Question about AN85951 CapSense Design Guide

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NaMo_1534561
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Distributor - Macnica (Japan)
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Hi,

I refer the "Table 5-6. Recommended values for the Threshold parameters" for manual tuning.

NaMo_1534561_0-1645003747946.png

Recommended values for the Noise Threshold is 40% of signal on this table.

Please explain why 40% was defined of this value.
The gap between 40% to 80%(Finger Threshold) is no action taking of this method.

What made you decide on this value? Please let us know if you have a design policy or design concept.

Best Regards,

Naoaki Morimoto

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

Hi @NaMo_1534561 

 

In the CapSense operation, if the difference counts, which is raw counts - baseline, is less than noise threshold, the baseline tracks the raw counts so that the effect of noise can be avoided. In case we keep the noise threshold very close to the finger threshold, then the baseline will still keep tracking all the way to the finger threshold and the difference counts will not be greater than finger threshold. This will result in the sensor not working in certain conditions where the system is not very sensitive. 

Another example is the proximity sensor operation. In a proximity sensor, there is a gradual increase in difference counts. If the baseline keeps tracking the raw counts, then the proximity threshold will never reach and the sensor will not be triggered. 

Even in the case of sliders, we use the signal level from neighboring sensors, so the baseline should not be tracking the raw counts after the noise level is crossed. 

 

40% was chosen as a nominal number considering the noise levels we see in the majority of the systems. 

 

Best regards, 
Hari

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

Hi @NaMo_1534561 

 

In the CapSense operation, if the difference counts, which is raw counts - baseline, is less than noise threshold, the baseline tracks the raw counts so that the effect of noise can be avoided. In case we keep the noise threshold very close to the finger threshold, then the baseline will still keep tracking all the way to the finger threshold and the difference counts will not be greater than finger threshold. This will result in the sensor not working in certain conditions where the system is not very sensitive. 

Another example is the proximity sensor operation. In a proximity sensor, there is a gradual increase in difference counts. If the baseline keeps tracking the raw counts, then the proximity threshold will never reach and the sensor will not be triggered. 

Even in the case of sliders, we use the signal level from neighboring sensors, so the baseline should not be tracking the raw counts after the noise level is crossed. 

 

40% was chosen as a nominal number considering the noise levels we see in the majority of the systems. 

 

Best regards, 
Hari

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

Hi Hari-san,

Thank you for explaining some cases where CapSense doesn't work.
I understand the need to have a gap between FingerThreshold and NoiseThreshold.
This question has been resolved.

 

Best Regards,

Naoaki Morimoto

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