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LDC0851: LDC0851 - detection latch problem

Part Number: LDC0851

Hi everyone,

i currently working on design for sensing a metal with LDC0851. My problems is this: Output on LDC0851 is normal high, when I get close with metal bar or rod, it goes down to logic low, like it should. But when the metal rod is removed the output stays low. Why is that ? It should go back to logic high.

My design is from datasheet and from proto test board LDC0851EVM.

Design details: Working voltage - 1.8 V, Coil (Stacked 4 layer, 20mm diameter, Adj Value = 0, trace width 10 mils, Capacitor=68pF, 16 turns,)

Detection distance should be around 3 mm to 5 mm.

Please for some help.

  • Blaz,

    The first thing to do is take a look at the signals on the oscilloscope. When you get close with the metal bar and then remove it, do you see a difference in the waveforms between:

    - before the bar is brought close
    - after the bar has been brought close and then removed
  • Here you go. First picture is COM_COIL without any metal bar near. Second is with metal bar. Then the third is SENS and REF coil without metal bar and the last picture is with metal bar.

    Regards

  • Hi Blaz,
    I will run a similar test in lab tomorrow. And I will follow-up with you on what I find.

    Best Regards,
    Bala Ravi
  • Hi,

    ok great, I'm looking forward what you will find out.

    If it helps you, I did try on three out of ten board to change COM capacitor from 68pF to 560pF. Switching frequency has drop to 2 to 3 Mhz.

    The result was two working PCB panes. I did the same thing on third board and there was no change. It's still works like a latch.

    Regards.

  • Hi Blaz,
    I tried this in lab today. I got similar results except when I pulled the metal away from the sensor, my LDC0851 EVM detected it. First off, is there a reason why you're looking at LCOM pin instead of the OUT pin? If you look at the OUT pin, it should be high (Vcc) when the metal is away from the sensing range, and low (GND) when the metal is in the sensing range. From your setup it looks like ADJ pin is causing the issue (I am assuming R401 is open in your case). If you look at Figure 20 in the datasheet, as ADJ value goes lower, LRef becomes very close in value to LSense. So when the metal is brought close to the sensor, LSense goes lower in value and the change is detected. But when the metal is taken away LSense and LRef are very close in value that the change is not detected. I would recommend changing the ADJ code to somewhere between 3 to 10 (Refer Section 8.3.3). Hopefully that should fix your issue!

    Best Regards,
    Bala Ravi
  • Hello Ravi,

    very much thank you for your time. The ADJ value in our case is 0Ohm so that is directly connected to GND. Id We will try to test with different ADJ code.

    I will let you know, if that will fix our problem. 

    Regards

  • Hello again,

    so I did test with different values of R1 and R2. R1 is as it is, so 49,9kohm and for R2 the best for no latching is 1kohm. That'h give me around 1.8 mm to 2.2 mm of detection distance. I did't change value of COM capacitor. I did try but, the result of changing of capacitor was worse detection distance.

    I did this on three circuit and the result was always the same. So this problem is kinda resolve. In final version, I will probably change size of coil, to increase the detection distance to at more then 2.2 mm.

    Thanks again for help. 

    Regards