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LDC1000: LDC1000 strong temperature dependency

Stefan Thiel
Prodigy 50 points

Part Number: LDC1000

Hi,

after collecting a lot of experience with the the LDC1000 and the Eval Board, I finished my first own design. When I did a test in the temperatur chamber of the design, I found out, that the proximity value drifted largely away from the starting value at room temperature, when the enviroment was heated up to 60°C. Hence, the targetposition was not changed.

-> I started to investigate the problem and built a new pcb sensor, double checked the usage of C0G capacitor and measured the resonace frequency over temperature. I found no significant change in resonance or amplitude. So I assumed it is not the resonant circuit itself.

-> I made a setup with the LDC1000 Eval board and the same sensor pcb and put them both into the temperature chamber.

Here I also found a big change in proximity when temperature increased. 2800@21°C to 15000@60°C without moving anything. -> this means, that the same problem occurs with the eval board.

Any hints on this?

Any other experiences to share regarding temperatur dependencies?

Any help is appreciated,

thanks,

Stefan

  • 0
    TI__Genius 14270 points
    Hello Stefan,

    This type of issue occurs with Proximity measurements due to the change in conductivity of the target - for example, Copper has a 3900ppm/degree C shift. This effect is inherent in the measurement; for this reason we generally recommend using L measurement, which can have a shift <1% of the proximity measurement.

    Are you working on automotive applications or metal identification? These are really the only reasons to use the LDC1000 - the LDC1101 is a cheaper part that is superior for almost every other set of applications.

    If you can use an L measurement, you may consider using the LDC1312 or LDC1612, which measure L and are multi-channel devices that enable you to use a reference channel to compensate for most other environmental shifts.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 50 points

    Hi Chris, thanks for the information, however I do not fully understand.

    Well, I am using the LDC1000 because we started our design when it came out some years ago, when the other chips have not been availiable. The project was sleeping for a while :-)

    So, I am doing a linear position measurement where a steal target is moving parallel to my pcb coil. It will be used in an harsh enviroment -40 to + 125°C.

    When I understand correct, you say, that temperature is changing the conductivity of my target ( steel ) and this is causing a change in proximity even when the position of the target is not changed, right?

    Is the reason that the increased conductivity is changing the flow of eddy currents in the target material?

    And this influence is not seen in measuring inductance? But doesn't the change in conductivity also affect the copper of my coil and also the wires to the coil? Does this affect inductance, or only proximity because of the changing resistance?

    I will do some trials with the LDC1614EVM.

    Can you say anything about the parts being around the LDC1000, e.g. what if the capacitor on CFA/CFB or CLDO changes with temperature.

    As I see in the BOM of LDC1000EVM C14 is a X7R

    Thanks,

    Stefan

  • 0 in reply to Stefan Thiel
    TI__Genius 14270 points
    Hello Stefan,

    Your understanding is correct on the conductivity shift and proximity. For the inductance measurement, the shift is much smaller, as the eddy currents will still flow in the target, however with a deeper skin effect when the conductivity decreases. As for the resistance of the wires to the sensor, they will affect the sensor signal amplitude, but not shift the sensor resonance frequency very much.


    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    TI__Genius 14270 points
    Hello Stefan,

    I would like to add that for an absolute linear position sensing application, use of a differential sensor construction with two sensors is recommended. Otherwise, Z variations and temperature variations may appear as linear position shifts.

    Regards,

    ChrisO