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Original question:

LDC1101: non metal sensing

LDC1101: Although it is an inductance-to-digital converter, why it measure an impedance instead of an inductance

Mabel
Prodigy 70 points
Part Number: LDC1101

Hi,

Thanks for your question.

You answered " the capacitive sensor could be triggering the LDC1101 sensor. "earlier, and this concept is not clear to me..Could you explain the mechanism of how the capacitive sensor could be triggering the LDC1101 sensor?

And I have another question.

when I read the LDC1101 data sheet at 8.1 Overview section, it describes that LDC1101 measures the impedance and resonant frequency. However, as far as I know, LDC1101 is a converter for measuring inductance, not for impedance. Could you explain a bit more about this?

  • 0
    TI__Genius 17440 points

    Hi Yeeun,

    Two different phenomena could be occurring here:

    1. If your hand gets very close to the sensor coil, then you can cause the sensor capacitance to increase. Because L-measurement is done by measuring the resonant frequency of the LC tank, shifts in sensor capacitance will look like shifts in sensor inductance. Inductive sensing requires a very stable capacitance in order to be reliable. This is why we recommend using COG/NP0 grade capacitors. 

    2. It's also possible that your sensor frequencies for your capacitive sensing system and your inductive sensing system are too close together, and they are interfering with one another. It's possible that this is exacerbated when your hand shifts the capacitive sensor frequency. If this is the case, the simplest solution is to place the devices in sleep mode when the other is sensing. This ensures the sensors are not actively driven at the same time. You could also shift the sensor frequencies away from one another or shift the sensors physically apart.

    For your second question, the LDC1101 measures metal proximity in two ways: L-measurement and Rp-measurement. L-measurement measures the change in sensor inductance by measuring the resonant frequency of the LC tank sensor. Rp-measurement measures the losses in the sensing system due to interaction with the metal target. Both L and Rp change with the interaction with the metal target. If you'd like to know more about Rp, I recommend reading LDC Sensor Design. 

    Best Regards,