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PCB with 4x LDC1000 giving different results

Henry3374
Prodigy 190 points
Other Parts Discussed in Thread: LDC1314, LDC1614

I constructed a PCB with 4x LDC1000 with identical PCB loop inductors and Ctanks in 4 corners of a PCB.  I assembled 2x boards and both boards have 1x sensor which is significantly different reading than the other 3.  These are in different locations on the two boards.

I measured resistance of the pcb coils and they are also quite a bit different ranging from 110 ohms to 134 ohms though oddly enough.  The PCB looks ok and the spiral inductors were generated from the TI Webbench utility.

Is there a <$1000 tool that can measure inductance for me to do a sanity check on the PCBs?

What other factors would I see different readings from otherwise identical circuits on the same board?

  • 0
    TI__Expert 6550 points
    Hello Henry,

    LDC1000 would have part to part variations. What measurement are you using? If you are using only L (not Rp), please consider switching to LDC1314 or LDC1614. They are 4 channel multiplexed devices which will allow you to reduce the variations.

    Best Regards,
    Natallia Holubeva
  • 0 in reply to Natallia Holubeva1
    Prodigy 190 points

    Yes I only use L.  Rp doesn't seem stable and I'm not sure why one would use Rp instead of L.

    LDC1614 is a better chip for my needs and also seems it needs less external components.  Is this a new part as I can't order it nor the EVM on Digikey or mouser.  Where can I get samples?

  • 0 in reply to Henry3374
    Prodigy 190 points
    I found I can order samples so that's ok. This appears like a very new part with Datasheet written in Dec 2014. Any idea when chip availability will be commonly available in larger quantities and commonly through distributors like Digikey?

    Also on the EVM, the schematic shows C20/C23/C42/C44 18pF caps on the IN0B and IN0A lines to ground. What is the purpose of these? Are they just for decoupling? Are they necessary as LDC1000 measures IN0A and IN0B with direct connection to CTank+PCB inductor.
  • 0 in reply to Henry3374
    TI__Expert 6550 points
    Hello Henry,

    I am glad the new chip would fit your system better. The IC should be available now in small or larger quantities. Please, contact your distributor for more information.
    Keep in mind that trace capacitance from the LDC to coil is different from coil to coil. On a typical PCB, the difference in trace length can make up tens of pF (which is a lot if you use a 100pF sensor cap). To avoid the mismatch between the channels, please keep the trace lenght same for each channel.
    You are also correct about the caps. In your case they might not be necessary.

    Best Regards,
    Natallia Holubeva
  • 0
    Prodigy 20 points

    I use a SARK110 for measuring inductance (and many other factors) for PCB RFID coils. It should work great for LDC coils too as long as they work at 100kHz upwards (the lower limit of the SARK110). Bought from Seeed studios. Apparently there is a new disty for USA just appointed.

    [Edit] Sark110 since September 2014 will operate down to 5kHz.  A hardware mod is available for earlier versions to do the same - mine is going to get a rework very soon!

  • 0 in reply to Andrew Larkin
    TI__Expert 6550 points
    Andrew,

    Thank you very much for the information.

    Best Regards,
    Natallia Holubeva
  • 0 in reply to Natallia Holubeva1
    Prodigy 190 points

    Is HP 8751A suitable for making appropriate measurements useful for LDC1000?  We have one of those in the lab.

  • 0 in reply to Henry3374
    TI__Expert 6550 points
    Hello Henry,

    The appropriate measurements would be the inductance, series resistance, and quality factor of your sensor.

    Best Regards,
    Natallia Holubeva