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LDC1000: 5 V dc on cfb pin

Markus Schyma
Prodigy 70 points

Hello,

im having a hard time finding the correct filter capacitor value to obtain the 1 Vpp on the cfb pin.

My Setup is as follows:

  • LDC1000 eval board
  • ferrite coil, L0 = 2,6 mH, ferrite core U67/27/14 (L0 is inductance without target)
  • tank capacitor C = 96 nF
  • Rpmin = 820 Ohm (measuerd with impedance analyzer)
  • Rpmax = 10 kOhm (measuerd with impedance analyzer)
  • hence the resonance frequency is somewhere around fres = 10 kHz

In the GUI:

  • Rpmax set to 21.547 kOhm
  • Rpmin set to 0.798 kOhm
  • Sensor frequency set to 10.16 kHz 
  • Response time set to 6144
  • Amplitude set to 4 V
  • Sensor Capacitor set to 96000 pF

When i probe the cfb pin, all i can see is 5 V dc. I tried different filter capacitors from 330 nF down to 3.3 pF but all gave me 5 V dc on the cfb pin.

To check my measurment equipment, i used the eval board with its original pcb coil an tank capacitor, here i can see the 1 Vpp Signal, so i assume my scope and probes are fine.

Another thing i recognized is, when i switch between the original eval board and my test setup with the ferrite coil, the field in the GUI that is named "Status: Data Ready (DRDY) Status" changes from "Data Ready" to "Data not Ready".

Kind regards,

Markus Schyma

  • 0
    TI__Genius 14270 points
    Hello Markus,

    You need to set the timescale of the scope so that you can see around 100 cycles of the sensor frequency. So you will need to use a very low sample rate on you scope. When you say you are setting the sensor frequency to 10.16kHz, are you referring to the Min Sensor Freq register setting? If so, that may not be a low enough - you should set the min freq setting well below the minimum sensor frequency.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 70 points

    Hello Chris,

    in the LDC1000 EVM Quick Start Guide it is recommended to set the oscilloscope to about 1/5 of the oscillation period instead of 100 cycles!?

    I was able to achieve the desired waveform on the cfb pin by adjusting the tank capacitor so that the oscillation frequency is about 25 kHz.

    When i said i set the sensor frequency to 10.16 kHz i was referring to the drop down menu in the gui. I have now found a document that says this frequency should be set about 20% below the resonant frequency, that wasn't obvious at first.

    Regards,

    Markus Schyma

  • 0 in reply to Markus Schyma
    TI__Genius 14270 points
    Hello Markus,

    The amplitude on the CFB pin needs to be viewed over a full modulation cycle of the sensor. By looking across 100 cycles of the sensor, you should have multiple modulation cycles, and have a good measurement of the CFB pin range. My apologies for the unclear instructions in the quick-start guide.

    Regards,

    ChrisO