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Dear SIrs and Madams,
We are considering configuring the CapSense sensor that connects to PSoC with ITO.
ITO seems to have a higher resistance component than copper wiring on a general board.
Since it is necessary to make a fine structure this time, the resistance component of the sensor is about 7k ohm.
I tried the simulation as below, but it seems that the resistance component of the sensor is not affected so much.
Is it a problem that the sensor resistance (or resistance to the sensor) is high?
Regards,
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PSoC 4 MCU
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Hi @MiNe_85951 San,
The resistance of the sensor will not be an issue but Cp has to be in the supported range. Based on that you'll have to follow 10*R*C rule (as you already know). The calculated frequency based on R and C should be able to completely charge and discharge the sensor, as long as this happens, There won't be an issue with CapSense. For CSDv2, Sensitivity will not be affected by Fsw, Scan resolution affects Sensitivity and Scan time is affected by Fmod and Scan Resolution (Section 3.2.4.2 Modulator clock)
So to summarize, for a given R and C (Cp must be in supported range) the calculated Frequency should be able to completely charge and discharge the sensor (as pointed by the design guide)
Kind regards
Arpit Srivastav
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Hi @MiNe_85951
For CapSense we follow the 10*R*C rule. where R is the total series resistance and C is the sensor parasitic capacitance. The sense clock frequency should be selected so that the sensor will charge and discharge completely in each sense clock period. So FSW (Max) = 1/(10*R*C) where C is the sensor parasitic capacitance, and R is the total seriesresistance, including the 500 Ω resistance of the internal switches, the recommended external series resistance of 560 Ω and trace resistance if using highly resistive materials (example ITO or conductive ink);that is, a total of 1.06 kΩ plus the trace resistance. These factors dictates the sense clock frequency calculation. Also, the Cp of the sensors should stay in the supported range. For more details please refer to https://www.infineon.com/dgdl/InfineonAN85951_PSoC_4_and_PSoC_6_MCU_CapSense_Design_GuideApplicati...
Did you check the Cp of the sensor? Is it between the supported range?
When you say "the resistance component of the sensor", I believe you are talking about the resistance of the sensor that you are using, Is my understanding correct?
So as per the image attached are you trying to use two different sensors with different sensor resistance and comparing the output i.e. charging and discharging of the sensor?
Thanks!
Kind regards
Arpit Srivastav
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Arpritsan,
I will answer your question.
[Q1]
Did you check the Cp of the sensor? Is it between the supported range?
[A1]
We are still in the investigation stage, and we do not know the actual resistance and capacitance values of the sensor.
[Q2]
When you say "the resistance component of the sensor", I believe you are talking about the resistance of the sensor that you are using, Is my understanding correct?
[A2]
You're right.
I have heard that if the wiring to the sensor by ITO is 0.15mm wide, it will be equivalent to 7k ohm, which leads me to this question.
I have often discussed the capacitance value of the sensor, but I have never discussed the resistance value of the sensor (the resistance value to the sensor).
That's because even AWG 38 is 1.4kohm/km at 0.0137mm^2 with copper wire.
[Q3]
So as per the image attached are you trying to use two different sensors with different sensor resistance and comparing the output i.e. charging and discharging of the sensor?
[A3]
I did a simulation because I could not judge whether it would work even with such a high resistance value.
I did a simulation to compare low and high resistance values.
Since the sensor is floating and not connected to anything other than the PSoC, I thought that if there was only a resistance component, there would be no phase shift, and if the capacitance was low, it would probably work.
I also consider FSW (Max) = 1/(10*R*C) and in the case of R=10k and C=20p, it is calculated that it can operate at about 500kHz.
I'm not familiar with ITO.
I asked this question because I heard that ITO has such a high resistance value when the wiring is thin.
Do you think a sensor with such a high resistance value can be used as CapSense?
Regards,
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I understand that the resistance and capacitance values of the sensor are subject to the constraints of the above formula.
If the resistance component of the sensor is about 7.5k ohm and the capacitance is 20pF,
Rtotal is 8060 ohm when the damping resistance is 560 ohm.
The maximum frequency is calculated as 620kHz.
Also, if the resistance component of the sensor is 500 ohm and the capacitance is 140 pF, the maximum frequency will be 670 kHz as well.
From this, we concluded that even if the resistance value is high, operation can be achieved if the capacitance value is low.
However, since this 600kHz requires scanning time, we expected that sensitivity would become a problem, and we determined that tuning would be even more difficult if the actual ITO had a high capacitance.
Regards,
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Hi @MiNe_85951 San,
The Cp (Parasitic capacitance) of the CapSense sensor has to be in the supported Cp Range. For more details you can refer to Table 2 Comparison of CAPSENSE™ architecture for CSD and CSX in https://www.infineon.com/dgdl/InfineonAN85951_PSoC_4_and_PSoC_6_MCU_CapSense_Design_GuideApplicati... So you'll need to calculate the Cp of the sensor and check if lies between the supported range.
I would recommend you to refer to https://github.com/Infineon/mtbexamplepsoc4capsensecsdbuttontuning for more details on manual tuning .
Kind regards
Arpit Srivastav
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Hi @MiNe_85951 San,
The resistance of the sensor will not be an issue but Cp has to be in the supported range. Based on that you'll have to follow 10*R*C rule (as you already know). The calculated frequency based on R and C should be able to completely charge and discharge the sensor, as long as this happens, There won't be an issue with CapSense. For CSDv2, Sensitivity will not be affected by Fsw, Scan resolution affects Sensitivity and Scan time is affected by Fmod and Scan Resolution (Section 3.2.4.2 Modulator clock)
So to summarize, for a given R and C (Cp must be in supported range) the calculated Frequency should be able to completely charge and discharge the sensor (as pointed by the design guide)
Kind regards
Arpit Srivastav
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Arpitsan,
Thank you for your reply.
I understood this issue.
I have no experience operating a switching capacitor at a frequency as low as 500kHz, so I will be careful about sensitivity.
Regards,