recommended for noise suppression and EMI/EMC performance. 

Is there an overlay being used in the design? Note that slider performance (linearity) will be better with an overlay as it will not lead to signal saturation. 

Best regards, 
Hari

[HuEl_264296]

Hi Hari,

Here's the back of the PCB. It's only 2 layers. You can see that there are no components or ground planes directly under the capacitive touch pads, although there are some tracks.

BTW, the capacitive touch pads are only the 8 pads in a circle, and those labelled A, B, OK, Menu, Back.

Apart from the lack of an overlay, is there no other reason that these sensors shouldn't work?

Hugo

[Hari · Accepted solution]

Hi @HuEl_264296 

Would it be possible to run the CapSense Tuner Helper on this board so that we can visualize the performance of the sensors? Please note that for a slider, we require some interference from neighboring sensors for accurate position measurement and the centroid algorithm takes care of converting this to the right position. 

Best regards, 
Hari