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Original question:

FDC2214: Soft robotic sensor capacitive sensing application

FDC2214: FDC2214

Mark TUCKER
Prodigy 150 points
Part Number: FDC2214
Other Parts Discussed in Thread: FDC1004

We are working on a capacitive sensing application that has 8 separate capacitive inputs. We are using the 4-channel FDC2214 to sense these elements.

PCB space is very tight in this design. Because space is at a premium, we are considering putting a quad 2:1 analog mux (TMUX1574RSVR) in front of the FDC2214 to allow us to sequentially monitor 8 channels with the 4-input FDC2214. This mux idea saves chip area (one FDC2214 rather than two) but more importantly reduces the total number of very large 18uH inductors from 8 to 4. 

Section 10.3 of the FDC2214 data sheet explicitly warns us not to hot-swap sensors in front of the FDC2214. However, Ti's response to this e2e query from three years ago:

https://e2e.ti.com/support/sensors-group/sensors/f/sensors-forum/871438/fdc2214-soft-robotic-sensor-capacitive-sensing-application

Leads us to believe that the 2:1 muxing in front of each FDC input may be acceptable if we maintain the connection from sensor to FDC input solidly in place during the time the FDC is sampling that channel. Switching the mux to bring in the next four channels would take place between the active sampling intervals. [Not quite sure exactly how we do this but - one thing at a time.] 

Does this idea seem feasible if we switch the muxes only during non-sampling times?

  • 0
    TI__Mastermind 33190 points

    Mark,

    With the constraints you have to deal with, it sounds like the only workable alternative if you are set on using the FDC2214.
    And just so you know, I have not encountered anyone using this technique before. It may be out there, but it hasn't filtered its way back to us in the product team.
    As a first-order consideration, you will need to configure the timings and so on to account for the MUX switching.

    All that being said, our customers report very little success with any of the FDC2xx family of parts.

    Would the FDC1004 support your application?
    It is also a capacitive sensor, and does not require an inductor because its front end does not rely on a resonant circuit.
    It has 16b of dynamic range and a ±15pF direct measurement range which can be extended to a 0 to 110pF range with internal settings.
    More information on the FDC1004 is available at the E2E FDC1004 Frequently Asked Questions.

    Regards,
    John

  • +1 in reply to John Miller - Sensing
    Prodigy 150 points

    John - thanks for answering my question and providing comments about the usefulness of the FDC2214. After I marked this resolved, I thought of one more general question to ask regarding the overall usefulness of the FDC2214.

    This customer will be using this part in a Class II, healthcare monitoring application (i.e. intermediate risk but not life-sustaining). This product will require standard FDA approval for a Class II device. That will entail many man-months of verification and test time plus the calendar time required by FDA for approval. Also, the customer will want to maintain production of this device for a number (5?) of years. 

    My question to you is - are we betting on the right chip from a product sustainability standpoint? The customer made their application work well enough using the FDC2214. However, we don't want to be swimming upstream by choosing the 'wrong' sensing technology. The FDC2214 is clearly marked "in production". Maybe that's all you can confirm for us.

    That said, we're about to undergo a time-intensive development effort using the FDC2214. If you have any insights as to this part's ultimate viability, we'd sure like to know your concerns sooner than later. 

    Mark  

  • 0 in reply to Mark TUCKER
    TI__Mastermind 33190 points

    Mark,

    The FDC2214 is indeed in production and there are no plans to discontinue that device, or any of the other devices in the FDC2xxx product family.

    The main challenge for many of our FDC2xxx customers is EMI. The devices do not have built-in active shield drivers, so EMI management can be challenging. 

    Without knowing more, that's about all that can be said for now.

    Regards,
    John