Sensing Technologies

[GiSp_4591751]

Hi everybody,

I'm new to the capacitive touch world and I'm designing a touch panel that must work in a very noisier environment and I have some questions about properly configuring the CY8MBR3110 chip.

I have already a prototype board with spring sensor (I don't have the cypress demo board) and I'm reading the debug information from the chip. Here some questions:

1. The chip is reading Cp values (for the sensors enabled) that go from 5pF to 7pF. So I decided to put an extra capacitor of 10pF (see figure below). Is that right or I'm doing something wrong for the correct functioning of the chip?
2. In the document AN64846 (paragraph 3.2.3) there are some consideration about using ESD protection device. I'd like to know if there is also a recommendation on where to put these devices. Is better to put these devices near the sensor pad or near the input chip?
3. I haven't found any considerations about how to design a spring sensor. There are some tips to follow when designing this type of capacitive sensors (like dimensions, material, compression, ect.)?

Thank you everybody that like to share their experience.

Best Regard,

GS

[Hari · Accepted solution]

Hi GiSp_4591751​

->The chip is reading Cp values (for the sensors enabled) that go from 5pF to 7pF. So I decided to put an extra capacitor of 10pF (see figure below). Is that right or I'm doing something wrong for the correct functioning of the chip?

-Yes, this is recommended for designs with Cp lower than 5pF. This will improve the noise immunity by lowering the sensitivity of the sensor.

->In the document AN64846 (paragraph 3.2.3) there are some consideration about using ESD protection device. I'd like to know if there is also a recommendation on where to put these devices. Is better to put these devices near the sensor pad or near the input chip?

- It is recommended to place these components close to the IC (within 10mm)

->I haven't found any considerations about how to design a spring sensor. There are some tips to follow when designing this type of capacitive sensors (like dimensions, material, compression, ect.)?

- Recommendations regarding dimensions of spring can be found in the spring section of the same document, Getting started with CapSense, page 109.

Best regards

Hari

[GiSp_4591751]

Hi Hari,

Thank you for your suggestion.

I'd like to ask you some some questions:

->In the document AN64846 (paragraph 3.2.3) there are some consideration about using ESD protection device. I'd like to know if there is also a recommendation on where to put these devices. Is better to put these devices near the sensor pad or near the input chip?

- It is recommended to place these components close to the IC (within 10mm)

• It's better to put the ESD devices in parallel to C52 and C53 or on the other side of the series resistors?

->I haven't found any considerations about how to design a spring sensor. There are some tips to follow when designing this type of capacitive sensors (like dimensions, material, compression, ect.)?

- Recommendations regarding dimensions of spring can be found in the spring section of the same document, Getting started with CapSense, page 109

• In reference to figure 5-8 (pag. 110 of AN64846 document) it's possibile to design a spring with a maximum diameter of 20mm, but the design-toolbox doesn't accept sensor bigger than 15mm. Which one indication is ok for a robust design?
• What are the performance of the driver if I put two leds in the center of a capacitive pad? It's a design possibility?

Best Regard,

GS

[Hari]

Hi GiSp_4591751​

• It's better to put the ESD devices in parallel to C52 and C53 or on the other side of the series resistors?

-> No. These devices must be placed on the same side. That is, it should be placed between the IC and the series resistor.

In reference to figure 5-8 (pag. 110 of AN64846 document) it's possibile to design a spring with a maximum diameter of 20mm, but the design-toolbox doesn't accept sensor bigger than 15mm. Which one indication is ok for a robust design?

-> The maximum recommended dimension for a button is 15mm. it is recommended to design a sensor within this size. The minimum size of a sensor is 5mm. You can find more details in CapSense Design Guide under Layout rule checklist section

• What are the performance of the driver if I put two leds in the center of a capacitive pad? It's a design possibility?

-> This can be done. However, care has to be taken to ensure that CapSense (or any analog trace) does not run parallel to the LED traces without a ground separation as this might induce noise in the analog traces.

Best regards

Hari