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CAPSENSE™ & MagSense Forum Discussions

mbastida
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Hi,

Having read some threads in this forum I have come up with this layout, we need inmunity to liquids.

mbastida_2-1669906441344.png

 

(the dimesions are aproximate).

The shield will be on an intermediate layer (4 layer PCB). The guard and proximity sensor will be on the bottom layer.

The shield will have the pattern described in AN90071.

mbastida_1-1669906215178.png

 

Questions:

1. Is it better to have a square pattern instead of a rectangular one?

2. The location of the wireless module is adequate? Should I place it in the top or bottom (same as cap sense) layer?

3.  Is it important to avoid having solid ground planes around the sensor? From my experience it is a thing to avoid. But the AN90071 seems to say otherwise. I say that because in the proximity sensor design they recommend placing a ground trace around the sensor.

4. How would you improve the design?

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PandaS
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Hi @mbastida ,

The twin guard sensor placement is correct and good for water tolerance. Coming to the ground hatch: it has its own trade off. It will increase the noise immunity but also reduce the proximity distance. So, I would recommend if you prefer having a hatch you can have it between the inner guard ring and the sensor layout.

There is also a firmware technique which can also help, it is Driving the inactive sensor connection (Guard rings) with shield signal. It can protect the proximity from noise coming from the wireless module up to a certain extent. But we need to test it in hardware. If that does not work, we can also set the inactive sensor connection to Ground as well. We will have to verify the performance of both on hardware.

Warm regards

Sobhit

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PandaS
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Hi @mbastida ,

CapSense performance will drop due to the presence of Wireless Module. Place it away from the CapSense design.

Placement of Guard sensor seems to be good for liquid tolerance feature. 

I would recommend having ground hatch on the top layer, if possible.

Shield may or may not help in this regard, we need more information about the stack-up and PCB designs. Can shield layer be placed between Wireless module and CapSense Sensor?

Warm regards

Sobhit

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Hi @PandaS and thanks for the response.

 

The module has to stay there because it has to interact with the user (the same way the capacitive detects the user).

The ground hatch in the top layer is possible. But I have still the doubt if it should be connected to the shield pin or GND.

The PCB has four layer, but in the capacitive zone there are no ground nor power planes*. So the layers are "empty". If you need more information please don't be hesitated to ask.

And yes, it would be possible to place a shield between the wireless module and the cap sensor. What would you recommend? hatch plane or solid plane? How much spacing from the cap sense?

 

*It is done like so because previously we were using the MTCH102 from microchip. This sensor worked the best when no GND planes where nowhere close to the sensor. 

This is why I'm confused when in the aplication note or you recommend having GND planes next to the sensor. Isn't this bad for the range?

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PandaS
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50 solutions authored 100 replies posted 100 sign-ins

Hi @mbastida ,

The twin guard sensor placement is correct and good for water tolerance. Coming to the ground hatch: it has its own trade off. It will increase the noise immunity but also reduce the proximity distance. So, I would recommend if you prefer having a hatch you can have it between the inner guard ring and the sensor layout.

There is also a firmware technique which can also help, it is Driving the inactive sensor connection (Guard rings) with shield signal. It can protect the proximity from noise coming from the wireless module up to a certain extent. But we need to test it in hardware. If that does not work, we can also set the inactive sensor connection to Ground as well. We will have to verify the performance of both on hardware.

Warm regards

Sobhit

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