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Original question:

2.66mH Inductor availability and substitutions (TMS37157)

TMS37157: 2.66mH Inductor availability and substitutions (TMS37157)

William Borsum
Prodigy 215 points
Part Number: TMS37157
Other Parts Discussed in Thread: MRD2EVM

Hi All....

Referring to the original locked thread, I'm having the same problems identifying a usable antenna for a new smart tag design. With all due respect to TI, I would appreciate knowing the reasoning for effectively dropping support for this product! The ez430-TMS37157 is flat out NOT available anywhere (China sources are all SCAMS!), The READER in the eval kit shows up on the TI store as  RI-ACC-ADR2-10 for $199.  I just confirmed that it will be in stock in sometime in September, but it does not have the smart tag in the kit....so I am designing a replacement using an MSP430FR5994IRGZ chip rather than the -2744. This may be available for sale by me at some point (kborsum@dascor.com). Also, the MRD2EVM reader kit is available from Mouser for $70 or so, and appears to support the same functionality, but without the big air coil.

I had a long chat with Allan Burk (sp?) at ECIworld.com this afternoon, and he confirmed that all of the Neosid antennas are special order, and that Neosid probably doesn't stock them either--and he implied that Neosid really builds everything to order, so there may not be anything "in inventory." ECI does have a 3-axis part A3D4-2506 listed as stock, and the three axes in parallel work out to abut 2.48 mH....so this might be a usable solution--and give you three-axis support as well.  I'm still waiting on price and availability from ECI, and from Neosid in Germany as well.

Again, referring back to the original thread, the last post mentioned that he had reworked the design for a different inductance and capacitor. It was indicated that the tuning range for the capacitor in the TMS was 30uF....so I would like to confirm that the external cap should allow for a 15 uF internal cap in parallel with the external cap to allow maximum adjustment range? Discussion please!

Finally, if anyone out there has successfully implemented a design using parts from Coilcraft, or Toko, I'd sure appreciate your input. Particularly specific part numbers and capacitance values used.  

  • 0
    TI__Guru*** 128255 points
    Hello William,

    I can confirm that the ez430-TMS37157 is not being made by TI anymore. While I can't go into details or share all the reasoning, what I can say is the decision was purely due to costs to continue producing the EVM long term.

    Regarding the question about tuning, first off the unit is not uF, but pF. But yes I agree I think he was talking about the internal cap there. Yes that needs to be accounted for when choosing the external cap.

    The core of the matter is just getting the right resonance frequency which is based on the formula on Page 9 of this app note: www.ti.com/.../swra284.pdf

    I think that app note will be very useful as a resource for you while antenna tuning. This app note also mentions the internal capacitance and indicates it is part of the number resulting from the Cres calculation.
  • 0 in reply to Ralph Jacobi
    Prodigy 215 points
    Hi Ralph..
    I'd like to leave this thread open for a while as there are going to be a few more relevant questions come up. The app note you reference is a good one for this issue. SWRA382A is good too, but focus's on non-available Neosid parts. SCBA027 is another good one on passive antenna tuning.
    For example, directional sensitivity. I am assuming that one coil on one axis is going to be more sensitive on its axis that to either of the cross axes. This implies three orthogonal antennas, which means three inductors in parallel--such as the ECI part mentioned before. The issue is then getting the correct total inductance and the right capacitor to resonate close to the target frequency. I can do almost the same thing with three separate antennas from CoilCraft. Do you see any issues with doing this? And are there any benefits or drawbacks to the three axis approach, such as and overall loss or increase of sensitivity?
    Next is the target frequency. If I understand the operation of the TMS chip, it really operates at two frequencies....~135 and ~125 Khz. Should resonance target one or the other of these frequencies, or somewhere in between?
    Best Regards,
    Kelly
  • 0 in reply to William Borsum
    TI__Guru*** 128255 points
    Hello William,

    No problem with leaving the thread open :)

    The focus are on Neosid parts but that isn't what is important. The antenna maker just gives a completed product, but what affects the tuning is the inductance of the coil. It doesn't matter what vendor a coil comes from as long as it is tuned correctly. Antenna tuning is the key for performance.

    As far as what you are talking about with three coils, I don't think you have the right idea there. Or rather, I've never heard of such a configuration as you are describing it. What you could try and do to use three coils would be to use a multiplexer to switch between each of them? But even that is unheard of for a transponder! Only readers go to that level of effort at times, and even that is rare. Much more common is the base expectation with RFID that a transponder needs to be put within a certain area to be read. To actually use on the transponder, which needs to find itself inside a reader RF field, some multiple antenna configuration makes no sense to me. The only tangible benefit would be to not need to orient the tag to maximum read distance, but that is hardly worth the size and cost to boast multiple antennas! Keep in mind the transponder is going to load modulate the reader RF field, not make it's own.

    For target frequency, you want 134.2 kHz. The ~124 kHz number refers to the high bit frequency for the FM response. But the reader specs in the datasheet state clearly it should be used with a 134.2 kHz reader, and that is what you want to resonate with and therefore what your center frequency should be based on.