This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LDC1612: Optimal design for close proximity target distance measuring with high immunity to temperature drift

Part Number: LDC1612

Hello Community,

we are designing a sensor to measure the flexing of a circular stainless steel membrane under changing pressure conditions in a closed container.

The membrane is of circular shape with a diameter of 55 mm. The membrane will flex according to the pressure difference inside and outside of the container (in the container the pressure will be varying between 0.1 mbar and 1000 mbar).

The maximum distance in the center of the membrane from the sensor coil will be about 0.9 mm the minimum distance will be 0.5 mm.

So our goal is to maximize the resolution of the measurement for the delta of 0.4 mm between both membrane distances.

The movement of the target (the stainless steel membrane) is orthogonal to the sensor plane. The PCB Coil and the membrane are positioned centered above each other.

The measurement frequency does not really matter for us, we would be fine with 10 Hz or even less since the application is more long-term monitoring the pressure curves with alarms being set when defined thresholds are exceeded.

We did a smaller scale prototype with a 25mm membrane and a 14mm coil on the PCB (single layer coil on the side of the PCV that faces the membrane) with 18 Turns and a calculated oscillation frequency according to the we-bench tool of 5 Mhz (with a 400pF capacitor). The prototype showed that it is absolutely a working solution yet we would like to increase the resolution as much as possible in general and also perform everything possible to compensate for temperature drifts. The planned temperature range for the sensor that might be exposed to external conditions could be anything from -10 to +80 degrees °C.

While digging through the excellent application notes for LDC sensors we already gathered a lot of input, yet we still could not find the right answer in regards to coils size when you only want to measure a rather narrow band of distances.

Application notes say to chose the largest possible outer diameter of the coil and keep the inner diameter also rather large (ratio dinner / douter> 0.3) but then there are also some excuses from the rule where a smaller inner diameter seems to provide a higher resolution (as an example close proximity metal-touch-button applications are mentioned but without further details).

So we could increase the outer diameter to something like 40mm if needed, yet we actually want to focus on the movement of the center of the membrane since the change in distance will be the highest there (the closer we come to the border of the membrane, the less (orthogonal) movement / change in distance we will have).

So here are finally my questions:

1.) what is roughly the recommended outer and inner diameter for the PCB Coil when we want to measure distances from 0.5 mm to 0.9 mm only? Is it beneficial in general to build the sensor in a way that the minimum expected distance to the target is as low as possible (in our case 0.5mm) or would it be better to increase the minimum distance a bit?

2.) from the application notes I figured that we should use an external Clock Signal (40 Mhz for two channels/35Mhz for a sig) rather than the internal Clock generator and try to reach a max coil frequency of about 3-5 Mhz to still keep a somewhat high sensor resolution (high amount of individual output values / steps)

3.) Should we go for a single PCB Coil (on the PCB layer facing the target) or would it make sense to use a two-layer coil (bottom and top layer of the PCB) instead?
3.1) Also when talking about temperature compensation it seems more suitable to only use a single layer coil to eliminate coil distance changes due to temperature changes

4.) In general, I checked the Application Report SNAA212A (LDC100x Temperature Compensation) for input on the reduction of temperature drifts, my "conclusion" was that the following should be considered:

  • use a single-layer coil to eliminate the influence of the PCB expansion over a wide temperature range
  • use an external clock signal
  • use capacitors with a low-temperature variation (C0G/NP0 capacitors)
  • use the same material for Coil and target (not an option for us, we have to stick to a stainless steel membrane)
  • the distance from the coil to the target seems to be relevant in general too (quote from AR "LDC100x Temperature Compensation"):
    When a target is in proximity of the coil (<50% of the coil diameter distance), temperature effects on the
    mutual inductance need to be evaluated.
    A temperature variation changes resistivity, and consequently eddy current distribution in the target. This
    change in eddy current distribution impacts mutual inductance. The magnitude of the impact depends
    greatly on the distance to target as well as frequency, and is on the order of tens of ppm when the target
    is very close to the coil, quickly dropping to single-digit ppm when the target is at a distance greater than
    20% of the coil diameter.
  • So it seems we should try to adapt the design to ensure that we have a minimum distance between target and coil of 20% of the outer diameter. Currently, we have a 0.5 mm minimum distance but it should rather be 4mm for a 18mm coil if I get this right, which might not be an option for us since we need to build a rather flat sensor. We can probably go to a minimum distance of 2mm, but we are not sure if this actually negatively impacts the resolution of the sensor (the max delta between the highest and lowest distance would still be 0.4mm)
  • Also, the frequency should be rather lower than higher in general. According to the AR 1 Mhz has a much lower temperature drift than e.g. 5 Mhz 
    • On the other hand, due to the target material being stainless steel, we cannot go to low in the frequency to still get a suitable skin depth and general sensibility. I would currently aim for a frequency of maybe 3 Mhz as a compromise
  • Implement a mapping table in software in combination with a temperature sensor on the same PCB to compensate for temperature drift (this would of course be the last resort, but is certainly doable)

Are there any other recommendations other than that from your experience to reduce the influence of temperature on the measurement?

Ok, wall of text, thanks to anyone who made it this far :-) Any reply is highly appreciated.

Kind regards


  • Paul,

    Thank you for your inquiry, your interest in TI products, and your kind words on our app notes.
    I will look into your excellent questions and provide an update to this thread by COB tomorrow.


  • Paul,

    You have done a great job thinking thru the trade-offs in your application. and there aren't many comments or suggestions left to make.
    If the target material is locked in, and it becomes an issue, plating it with a metal with conductivity closer to that of the coil might be worth considering.
    Thank you again for sharing your thoughts, comments and questions.
    Please let me know if you have any questions.


  • Hello John, 

    first of all sorry for all the typos in my initial post, I submitted it a bit to quickly before performing a proper review. Unfortunately, it does not seem to be possible to edit posts, once submitted. Also thank you for taking the time to read my post and to reply.

    We thought about galvanic plating the stainless steel membrane with some copper, yet it would significantly affect production complexity and by that costs of the membrane. So currently this is not a path we are going to take. Also, I fear it might only partially address the problem of temperature drift, since we would still be using stainless steel as the main carrier material of the membrane, with the different temperature coefficients. But I am well aware that it might give us the option to potentially lower the frequency.

    Anyhow, lets, for now, say the material choice is locked, then I still do have basically two questions that remain unanswered:

    1.) Would you suggest increasing the minimum distance between coil and target (currently 0.5mm) or will this not be beneficial?

    2.) Would you rather suggest a larger or a smaller coil diameter in general for our specific application (with the rather small distances [0.5-0.9mm] involved compared to the outer diameter (14mm) of the coil we currently use)?

    I regard to the coil size I only found information in the AR about the outer coil diameters and the maximum measurable distance, but I did not find many details about minimum coil sizes especially for close distance measurement with high resolution.

    Kind regards


  • Oauk,

    It is very difficult for us to provide guidance on high-level parameters for the sensor coil and target.

    As an alternative - if you haven't tried it already - please take a look at our design tool. You can download from here.
    It allows a user to specify the coil parameters and then apply them to a system based on several of our devices.
    There is a tech note that gives some guidance on using the tool.
    Apologies for not suggesting this right away.


  • Hi John,

    the provided link to the design tool is not working (but I assume you are referring to the Excel Spreadsheet for calculation, I am aware of that and it was already of good help during the design of our initial PCB design calculations).

    Also, may I point out in general that the link to the datasheet on the product page of the LDC1612 also seems to be broken. (never mind, works now some minutes later)

    Sidenote: just figured out that it actually is possible to edit posts in this forum using the "More" -> "Edit" link below your own posts :-)

  • John, one last question: could you help me understand the differences between the LDC1612 and LDC1612-Q1? I can see that the Q1 is AC-Q100 certified and only seems to be operating on 3.3V (at least according to the cover page of the datasheet, later on in the DS it is stated that 2.7 - 2.6V is recommended) rather than the wider voltage range of the LDC1612 (2.7 V to 3.6 V), but not sure if there are any other differences?

    Can we in general just interchange the two depending on availability at our supplier?

  • Paul,

    The parts support the same Recommended Operating Conditions - supply voltages & operating temps - according to the two data sheet tables (6.3 & 7.3). If your application requires the reliability certifications you mentioned, then the Q1 is the better fit. 
    If not, it might be good to compare your critical specs for the two parts to be sure they are the same.
    If they are, then it is probably okay to use either device.