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FDC2114: Waveform seen on the sensor inductor and sensor?

Peter Prince
Prodigy 30 points
Part Number: FDC2114
Other Parts Discussed in Thread: , LDC2114

Re: FDC2114 and waveform seen on the sensor inductor and sensor.

Referring to the datasheet Section 9.1 sates “to an oscillation of a LC resonator” which I assume means a sine wave.
Referring to the datasheet Figure 13, this drawing shows a sine wave for the activation, settling, etc periods.
I quote from just after Table 9
“The CHx_IDRIVE field should be programmed such that the sensor oscillates at an amplitude between 1.2Vpk (VSENSORMIN) and 1.8Vpk (VSENSORMAX). An IDRIVE value of 00000 corresponds to 16 µA, and IDRIVE = b11111 corresponds to 1563 µA” Which again I assume means a sine wave.

A sine wave, I do not get. I see only the positive half of a sine wave the negative side is stuck at 0 volts. This looks as if the the waveform has passed through a rectifying diode.

Please note this is NOT something I have done wrong. The effect is in my development FDC2114 system AND In T.I.s unaltered FDC2114EVM evaluation module.

I believe it is not the connection of the scope causing this. I have treated the sensor as a RF radiator and "received" the waveform without making an galvanic connection to the FDC circuit.


My Questions: Is this normal and not a sine wave or is there a special way to view the waveform developed in the FDC? If it is not a sine wave and is of a rectified nature how is this for EMC radiation? Does the set up of the "1.2 ~ 1.8 volts pk" drive current need clarification due to it being an asymmetrical wave?

  • 0
    TI__Expert 6480 points
    Hi Peter,

    This is expected as the output for the FDC2114. The half-sine wave is normal and the 1.2-1.8 pk-pk voltage is consistent with this behavior. In terms of EMC, is it failing for susceptibility or for radiated EMI?

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 30 points
    Re: More on FDC2114 Waveform seen on the sensor inductor and sensor?
    Thank you for your quick reply.

    For me to run with this device in a product I have to know exactly what is going on, in it. Therefore the information you have given me is lacking in detail.

    You say “This is expected as the output for the FDC2114.”, as a simple statement this is not correct. The output to the LC tank IS a sine wave and can be viewed by using an close coupled inductor to the tank inductor connected to the scope input. There is a sine wave as one would expect, across the resonating LC network.

    Therefore how does one only see a “rectified wave” when looking at the sensor? I suspect it is actually a sine wave and the rectification effect is due to the floating FDC connections to the tank. Connecting a scope which is earthed and with the FDC Vss being earthed, then I can see a diode path through the floating InxA & B pins and FDC Vss. My problem is I can’t prove this and need to know the truth about this.

    You say “The half-sine wave is normal and the 1.2-1.8 pk-pk voltage is consistent with this behaviour.” This statement is meaningless to me.

    As I can only view the tank waveform as a half, then is the pk to pk voltage set to 1.2-1.8 pk-pk, OR half of this voltage? In other words does the given calibration value refer to the seen half wave or the actually sine wave on the tank? It is vital to have an answer to this.

    You say “In terms of EMC, is it failing for susceptibility or for radiated EMI? “ I haven’t got that far yet, but this will have to be approved, so this is why I need to know what is going on, so I can deal with any non-conformity. If there is a “rectified” wave on the sensor I wonder if that is a problem for emissions, so this is why I need to know the truth about this.

    If you do not answer the specific points then I suspect this project will die and you will loose a customer.
  • 0 in reply to Peter Prince
    TI__Expert 6480 points
    Hi Peter,

    The FDC is driving the LC tank with a half-sine wave, this is the standard behavior of our device. The process by which this is achieved is proprietary information which we cannot release.

    In terms of the pk-pk voltage, you do not need to divide the specification by half - the datasheet was written so that the specifications are of the expected half-sine wave output, not a full sine-wave.

    It's difficult to give a hard answer for EMI performance since it is system-dependent. The size of the sensor, the system set-up, the final PCB layout, etc. will all affect EMI emissions.

    If you would rather work with a device that outputs a full-sine wave, you can also take a look at the LDC2114 which can also be used as an FDC device in capacitive sensing applications.

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 30 points
    Thank you again for another quick reply.

    It would appear the LC tank is hit with 2 anti-phase half sine waves and the problem is both of these will contain harmonics. The sensor that is connected to one of these half sine waves will radiate these harmonics. It remains to be seen if these are significant as the power is very low. This explains why the evaluation module PCB includes the option of 4 capacitors , a common mode choke and mentions their use if EMC “problems”
    Fortunately I have included these on my development board!
    I note your comment on the LDC 2114.

    I must say the datasheet is very misleading and lacks the detail that has been discussed here.
    Thank you.