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LDC1614EVM: URGENT - Tuning of kit

Steve Jarvis
Intellectual 600 points
Part Number: LDC1614EVM
Other Parts Discussed in Thread: LDC1614, , LDC1314

Hi,

we are building a new instrument for array testing. Each system uses 32 of the LDC1614 (one for each channel). In order to test if the LDC1614 provides good performance we bought a Eval Board (LDC1614EVM) but so far we could not get it to work correctly.

Main issue: the value displayed by the EVM PC software (TI's Sensing Solution GUI) only varies few hundred points instead of the full 12 bit range. We tried following the instruction of the datasheet but maybe done something incorrectly.

Goal: to have the output (on the TI's Sensing Solution PC software) to swing full 12-bit scale with out inductor's value ranging from 79 to 81 uH

Can you please:

- outline the detailed configuration of the guy and the EVM

- the capacitor value to use in parallel with the inductor

- how to set the frequency ideally to 200KHz

Here are the full details of the coil and of the applicaiton/system:

- coil details: 8mm ID diameter - 79 turns of 0.8m wire - 82 uH inductance (at rest) - 7.9 ohms

- the inductance variation we are trying to measure varies from 79 to 81 uH

- ideal working frequency 200KHz

- capacitor value: please advise

Because of the urgency and tight timescales, in order to avoid going back and forth with additional questions could I ask you to please outline all the calculations/procedures. Please not just a cut and paste from the datasheet because we already tried all described in there and clearly could not get it to perform correctly.

Many thanks

Riccardo

  • 0
    Intellectual 600 points

    Hi,

    can someone please answer this? As mentioned in the title it is urgent and we have been working through New Year's eve and New Year's day trying to solve this. Since we posted this over 80 threads in this Sensor forums have been answered by TI employees but not this one. 

    Much appreciated. :)

    Many thanks 

  • 0 in reply to Steve Jarvis
    Intellectual 600 points

    Anybody... PLEASE??? It is holding up several departments. 

  • 0 in reply to Steve Jarvis
    TI__Genius 17440 points

    Hi Steve and Riccardo,

    We were on vacation over the holidays and unable to answer your questions. I am back in office now and can help you.

    First, can you clarify if you are using the LDC1614 or the LDC1314? The LDC1614 has 28 bits of resolution and the LDC1314 has 12. They share the same datasheet. 

    In general, if you are using the LDC1614 and seeing a poor target response, either your drive current is set incorrectly or your target characteristics (including sensing distance) are not ideal. Drive current is discussed below. I recommend reading LDC Target Design in the FAQ for more help with target characteristics.

    I also strongly recommend reading the inductive sensing FAQ in my signature below. It should answer many followup questions and includes helpful tools like the LDC Tools Excel Calculator.

    - outline the detailed configuration of the guy and the EVM

    For the most part, the default settings in the GUI will work for most applications. When changing the sensor coil, the most important thing to change is the sensor drive current. This application note should help with how to set that. You should also change the deglitch filter so that it is the smallest value that is greater than your maximum sensor frequency. Also change the sensor capacitance value to match the value you are using. Finally, change RCOUNT to satisfy whatever sample rate you need. The equations relating RCOUNT and sample rate can be found in the datasheet. Note that the device sample rate will change, but the rate that the MCU reports data to the GUI will not change. I also recommend this resolution application note for fine tuning.

    - the capacitor value to use in parallel with the inductor

    This depends on your coil characteristics. Selecting the capacitor will set both your sensor frequency and initial sensor Rp. I recommend using the Spiral Inductor Designer to estimate your sensor frequency and Rp, both of which must satisfy the range specified in the electrical characteristics table in the datasheet. You can also measure your sensor Rp using these methods. Note that resolution is maximized when the sensor frequency is between 400kHz and 6MHz.

    - how to set the frequency ideally to 200KHz

    Select a capacitor value that sets the resonant frequency of your LC tank to 200kHz. You can use the quick calculator in the table of contents page of the LDC Tools Excel Calculator. Note that high sensor frequencies will yield better resolutions, and that the sensor frequency will change as the sensor inductance changes.

    Best Regards,