MTCH101 - metal housing / button sensitivity

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Pawel_Z
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2019/02/18 08:51:21 (permalink)
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MTCH101 - metal housing / button sensitivity

Hi!
 
In my solution the MTCH101 is used as a main button (only one button for device). The device is powered by Li-Ion cell. The MTCH-101 is supplied by 2,8 V with capacitors 100 nF and 10 uF placed near to the chip.
 
The pad sensor on PCB has a round shape of 7 mm diameter. According to the datasheet the pad sensor on PCB is divide for two similar pieces. One pieces is connected to the MTCH and the second is connected to the ground. Around the sensor the pcb is free from the gnd.
 
The PCB is placed in the metal housing (aluminium). The distance between housing and the pad sensor on PCB is 0.2 mm. The MTCH should detect bending of the metal housing.
 
The metal housing is connected to the ground (gnd from cell).
 
I have got problem with low sensitivity of the button. Especially if the button is pushed for few seconds. If the housing is pressed then MTCH detect this but if the housing is pressed for longer time then the MTCH will lost the signal.
 
What is the reason of this situation? In addition I have detected that if the housing and gnd on the pcb is connected to the gnd from power supplier or usb cable the sensitivity of the MTCH works much better.
 
Best regards
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    PStechPaul
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    Re: MTCH101 - metal housing / button sensitivity 2019/02/18 16:49:06 (permalink)
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    I'm not familiar with this device, or capacitive touch sensing in general, but I think it senses the presence or absence of an external object by a change in capacitance between the sensor and its ground reference. If you hold your finger on the sensor for a while, the floating ground of the metal case and ground will approach the potential of the applied object (your finger), and will retain that charge for a while when released. But with a better earth reference through the USB shield or power supply ground, the case remains at earth potential, and capacitive sensing will be more reliable. If the device will be fully floating, perhaps some sort of differential proximity sensing could be used.
     
    http://ww1.microchip.com/...eviceDoc/40001664B.pdf

     
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    Pawel_Z
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    Re: MTCH101 - metal housing / button sensitivity 2019/02/19 07:02:58 (permalink)
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    Thanks for your answer Paul!
     
    I think you are right. If the device is held in hand the electrical potential on the housing and inside the battery is the same and sensor does not detect the capacity changes.
     
    It could be detect impuls, but if the housing is pressed for long time than sensor will lost the signal.
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