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FDC2214EVM: Drive_current and Rcount Settings

Guoxing Gu
Intellectual 590 points
Part Number: FDC2214EVM

Hello Application Engineers,

In order to obtain the highest resolution in a 4-channel sequential operation at the highest data readback rate possible, I used 0XFFFF (or 65535), the highest value, for Rcount setting on the FDC2214EVM. 

As for the Drive_current setting, it works fine with a wide range of current value 0.05 - 1.5 mA. The data sheet says Set such that 1.2V ≤ sensor oscillation amplitude (pk) ≤ 1.8V, however, I do not have an oscilloscope to measure the amplitude.

The application report, SNOA943, shows a chart below, a Rcount value of 65535 requires 1.5 mA current setting.

Question: Should I set  Drive_current to the highest value of 0XF800 (or 1.57 mA)?

Thanks,

Gu

  • 0
    TI__Mastermind 18545 points

    Hello, 

    The chart is showing the current consumed based on the active time of the sensor. With a higher RCOUNT setting, your sensor will be active longer and will increase the average current of the device during one sampling period. The sensor drive setting is independent of RCOUNT and does not have to be adjusted based on this setting. 

    For setting the sensor drive current, without an oscilloscope, you could try to estimate the required setting by measuring the Rp/Rs of your sensor with an LCR meter or network analyzer. Alternatively, you could try to get a decent measurement with a multimeter to help set the current drive. 

    Best Regards, 

    Justin Beigel

  • 0 in reply to Justin Beigel
    Intellectual 590 points

    Hi Justin,

    I found that different current setting gives different capacitance reading while other settings are kept the same, I graphed it as below. There is a minimum point at 169 uA, I think that the minimum point is the current I should use. The reason is that data will be more stable to current fluctuation.  Is my understanding correct?

    Thanks.

    Gu

  • 0 in reply to Guoxing Gu
    TI__Mastermind 18545 points

    Hello, 

    The lower end of the voltage recommendation is to ensure you have better SNR in your reading and that the oscillation is strong enough for the FDC to measure the sensor. The upper end is to ensure that you have consistent results across temperature. Picking a current drive based on the minimum point in your graph doesn't guarantee meeting these conditions but if you can test across temperature and still get consistent results then you should be fine to keep the current value at 169uA. 

    Best Regards, 

    Justin Beigel