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LDC1614: LDC 1614 Inductance Limit & Coil Shape

Sam Ref
Expert 1610 points
Part Number: LDC1614


Hello

We purchased one of your LDC1614.

We had a large iron-core coil has an inductance of ~1.4H.....when we get it close to a piece of metal this inductance reaches 1.9H. This experiment failed, while using a PCB spiral coil managed to give more useful results.

Questions;

- Do we have to use spiral coil with your LDC or can we use any coil?

- Is there a limit on the inductance we can measure using LDC1614?

- We understand that this LDC can detect a change of frequency of ~0,2Hz. We understand also that there is a range for the resonance frequencies that this LDC can work with and hence we choose the correct capacitor. Do you have any explanation for the lack of success (apart from the iron-core coil and large inductance, if they were a reason).

Regards

Sam  

  • 0
    TI__Expert 3665 points

    Hi Sam,

    Thanks for reaching out. I dont think we have ever tried with an iron core coil all of our EVMs and design examples assume some type of PCB coil. Both size and shape really depend on the application.

    The limiting factors when designing a coil for the LDC1614 can all be found here in this application note, i would recommend you review this material.

    Sensor Design: https://www.ti.com/lit/an/snoa930b/snoa930b.pdf?ts=1595959316708&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLDC1614

    App Note on Common Applications:https://www.ti.com/lit/an/slya048a/slya048a.pdf?ts=1595959312953&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLDC1614

    When designing a coil you must verify 2 things, that the resance frequency of your LC tank is withing the operable range with and without a target, for LDC1614 thats 1kHz to 10Mhz. The Rp impedance of the sensor is also within the recommended range 1kOhm to 100kOhm. The challenge i see with such a large inductance is the Rp resistance being too high that the IC driver cannot source enough current and that you will need a very small capacitor to maintain a resonance frequency greater than 1kHz with a target in place. 

  • 0 in reply to Julio Apps
    Expert 1610 points

    Hi Julio...many thanks for the clear answers. Regards, Sam

  • 0 in reply to Julio Apps
    Expert 1610 points

    Hi Julio,

    One more question; ..if we place the coil far from the LDC IC would that be ok?  We want to place them about 6m apart

    Regards

    Sam

  • 0 in reply to Sam Ref
    TI__Expert 3665 points

    Hi Sam,

    6m is a very far distance, i wouldn't recommend LDC for this application if the coil acts as a remote sensor.

  • 0 in reply to Julio Apps
    TI__Expert 3665 points

    Sam,

    Can you give me more details about the application? Maybe i can point you to an alternative solution.

  • 0 in reply to Julio Apps
    Expert 1610 points

    Hi Julio,

    • If 6m is too far...what is the maximum distance between the coil and the MCU that you recommend? and what is the reason behind this limitation?
    • Another question; we are now using another iron core coil with 60mH inductance and we ensured impedance of ~16 KOhm within the range you recommended (1 KOhm to 100 KOhm).. However, we had no success. Any explanation?
    • One strange observation. In experiment we did the following;

    1. We used inductometer to measure the inductance of the coil and we found it to be ~60mH. 
    2. We placed the coil close to a piece of metal and we measured the inductance again using inductometer and found the inductance to be ~80mH
    3. We connected the coil to your LDC (without any capacitors yet) and we repeated step 1 and step 2 and we found that inductormeter readings changed dramatically (~60mH became ~20mH and ~80mH became ~25mH).

    Any explanation?

    Regards

    Sam

  • 0 in reply to Sam Ref
    TI__Expert 3665 points

    Sam,

    6m as in ~20ft? The magnetic field present 20ft away from the coil will be very weak and you will see little to no change. We get the best results when the measured distance is within 20% of the sensing coil diameter. I really cant comment on your issues, this is something we have not tried before so i am going to give you the resources necessary for you to test. 

    Have you used the calculator tool to adjust the capacitor and or settings of the LDC1614? You can find this in the product folder for this device but i really dont think you will make this work trying to measure a target 20ft away.

  • 0 in reply to Julio Apps
    Expert 1610 points

    Hi Julio,

    There is misunderstanding.

    The coil is 6m away from the LDC1614-Evaluation Module.....that is, the coil and the LDC1614-EVM are 6m apart and they are connected with 2 wires that are 6m long each.

    The airgap between the target and the coil is 20mm. We respected the ratio between the coil diameter and the air-gap. The iron-core coil external diameter is 60mm.

    Can you please address our last set of questions again after we cleared that misunderstanding.

    Regards

    Sam

  • 0 in reply to Sam Ref
    TI__Expert 3665 points

    Hi Sam,

    Ahh ok thank you for verifying, the 6m leads back to EVM become part of the coil and will affect the reading. The device is not meant to drive a remote sensor that far away but lets check a few things first.  Can you check the inductance and Rp of the coil again including the 6m leads. Also, where are you placing the capacitor of the coil, is it at the inductor or at the EVM? It would be best if it was at the inductor when you do these measurements.

    Can you also take a few scope plots of the sine wave at the coil  and at the inputs of the device on the EVM.  Please add a frequency and amplitude measurement on the scope.To do this measurement you will have to add a 1kOhm resistor in series with the probe to not load the LC tank.

    Please verify that you are providing me the correct Rp value it will be easier to do tha calculations in our excel tool. Rp is described in section 1.2 of the following document: https://www.ti.com/lit/an/snoa930b/snoa930b.pdf?ts=1596475791947&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLDC1614-Q1

    With those measurements we should be able to tell what is going on and it will allow me to analyze it further to address your questions. Please let me know if you have any questions on this.

  • 0 in reply to Julio Apps
    TI__Expert 3665 points

    Hi Sam,

    I cam across this E2E post from a previous engineer that i think will be very helpful for you. Please take a look:

    https://e2e.ti.com/support/sensors/f/1023/t/683289?LDC1614-Noise-impulses-in-the-output-values

    Seems like a larger RCOUNT setting and placing a cap at the EVM so that the long wires become part of the LC tank was best solution to reduce noise.

  • 0 in reply to Julio Apps
    Expert 1610 points

    Hi Julio

    That link was useful. Thank you. We will do some experiments before further discussion.

    In the meantime I have two questions; In earlier correspondence with you mentioned recommended range of RP. Can you please give us an IT document that mentions that recommended range?

    Also in the above-mentioned link there is a recommended range for the capacitor to be used. Can you please give us an IT document that mentions that recommendation?

    Regards

    Sam

  • 0 in reply to Julio Apps
    Expert 1610 points

    Hi Julio,

    Thanks for your response and for the link. They were useful.

    • We will do some testing and we will revert back to you.
    • In the meantime;

    1. In your response above there is a recommended range for Rp. Can you please refer us to a TI document that states that recommended Rp range?
    2. In the link you sent us I found that there is a recommended range for the Capacitor. Can you please refer us to a TI document that states that recommended capacitance range?
    3. We are confused about the definition of Rs. We assume that it is the resistance of the coil, the normal resistance that is usually constant and we usually measure it using multi-meter. However, here because we have high frequency signals skin effect is present. Therefore, the resistance changes with frequency. Correct? If so the change in that resistance due to skin effect is usually a percentage only of Rs and in case of coil wires that is 1mm in diameter will be even less of that percentage. The question is; can we consider Rs in that case to be the static value of Rs. If not can you please refer us to a document that allows us to find Rs in a straight forward manner, as we have different coils.

      Regards,

      Sam

  • 0 in reply to Sam Ref
    TI__Expert 3665 points

    Hi Sam,

    The Rp valid range can be found in page 6 of the LDC1614 datasheet under the sensor specifications. There is no hard range for the Capacitor so long the capacitor type is stable over temperatures and the resonance frequency of the LC tank is within the 1kHz to 10Mhz range.

    In regards to your third point you are correct, we like to look at the parallel resistance which is easier to determine what sensor current is needed to for a given sensor peak voltage. The LDC Calculator in the product folder can help calculate this value and if you need clarification on how it is take into account the document below on Sensor Design starting in page 3 is a great resource.

    I look forward to the result from your testing.

  • 0 in reply to Julio Apps
    Expert 1610 points

    Hi Julio,

    Thanks for your answers. They provide further help. It seems I am still missing something though.

    I understood that Rp must be confined within a limit. To calculate Rp we need to know Rs. The question is; Is there an easy way to know/calculate/measure Rs.

    I had a quick look at that excel sheet & the pdf you sent. I do not think there is a method described there to know/calculate/measure Rs. Is there?

    Regards,

    Sam

  • 0 in reply to Sam Ref
    TI__Expert 3665 points

    Hi Sam,

    Rp will be a function of frequency, you can try doing q static measurement like you mentioned before and calculate Rp as a estimate, the equation is give in the LDC Sensor Design PDF that i outlined in my last post on page 3. You can also think about it as provindg the coil enough drive current , if you are not able to measure Rp than you can change the IDRIVE value until the sensor signal amplitude is within the optimal range of 1.2-1.8Vp.