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LDC1001-Q1: "Remote" sensor

Supira Medical
Intellectual 270 points
Part Number: LDC1001-Q1
Other Parts Discussed in Thread: LDC1001

Tool/software:

Hi,

The datasheet says: Remote sensor placement (decoupling the LDC from harsh environments).

How far away can the LDC1001 and microcontroller be from the sensor? Does the sensor capacitor need to stay within a certain minimum distance from the inductor or the LDC1001? Any particular cabling requirements, such as the use of a shielded twisted pair cable between the sensor and LDC1001?

Thanks!

  • 0
    TI__Mastermind 37605 points

    Supira,

    Compared to just the sensor and capacitor, the cabling will present additional loading to the device outputs.

    Cabling losses need to be controlled so resistive losses aren't too high.
    Excessive resistance-based losses will cause a reduction in sensor waveform amplitude and a degradation in sensor/cable Q.
    Low sensor waveform amplitude will reduce your system's SNR/resolution, and a low Q could prevent the internal oscillator from working.
    The data sheet expresses this in terms of equivalent parallel resistance max/min values (3.93MΩ/798Ω), and for a remote sensor, these limits will apply to the sensor and cabling.
    Once you have your initial estimates for your sensor inductance and capacitance, you can calculate the equivalent series resistance max/min via well-known equations.

    Any parasitic L&C from the cable will need to be considered for the purposes of the LC oscillation (with respect to the LDC outputs) and the overall change in inductance you are trying to detect via the sensor..
    For example, it won't be desirable for the cable inductance to be much larger than the sensor inductance because the cable inductance will tend to dominate and could make the sensor inductance shift harder to detect.

    A shielded, twisted pair might be a good place to start since it should give the interconnect some EMC protection. 
    Hopefully the cable specs give the L/C/R parameters that can help pick the best options.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Intellectual 270 points

    Hi John,

    Thanks for the prompt reply. I agree that calculating it out is the best way to go. However, based on your experience and gut feel, what would you say is a typically acceptable cable length? Am I looking at something like six inches or a couple of feet?

    Also, your recommendation for placement of the tank capacitor - close to the inductor or with a short cable length, close to the LDC?

    Regards.

  • 0 in reply to Supira Medical
    TI__Mastermind 37605 points

    Supira,

    Its really hard to give a definitive answer.
    A few inches with a mid-grade cable is probably realistic.
    A couple of feet will take some more careful cable choices.


    What would best support your application?


    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Intellectual 270 points

    Hi John,

    A few inches would suffice to keep the PCBA away from heat sources. Would you recommend that the capacitor be placed close to the inductor or can it be on the PCBA?

    Thanks.

  • 0 in reply to Supira Medical
    TI__Mastermind 37605 points

    Supira,

    The best capacitor placement is close to the device pins on the PCB.

    regards,
    John

  • 0 in reply to John Miller - Sensing
    Intellectual 270 points

    John,

    Many thanks for the assistance. If I may add, I continue to be impressed with TI's speed of response on these forums and the quality of the design help.

    Regards.