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LDC1612: Oscillation stability

Pramod Variyam
Prodigy 20 points
Part Number: LDC1612

We are trying to use LDC1612 for distance measurement. Below are the settings

-- RCOUNT 0xFFFF

-- FREF Divider = 1

-- FREF = 40MHZ

-- FIN: Looking at the scope we are getting close to 550kHz

With these setting per some application notes we should get less than one HZ of resolution. But we seem to have code variation (repeatability) of +/-200 codes. Any way to get this more stable? 

-- We are using both sensors: Will using only one sensor make the performance more stable?

-- If there is inherent variation in LC frequency, what might be the root cause? Power supply variations? Or any other factors

-- If we reduce the parallel capacitance (currently at 390pF) it will increase the frequency, will it improve stability?

-- Any other ideas?

  • 0
    TI__Mastermind 33795 points

    Pramoud,

    How are you getting the +/- 200 code variation?
    In other words, how precisely can you position the target?

    You mentioned a 550kHz signal on the oscope. 
    Is that the sensor signal? 
    If it is, the data sheet recommends a 5MHz to 10MHz sensor signal, with an additional comment in the footnotes that it should be at least 3MHz. 

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 20 points

    I am moving my target to a complete stop against metal. So I think the target is at a constant place. There is some doubt about the stability of distance between the target and the PCB. Do we have any way to quantify the sensitivity of the Z distance to the sensor reading?

    550KHZ seen on the sensor

    Can you please let me know which page in the data sheet recommends the 5MHZ to 10MZ and minimum 3MHZ?

  • 0 in reply to Pramod Variyam
    TI__Mastermind 33795 points

    Sorry Pramoud.
    I was looking at the wrong data sheet.
    The LDC1612 data sheet does not have the sensor frequency specified, as you noted.

    What I meant about the precision is how precisely can you change to position of the target with respect to the sensor.
    For example, can you move a precise distance (such as 10um increments), and is it repeatable?
    In other words, can you:
    1. Position it at a distance from the sensor, say d0, note the output code (c0)
    2. Move it 10um closer to the sensor and note the code (c1).
    3. Move the target back to d0, and get the original code reading c0.

    You mentioned you are positioning the the target against metal.
    What does that mean? Can you send a diagram of your test set-up?

    Regards,
    John