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LDC1614: Inductive Sensor Array + Resonant Circuit Driver Signal

Thomas Locher
Prodigy 50 points
Part Number: LDC1614

Hi,

I am planning to design an inductive sensor array, similar as Texas Instruments did with the 16-Button Keypad, (see the documentation TIDU954A) , the following figure is from the mentioned documentation.

The multiplexed network of this 16 sensors (LC-Tanks) suffers strong crosstalk between sensors, so the application is based on threshold detection but it is not sensing Inductance with a precise resolution.

I am looking for ways to reduce this crosstalk, either take a separate MUX-channel for each sensor (not a network like arrangement, but multiplex multiple MUX) or add a low leakage diode to each LC_tank in this network arrangement. This would clip the oscillation signal as soon it goes below ground.

Here comes my 1. Question: Would the "Resonant Circuit Driver" still be able to excite an LC-Tank at its resonance frequency with a diode in series (clipped V_osc Amplitude)?

The reason why I came to this question is because in all the documentations I studied where V_osc is plotted it appears to be somehow clipped. Maybe the Regulation of I_Drive inside the "Resonant Circuit Driver" leads to an already clipped I_Drive signal. (Nice examples of clipped V_osc in SNOA950 p. 6 or SNAA221A p. 7)

2.Question: Why is V_osc clipped?

I know that the inside of the "Resonant Circuit Driver" is part of IP from Texas Instruments, so I will not ask on further details on that.

But still I would like to have a basic understanding of the signal coming from that "Resonant Circuit Driver" and driving the LC-Tank or whatever I connect to the IN0A and the IN0B pin.

3.Question: Could one assume that the "Resonant Circuit Driver" is approximately an alternating current source with an amplitude that is regulated to a constant level ( I_Drive ) and the frequency of this alternating current is self-adjusting to the resonant frequency of the circuit connected to it (amplitude might be clipped to positive values 2.Question).

Thanks for commenting / replying

  • 0
    TI__Genius 13650 points
    Hello Thomas,
    I do not see any sensor clipping clipping in the two images that you referenced. Note that the sensor output on this device is a half cycle of the sine wave; the other half is generated by the other pin (eg. IN1A, IN1B). The signal shape is different if you probe it from the center via of the inductor.
    Yes, it is essentially a current source with the current set as per the IDRIVE setting. The sensor oscillation frequency is determined by its L and C components, plus any changes in L due to the presence of metal in the proximity of the sensor.
  • 0 in reply to Ben Kasemsadeh
    Prodigy 50 points
    Hello Ben,
    Thank you for replying, I did use the wrong therm with "clipping" I meant to address this half cycle of the sine wave, that it does not go below ground on this graphs. Anyhow you answer clarified everything.

    Since March I have built a prototype circuit using one LDC1614 chip to sample 25 Coils.
    I posted a new question with the title "Nonlinear crosstalk when multiplexing sensing coils on one channel" , once the post is verified everyone that is interested to multiplex sensor coils on a LDC1614-channel should have a look at it, so they can see which challenges have to be solved.

    Best regards,
    Thomas