What is the recommended value for CapSense series resistors for PSoC4S Plus?

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NaMo_1534561
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Distributor - Macnica (Japan)
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Hello,


According to AN85951, the recommended value of series resistance placed on the CapSense sensor is 560Ω.

However, the CY8CKIT-149 uses a 2KΩ series resistor.

I looked at the schematics of some evaluation kits.
CY8CKIT-041 (PSoC4000S): 560Ω
CY8CKIT-145 (PSoC4000S): 560Ω
CY8CKIT-042-BLE (PSoC4200): 560Ω
CY8CKIT-149 (PSoC4100S Plus): 2KΩ

Why is only CY8CKIT-149 using a 2KΩ resistor?
Is the choice of 2KΩ resistors due to a special reason for this board?
Or does the PSoC4S Plus series have different recommended resistance values?

Best Regards,

Naoaki Morimoto

1 Solution
Hari
Moderator
Moderator
Moderator
750 replies posted 500 replies posted 250 solutions authored

Hi Naoaki-san, 

 

The reason we add a series resistor on CapSense traces is to provide a filter for ESD events. CapSense sensors are going to be placed in environments where the user touches the sensor with his/her finger and there might be an ESD on the sensor. The series resistor, along with the capacitance, acts as a low pass filter for this and the high current does not reach the IC. It also helps eliminate RF noise in the system. 

Hari_0-1614584913495.png

 

Note that as an added benefit, the series resistor also dampens the high-frequency switching signal on the sensor lines and reduces the noise emitted by CapSense. 

 

So considering this, it makes sense to add a higher value to make a better filter, but the disadvantage here is that the series resistor limits the maximum sense clock frequency as FSW < 1/(10RC). So, we need to choose a value such that it offers the best protection while not taking a toll on the performance. 

Therefore, we recommend a value of 560 ohms for typical designs. 

The reason we added a 2kohm series resistor on the 149 kit is that we have CSX sensors in the kit. CSX sensors typically work best with thin overlays (note that there is a recommended minimum thickness), and therefore, the chances of ESD events are higher due to this. So, to compensate for that, the higher series resistor was added.

 

The choice of series resistor is dependent on the application. If you are using a thick overlay and the end product is shielded from RF noise, then you can use a smaller series resistor such as 560 ohm. However, if you see any high-frequency noise in CapSense or if there is an increased threat of ESD, then it is recommended to go for a higher value.

 

Best regards, 
Hari

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2 Replies
Hari
Moderator
Moderator
Moderator
750 replies posted 500 replies posted 250 solutions authored

Hi Naoaki-san, 

 

The reason we add a series resistor on CapSense traces is to provide a filter for ESD events. CapSense sensors are going to be placed in environments where the user touches the sensor with his/her finger and there might be an ESD on the sensor. The series resistor, along with the capacitance, acts as a low pass filter for this and the high current does not reach the IC. It also helps eliminate RF noise in the system. 

Hari_0-1614584913495.png

 

Note that as an added benefit, the series resistor also dampens the high-frequency switching signal on the sensor lines and reduces the noise emitted by CapSense. 

 

So considering this, it makes sense to add a higher value to make a better filter, but the disadvantage here is that the series resistor limits the maximum sense clock frequency as FSW < 1/(10RC). So, we need to choose a value such that it offers the best protection while not taking a toll on the performance. 

Therefore, we recommend a value of 560 ohms for typical designs. 

The reason we added a 2kohm series resistor on the 149 kit is that we have CSX sensors in the kit. CSX sensors typically work best with thin overlays (note that there is a recommended minimum thickness), and therefore, the chances of ESD events are higher due to this. So, to compensate for that, the higher series resistor was added.

 

The choice of series resistor is dependent on the application. If you are using a thick overlay and the end product is shielded from RF noise, then you can use a smaller series resistor such as 560 ohm. However, if you see any high-frequency noise in CapSense or if there is an increased threat of ESD, then it is recommended to go for a higher value.

 

Best regards, 
Hari

NaMo_1534561
Level 5
Level 5
Distributor - Macnica (Japan)
100 replies posted 100 sign-ins 50 replies posted

Hi Hari-san,

My question has been solved by your answer. 

Thank you.

 

Best Regards,

Naoaki Morimoto

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