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LDC1614: Frequency setting, Settling time of the oscillating circuit & Setting IDRIVE current

Franz Schellhase
Expert 3605 points
Part Number: LDC1614

Hi Team,

Can you have a look at the following customer inquiry, please?

Frequency setting:
It must be possible to divide the frequency range (15 kHz to 150 kHz) into 5 to 6 frequency ranges. The data sheet states that the digital output value is proportional to the ratio between sensor frequency and reference frequency. Therefore, by changing the reference frequency, it should be possible to realize a frequency adjustment of the sensor frequency, since capacitance and inductance remain constant. When the reference frequency is changed (by a frequency generator), the sensor frequency does change briefly in the Sensing Solutions EVM GUI. After a short time, however, the output frequency is restored.
Therefore the question, if a subdivision into frequency ranges by changing the reference frequency could be implemented at all?

Settling time of the oscillating circuit:
The settling time measured on the oscilloscope matches the IDRIVE current displayed in the GUI. However, this settling time does not match the value calculated by the formulas in the data sheet. Example: Rs = 0.9 Ohm, C = 20 nF and L = 100 µH. Value read on oscilloscope approx. 400 ms, calculated value approx. 770 ms. This difference increases significantly with increasing inductance (at 900 µH: approx. 500 ms read and approx. 5 ms calculated). In addition, the measured resistance Rp does not match the calculated value.

Setting IDRIVE current:
An alternative to frequency adjustment by the reference frequency is a change of the capacitance. To achieve smaller frequencies (below 50 kHz) the capacitance has to be increased strongly. By increasing the capacitance, the amplitude decreases too much that the frequency change due to a metallic object above the coil is too small for object detection. The too low amplitude is due to the fact that the maximum current is reached e.g. already at an inductance of 100 µH and a capacitance of 60 nF (frequency 65 kHz). But much lower frequencies are needed. Is there possibly another possibility to keep the amplitude constant despite increased capacitance?

Thank you,

Franz

  • 0
    TI__Mastermind 18545 points

    Hello Franz, 

    For the frequency of the LDC1614, the device tries to drive the sensor at the resonant frequency for the LC circuit. The reference frequency determines how the sensor oscillation is sampled by the device. What is the goal with trying to separate the frequency range into multiple ranges? Can you share more details about the application in general? 

    In general, dividing the frequencies related to the LDC1614 can be shown in the clocking diagram below. More information on how each divider works can be found in the datasheet. 

    For the settling time, are you using full power activation or low power activation mode? 

    When you say: 

    Franz Schellhase said:
    The settling time measured on the oscilloscope matches the IDRIVE current displayed in the GUI

    I am not sure what you are referring to. The IDRIVE sets the amplitude of the sensor oscillation while the SETTLECOUNT register values determine the settling time. What value are you using in the SETTLECOUNT registers? 

    For the IDRIVE question, what is the motivation to lower the sensor frequency? With the lower frequency you are testing, is the device in HIGH_CURRENT_DRV mode? 

    Best Regards, 

    Justin Beigel