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LDC1101: LDC1101 maximizing effective Rp resolution and SNR

Maximilian Weber
Prodigy 60 points
Part Number: LDC1101

Hi,

we have been working with the LDC1000 and the LDC1101 for a bit now. The goal is to precisely measure a distance of around 1mm to a target that is moving in axial direction.

At the moment this is temporarily realized with the Rp measurement of the LDC1000. When switching to the LDC1101 we where not able to achieve the same effective resolution nor the signal-to-noise ratio that where achieved with the LDC1000.

The sensor coil is self-wound out of copper wire and paired with a 270pF capacitor.

The LC resonator circuit is oscillating at around 3.4MHz with the targest the furthest away and at around 3 MHz with the target in the closest position.

The sample rate has to be greater than 20ksps, therefor the response time was set to 384 cycles.

Rp_min and Rp_max where determined for each LDC Chip using the appropriate EVM GUI by making the Rp-intervall smaller until it is the smallest but not saturated in the lowest Rp nor highest Rp position. TC1 and TC2 Register Values where calculated and set accordingly.

However the resulting effective resolution is still quite low. Especially with the LDC1101. And with the response time set to 384 the Rp signal becomes very noisy in comparison to the LDC1000. Why is that? What can we do to make the effective resolution greater and reduce the noise?

What are the oscillators set by the time constants actually doing? Do they affect effective resolution and/or signal-noise-ratio?

What is the difference between Sensor Frequency (LDC1000 GUI) and Sensor FMin (LDC1101) ? How do they affect the measurement?

Does the reference clock have any influence on the Rp measurement?

Screenshots of the GUI settings and the resulting Rp Measurements for both LDCs in closest and furthest position are attached.

Thanks

  • 0
    TI__Mastermind 23810 points

    Maximilian,

    The LDC1101 is usually superior to the LDC1000, as explained in this e2e answer: 

    https://e2e.ti.com/support/sensors/f/1023/p/559490/2049390?tisearch=e2e-sitesearch&keymatch=LDC1101%25252525252520Rp%25252525252520resolution#2049390

    So let's double check your register settings to see if they can be improved. 

    - Did you choose your Rp settings based on the "LDC1101 Rp Configuration" section of the datasheet?

    - What did you measure your Rp to be at min and max target distances?

    - How did you measure the Rp?

    - Does the performance improve if you change RPMAX to 6kOhms, similar to your LDC1000 settings?

    The LDC1101 sensor FMIN is primarily used for the watchdog timer. If it is set too high, then the LDC1101 may incorrectly report that there is an issue with oscillation and attempt to restart the oscillator. It should not impact the resolution or noise performance.

    The reference clock is not used for Rp measurements, so it should not have an impact.

    Finally, have you considered using an inductance measurement to measure distance? We usually find that for distance measurements, measuring inductance with PCB inductors is the most accurate method.