LDC1000 minimum coil diameter for sensing human finder

Masaharu Katayama

Dear sir,

What is mimimum diameter of PCB coil to sense human finger? In order to use LDC for bottons on side of smartphone, small diamater coil has to be printed like on flexible cable. Assuming vey thin casecover (plastic material) is exiting between the coil and finger.

It is proably depending on how much PCB or flex-cable can accept how many turns, pitch and width.

It will be nice If few mm dia PCB coil will work with finger around less than just 1mm distance away.

Best regards,

Masa Katayama

over 11 years ago

0 Evgeny Fomin over 11 years ago

TI__Genius 9300 points

Katayama-san,

For small pcb coils multilayer design will be required to achieve required inductance of the coil.

for two layer it takes about 5mm at 4mil pitch/width. With more layers, diameter will decrease.

You will be able to detect human finger at 1mm away.

0 Masaharu Katayama over 11 years ago in reply to Evgeny Fomin

Expert 1420 points

Dear Evgeny,

Thank you very much for your calc. The 4mil means 0.5mm pattern width and 0.5mm space between patterns of coils making 5 turns on a side (10 turns as both sides) for 5mm dia?

Masa Katayama

0 Evgeny Fomin over 11 years ago in reply to Masaharu Katayama

TI__Genius 9300 points

Hello Katayama-san,

It will be 13-14 turns coil, 0.1 width/space for traces. Slightly more than 5mm dia.

I would not recommend it, unless absolutely necessary - higher temperature and/or presence of metal close to a coil will reduce Rp beyond operational range.

4-layer or even 6-layer coil will be much more robust.

0 Masaharu Katayama over 11 years ago in reply to Evgeny Fomin

Expert 1420 points

Dear Evgeny

Thank you very much for your correction and suggestion.

I made misstake 4 mil conversion to mm. The 0.1mm is the right.

I also realzed more layer coil layer recommendation to eliminate metal presence noise and by reducing Rp.

( LDC unique advantage of combined Rp and L info).

Best regards,

Masa Katayama

0 Evgeny Fomin over 11 years ago in reply to Masaharu Katayama

TI__Genius 9300 points

Katayama-san,

You are most welcome! Please keep us posted on your progress!

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LDC1000 minimum coil diameter for sensing human finder