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OSC status - oscillator overloaded and stopped

John Smith13
Prodigy 180 points
Other Parts Discussed in Thread: LDC1051

Hello,

In our application we use LDC1000 to measure rather small inductance (nH).

We have connected LDC1000 to MCU and are able to communicate over SPI.

We can read/write values and also read of defaults returns values that defined in spec.

We are able to measure inductor that supplied with LDC1000 EVB.

But when we connect our inductor we get status register 0xF0, that means "oscillator overloaded and stopped" and read all zeroes from frequency and proximity data registers.

We have tried to reduce RPmin to the minimum, but this doesn't helps.

The interesting part of this brainteaser, that if we connect our inductor to PC (EVM) we are able to measure inductance, e.g. we can read values and status is OK.

When we check frequency of osculation with both inductors we see the same picture on EVM and in our setup.

You can see below two snapshots, one form inductor supplied with EVM while another one from our inductor.

EVM inductor:

Our inductor:

And again, we see the same pictures on EVM, but in our setup we receive overload error, while EVM works without problem.

What we are doing wrong ?

Thanks,

John.

  • 0
    TI__Genius 13650 points

    Hello John,

    the overload flag means that the sensor impedance is too low. This is the result of a hardware fault (for example short circuit), or of an incorrect sensor. By looking at your scope plot, it seems that your sensor frequency is too high (ƒsensor_MAX is 5MHz for the LDC1051). Reduce the sensor frequency by changing the sensor capacitor. When choosing a sensor capacitor, also ensure that the minimum sensor Rp is within the specified range (798Ohm to 3.93MOhm for LDC1051). 

  • 0 in reply to Ben Kasemsadeh
    Prodigy 180 points

    Hi Ben,

    I understand what you are saying, but what I can't understand why the same inductor works without problem on EVM and refuse to work in our setup ?

    Do you have any thought what can be wrong ?

    Thanks,

    John

  • 0 in reply to John Smith13
    TI__Genius 13650 points

    Hi John,

    are you sure that the sensor capacitor has the same value in both setups? The sensor frequency is 1/(2*pi*sqrt(L*C)). Your scope plots show different oscillation frequencies (3.4MHz and 9.6MHz), which indicates that either the inductor, the capacitor, or both have different values. 

  • 0 in reply to Ben Kasemsadeh
    Prodigy 180 points

    Hi Ben,

    May be I have not explained this well in first post, but the screenshots are made for different inductors. And this the reason for different resequences.

    Please check description above the picture. First picture is for inductor supplied with EVM. Second picture for our inductor.

    In both setups I have the same capacitor (the same one that soldered on EVM board).

    So again, in our setup we change only inductor and it works for bigger value, but don't works for smaller one.

    On EVM both inductors can be measured without problem...

  • 0 in reply to John Smith13
    TI__Genius 13650 points

    Hi John,

    ok, I understand. 

    The issue is probably caused by exceeding the bandwidth of the LDC. Try reducing the oscillation frequency below 5MHz by increasing the capacitor size. Oscillation frequencies above 5MHz are not supported as per LDC1051 datasheet: