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FDC2114-Q1: General inductance/capacitance sensor questions

Mike Twieg
Intellectual 960 points
Part Number: FDC2114-Q1
Other Parts Discussed in Thread: LDC2114, FDC2114, LDC1614, FDC2214, LDC1101

Hello all, I'm looking at options for a proximity sensor, and have a couple general questions:

1. TI advertises both capacitance sensors (FDC2114) and inductance sensors (LDC2114), but both seem to be fundamentally measuring the resonant frequency of an LC tank, so both are sensitive to changes to either L or C. So what is the actual difference between these devices?

2. In my application, there will be bursts of very strong interference in the vicinity of the sensor. The interference is brief, and it is known in advance when they will occur. Do any of these sensors allow the user to control when conversions take place, so that I can ensure that none are corrupted by this interference?

Regards,

Mike

  • 0
    TI__Genius 17440 points

    Hi Mike,

    You can technically use the LDC2114 for both capacitive and inductive sensing. If you're interested in that, you can read more in this application note. However, if it's possible for you to use inductive sensing, we recommend that you do so. Inductive sensing is only sensitive to metal targets, while capacitive sensing will detect everything near the sensor. If you're still considering various proximity sensing technologies, this blog post may help.

    Both the LDC2114 and the FDC2114 have modes in which they do not drive the sensors. In the LDC2114 it's called "CONFIG_MODE" and in the FDC2114 it's called "sleep mode." Register settings are preserved in each of these modes. You could use these modes to manually control when the sensors are off.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Intellectual 390 points

    Hi Kristin, thanks for the fast reply.

    Kristin Jones93 said:

    You can technically use the LDC2114 for both capacitive and inductive sensing. If you're interested in that, you can read more in this application note. However, if it's possible for you to use inductive sensing, we recommend that you do so. Inductive sensing is only sensitive to metal targets, while capacitive sensing will detect everything near the sensor.

    Differences in sensor design aside, what about the LDC2114/FDC2114 suitable for inductive/capacitive sensing, respectively? Maybe the LD1614 is a better comparison, its documentation looks even more like the FDC2114.

    Also, the LDC2114 mentions a maximum Q of 30 for the sensor, why is that? I assumed higher Q would give better precision, so long as the settling time is long enough.

    Kristin Jones93 said:

    Both the LDC2114 and the FDC2114 have modes in which they do not drive the sensors. In the LDC2114 it's called "CONFIG_MODE" and in the FDC2114 it's called "sleep mode." Register settings are preserved in each of these modes. You could use these modes to manually control when the sensors are off.

    Great, thanks!

  • 0 in reply to Mike Twieg1
    TI__Genius 17440 points

    Hi Mike,

    If you look at the electrical characteristics tables for each device you can see that there are more useful specifications for either inductive or capacitive sensing, depending on the device.

    The sensor Q is related to both Rp and Fref, and the sensor Q limitations are based both on these limitations and the driver architecture.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Intellectual 960 points

    Hi Kristin,

    I think I see what you mean. For example, FDC2214 specifies a capacitance range, while the LDC1614 specifies an Rp range. But on every parameter which they both specify, the values are basically identical. So are they effectively the same device, just with different documentation tailored to different applications??

    What about other devices, like the LDC1101 or LDC2114? They have some slight differences from the LDC1614/FDC2214, like drive current and frequency range. But is it basically the same sort of analog front end?

    For the Q range, that makes sense. So as long as my sensor Rp is in the specified range, then it doesn't matter what my Q is? For example, Ls=500nH, Cs=2nF, and Q=100 would give me an Rp=1.58kohm, which is fine, right?

  • 0 in reply to Mike Twieg
    TI__Genius 17440 points

    Hi Mike,

    I can't comment on the specific design differences between various devices because this is proprietary information. Many of our devices do have significantly different driver architectures.

    You should still follow the recommendations for the sensor Q range. It's difficult to design a sensor that meets the sensor f and Rp ranges but not the Q range. However, it is possible, and you should make sure that all 3 values are within the recommended range.

    Best Regards,