LDC1614EVM: Inductance of the coil

Hi Chris,

Thank you for valuable response.

In this case conductivity of the metal and skin depth should not change the inductance and frequency values because I am not using any target (metal). The values which I have shown here are without any target interaction (as shown in setup). I agree that parasitic inductances and capacitances are not included in the calculation but does these parameters alters the values by more than 35% of the true inductance?

Is there any document which can guide me to choose the series inductance value for tiny coil?

Regards,

Tousseef

Hi Tousseef,

What is the tolerance of the fixed inductor? Did you measure it before inserting into the system?

We recommend using a shielded inductor for the fixed inductor.

In addition, did you adjust the Glitch Filter setting in the GUI - the nominal setting is 3.3MHz; for your sensor you should set it to 10MHz.

Regards,

ChrisO

Hi Chris,

Tolerance of the inductor is 2%. Inductor used for testing is 1206CS-561XGLB. I didn't measure the inductance but I tried with multiple inductors of same value still I am getting the same inductance and frequency values in GUI. Glitch filter setting had set to 33MHz and tried changing it to 10MHz but there is no change in the  L and freq values.

Hi Varn,

Frequency on the INx pin is exactly equal to the GUI frequency. It is 4.4MHz and amplitude is 1.64V. As you told the inductance of the coil Q is close to the value in GUI , is anything wrong in the calculation?

I hope it will not affect the performance of the LDC. One more query,

What should be the minimum change in inductance due to the target interaction to avoid false detection ?

In our application the change in inductance due to target interaction is 0.070uH . I guess it is very less. If so what should the minimum change?

Hi Tousseef,

The inductance change seen as a conductive target comes closer to the sensor is a factor of the sensor size, target size, target conductivity, and target distance (distance between sensor and target). You get the best sensitivity when the target distance is less than half the sensor coil diameter.

So to improve sensitivity you can either use a bigger coil (I am assuming the target is larger than the sensor?) or move the target closer to the sensor. Is the target copper or aluminum? Targets with higher conductivity will provide a better response.

As far as your setup is concerned, I would look at the raw output code from the LDC device and look at the code jump as the target moves into the detection range. If the code jump you observe is larger than the noise floor ( 6*std dev) then you are probably okay.
At 0.07 uH inductance shift you are still well above 10,000 PPM in inductance changes which is well within the measurement capabilities of the LDC devices.

Regards,

Varn.

Hi Tousseef,

It also looks like your inductor may not be shielded; it is possible that the higher inductance you are seeing is due to coupling between the sensor and the fixed inductor.

Regards,

ChrisO

Hi Varn,

Target used is rectangular steel dome of size 5.59mm x 4.19mm and the coil diameter is 6mm. Target is very closer to the sensor. We are using a thin (0.2mm) plastic sheet in between metal and coil. Dome can make a movement of 0.30mm. Picture shows here the raw code without switch press and with press.

First capture is for when pressing the switch and the other is for no press

Hi Chris,

Yes , series inductor might be the reason. I will test with shielded inductor once I get it and I will share the results.

Thanks,
Tousseef

Hi Varn,

Tried with multiple metal domes differing in diameter. With all the metal domes code jump is less than 6*std dev. Please find the below results.

Ls=1uH,C=220pF, Coil=6mm
Metal dome	Change in code	Std dev	Noise (6*std dev)
1	603667	219181	1315086
2	911173	359367	2156202
3	274870	115675	694050
4	180244	67748	406488
5	496400	193997	1163982
6	233238	96222	577332
7	434630	175895	1055370
8	332598	127080	762480
9	261697	101362	608172
10	584425	216604	1299624
11	1331569	508985	3053910

If this doesn't work, how to reduce standard deviation ?

Thanks,

Tousseef

Hi Tousseef,

Were these measurements done with a shielded inductor?


Regards,

Varn

Hi Varn,

Yes, these measurements are done with a shielded inductor.