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MSP430FR2676: Driver selection for 16-channel driven shield

Harald Noack
Prodigy 70 points

Part Number: MSP430FR2676
Other Parts Discussed in Thread: OPA354, TLV3544

Hello,

With reference to the following article:

https://software-dl.ti.com/msp430/msp430_public_sw/mcu/msp430/CapTIvate_Design_Center/latest/exports/docs/users_guide/html/CapTIvate_Technology_Guide_html/markdown/ch_design_guide.html#proximity-sensor-with-driven-shield

I need to design a 16-channel touch multi-slider sensor. Since the environment is poorly defined and I only want sensitivity above the PCB, I would like to arrange driven shields under the touch electrodes. 

The recommended OPA354 is only 1-channel and much too expensive for the application.

I am looking for a small 4-channel op-amp that is much cheaper and still works OK. 
Is the TLV3544 an option? (But still too expensive.) 

Is there a way to supply multiple buttons with one op-amp? Since there are only 4 measuring units, each with 4 channels, which are scanned one after the other?

Actually, the cost limit for all 16 channels together is £1 
without PCB space.

Where is the trade-off between bandwidth and effectiveness (interference induced by active shielding) of active shielding? 

Thank you for your help!

Kind regards,
Harald

  • 0
    TI__Mastermind 40195 points

    Hi Harald,

    The easy way is to add some GND under the buttons instead of using the driven shield. This is selected by other customers.

    Please let me know your concern. Why you choose active shielding.

    Eason

  • 0 in reply to Eason Zhou
    Prodigy 70 points

    That's not a solution, because it is reducing the resolution!


    The change in capacity caused by the finger is very small compared to the sensor (top layer)/ground (inner layer) capacity.
    Prepreg 140um at er=4.0 => large capacity

    This capacity must be compensated, and to do that I need active driven shielding.

  • 0 in reply to Harald Noack
    TI__Mastermind 40195 points

    Hi Harald,

    Sorry to miss the thread for a long time.

     As MSP430 don't have a shielder pin, I would suppose you can't use one shield with one buffer and 4 sensors. The problem is that different sensors are scaned at different time. The ideal structure will be like bellow, but it will generate cross talk between different channels. That means you need to add additional buffers for these 4 sensors. That is why you only see this mehod is only for proximity. It is not for touch sensors.

    My suggestion will be if you can check to optimize the GND layout and machinal strucuture to increase the sensivity.

    Eason

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