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jcsb1994
Level 4
Level 4
First solution authored 50 replies posted 50 sign-ins

Hi, I added ground traces between my CSX lines in my capsense pcb design.

1. Is there a minimum distance those traces should have from the sensors? And they create antennas (they are marked by small yellow circles in the image), this doesn't cause any problem, right?

pastedImage_0.png

2. The ground traces between the capsense lines are connected to the supply GND, I know it would be better to connect them to a capsense ground, but the CSX lines are all over the PCB. Is it a viable option to use supply ground, or should I create a capsense ground plane on the top layer and link it to my star ground with vias (there is no room on the bottom layer next to the capsense traces to do like it is advised in the datasheets). The ground plane would be placed where the white triangle is, to supply ground to the capsense ground vias

pastedImage_1.png

3. Is there a minimum distance between the supply ground and the capsense sensors? I drastically reduced it in my newest design (to around 5mm), and I didnt find this info in the datasheet.

pastedImage_3.png

4. I am connecting a grounded fabric on top of a compressible overlay, to detect any object with capsense. Therefore, I didnt put the hatched ground surrounding the sensors, as I thought its effect would be negligible compared to the grounded fabric. I am also trying a PCB where the CSX touchpad is at the edge of the PCB. Is there any major issue with this design? Thanks!!

pastedImage_5.png

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1 Solution
BragadeeshV
Moderator
Moderator
Moderator
First question asked 1000 replies posted 750 replies posted

Hi JeSi_4326976​,

1. a) It is not required to place ground trace between each trace for the CSX sensors. Only if the Tx and Rx trace are routed parallel to one another, you need to place a ground trace in between them to reduce unwanted Cm formation. It is recommended to have a trace to ground trace spacing of 10 mils at least.

    b) We recommend you to group all the Tx traces together and Rx traces together where ever possible while routing. You can have two Tx (or Rx) traces parallel to each other with 10 mils spacing between them. But just that the Tx and Tx should not be routed parallel to one another.

2. a) We would highly recommend you to create a capsense ground plane in the regions of the white triangle (preferably hatched) and have only a single point of connection to the supply ground / to the star connected ground. This ensures that any noise from the digital circuit doesn't affect the sensitive capsense section.

   b) Would it be possible to change to a hatched ground (white triangle region) at least in the places where the traces go? We might want to reduce Cp so that we could use higher Tx Clock frequency. Note that higher the Tx Clock frequency, lower is the scan time.

3.  The gap between the CSX sensor and the nearby ground (hatched preferably) should be at least equal to twice the gap between the Tx or Rx segments or equal to overlay thickness. What is the present clearance?

4. We do not see any issues with this design. If possible a ground loop surrounding all the sensors might help you get a good EMI/ EMC /ESD performance.

Regards,

Bragadeesh

Regards,
Bragadeesh

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3 Replies
BragadeeshV
Moderator
Moderator
Moderator
First question asked 1000 replies posted 750 replies posted

Hi JeSi_4326976​,

1. a) It is not required to place ground trace between each trace for the CSX sensors. Only if the Tx and Rx trace are routed parallel to one another, you need to place a ground trace in between them to reduce unwanted Cm formation. It is recommended to have a trace to ground trace spacing of 10 mils at least.

    b) We recommend you to group all the Tx traces together and Rx traces together where ever possible while routing. You can have two Tx (or Rx) traces parallel to each other with 10 mils spacing between them. But just that the Tx and Tx should not be routed parallel to one another.

2. a) We would highly recommend you to create a capsense ground plane in the regions of the white triangle (preferably hatched) and have only a single point of connection to the supply ground / to the star connected ground. This ensures that any noise from the digital circuit doesn't affect the sensitive capsense section.

   b) Would it be possible to change to a hatched ground (white triangle region) at least in the places where the traces go? We might want to reduce Cp so that we could use higher Tx Clock frequency. Note that higher the Tx Clock frequency, lower is the scan time.

3.  The gap between the CSX sensor and the nearby ground (hatched preferably) should be at least equal to twice the gap between the Tx or Rx segments or equal to overlay thickness. What is the present clearance?

4. We do not see any issues with this design. If possible a ground loop surrounding all the sensors might help you get a good EMI/ EMC /ESD performance.

Regards,

Bragadeesh

Regards,
Bragadeesh

BragadeeshV_41 wrote:

It is recommended to have a trace to ground trace spacing of 10 mils at least.

That means my ground traces between all lines are actually really bad because they are 7mil from each of my capsense traces... I will remove them. Of course, no capacitance can be created between 2 Tx or 2 Rx lines, but I thought it could help to reduce signal mixing.

I will also keep my TX parallel all together with a 10 mil spacing. I will do the same with Rx. Thank you for that. I still have issues regarding the point 2 and 3 however:

2 I am sharing a quick modif with the hatched ground (I did not remove the ground traces of the bottom blue layer between Rx lines, and between Tx lines yet, but at least their vias are in the capsense hatched ground for now. So now, the top layer gnd is pretty much hatched above the whole capsense trace area... This will help reducing Cp compared to a filled ground since hatched couples less with the traces? I am confused, because I thought CSX was not impacted at all by Cp, and from what I understand from your last message, reducing my Cp will actually let me increase my tx clock frequency??

pastedImage_2.png

3. The present GND to sensor clearance is 4.9mm, and the Rx-Tx pad clearance is 0.49mm so it is actually tenfold. The overlay should be around 2mm. I had seen these guidelines for the capsense ground in the datasheet, I was actually worried about the clearance from the supply ground. I am not sure I understand the advantage of having a hatch ground compared to no ground at all above the traces (I do understand that a full gnd plane would increase the Cp drastically) but in theory, having no ground at all wouldnt be better since there would be no coupling with ground at all, compared to a hatch (given the Tx and Rx are far enough from eachother)?

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BragadeeshV
Moderator
Moderator
Moderator
First question asked 1000 replies posted 750 replies posted

Hi JeSi_4326976​,

JeSi: I thought it could help to reduce signal mixing.

-> Inactive sensors (therefore traces) are connected to ground, therefore there will not be any signal mixing.

JeSi: I thought CSX was not impacted at all by Cp, and from what I understand from your last message, reducing my Cp will actually let me increase my tx clock frequency?

--> The CSX performance is not affected by Cp of the traces but it limits the maximum Tx Clock frequency that can be set. Please take a look at the CSX manual tuning section of the CapSense Design guide. The Tx Clock frequency should be set such that it charges and discharges the Tx Cp completely. Otherwise, the sensitivity will be reduced.

pastedImage_0.png

If Tx Clock doesn't charge and discharge the Tx Cp completely, then the sensitivity is reduced as per the following equation:

pastedImage_1.png

We would like the set the max Tx Clock possible because it directly affects the scan time and hence the response time.

pastedImage_2.png

JeSi: I am not sure I understand the advantage of having a hatch ground compared to no ground at all above the traces (I do understand that a full gnd plane would increase the Cp drastically) but in theory, having no ground at all wouldnt be better since there would be no coupling with ground at all, compared to a hatch (given the Tx and Rx are far enough from eachother)?

a) I'm not sure if I understand your 3rd question. Why do you want to remove ground  above the traces? They will provide good noise immunity to the traces. Please help clarify.

b) How are tx and Rx away from each other? Do you mean Tx and Rx traces?

Regards,

Bragadeesh

Regards,
Bragadeesh