FDC2214: Creating an Active Shield Output

***IMAGES DID NOT TRANSFER IN FIRSDT POST***

I am currently trying to design an active shield for use with the FDC2214.  The datasheet briefly touches on this, but gives no examples or tips:

My design uses a custom parallel finger sensor, constructed from etched double-sided PCB material.  Instead of liquid level, it will sense the movement of a metal bar in a similar fashion.  I first attached my custom sensor to an FDC1004QEVM in the following manner:

This sensor arrangement works very well, and the active shielding truly does direct the electric field as mentioned in the numerous application notes.  Despite the great results, the FDC1004Q will not work for my application due to sample speed and resolution.  I need the 4ksps and the 28-bit resolution of the FDC2214.  Furthermore, my application will be extremely noisy, and the narrow band of the FDC2214 combined with active shielding should give me the best SNR.

Next, I attached the same custom sensor to the FDC2214EVM.  Per the datasheet, I attempted to create an active shield by buffering the INxA signal.  My prototype looked like this:

This arrangement performed poorly.  The sensitivity was much lower than the first arrangement. When the metal bar was stationary, the data also had much worse noise issues than the FDC1004Q.

So my questions are as follows:

1)  Am I doing something blatantly wrong?

2)  Are there any reference designs, application notes, schematics, or other documents that deal with adding active shielding to the FDC2214?

3)   I have noticed the active shielding output of the FDC1004Q is "stair-stepped."  I assume this is a sine wave output from a DAC.  The INxA output on the FDCD2214 looks like a half analog sine.  Can I ever achieve the same level of active shielding with FDC2214, compared to the FDC1004Q?

Any suggestions are appreciated.

Thank you!

Hello,

Unfortunately I am not able to see the images you attached. Could you try reattaching them?

In general, the only reason to create active shielding for the FDC2214 is to create uni-directional sensing. The FDC2214 is highly noise immune and does not need active shielding to function well. In addition, the FDC2214 has a much wider input capacitance range than the FDC1004, so parasitic capacitances are not as big of a problem. 

In answer to your questions:

1) I can't tell without the photo. 

2) Most customers do not need active shielding with the FDC2214, so we don't have any documents detailing how to use it. This recent e2e post may help you (scroll to the end).

3) The output of the FDC1004's shield pins is identical to the output from the CIN pins. The FDC1004 uses a charge-based measurement and drives the sensor to multiple DC voltages. You are correct that the output from the FDC2214 is a half-rectified sine, used to drive the LC tank sensor at its resonant frequency. You can read more about those architectures here. 
Though it may be possible to achieve the same level of active shielding as the FDC1004 using external amplifiers with the FDC2214, it is generally not needed and I would not recommend pursuing it. 

Best Regards,

Hello,

I can see your images in your follow-up post. Your setup looks reasonable, though I cannot tell what the supplies are for your amplifier. You'll need to use dual supplies so the amplifier can effectively output 0V. 

Could you clarify where the metal target will be moving in your setup? Depending on the target movement it may be beneficial to use a different sensor setup.

Best Regards,

Hello Kristin,

Thank you for the blazing fast response. 

I did not add the power rails to the amplifier image because I initially did not think it was important for discussion.  However, I did run this as a single supply:

Although I used a rail-to-rail amplifier, you are very correct in stating that there should be some slight variation in buffered output around the limits.  I suppose even a slight change in shield signal would cause issues.  I will investigate this and re-post my results.

As far as the sensor and metal bar, the movement is like this:

The plate's mechanical range is from complete coverage of the sensor to no coverage at all.  It seems that the directionality created by the FDC1004 active shielding improved stability, resolution, and significantly increase dynamic range. 

1)  Do you have any other sensor arrangement suggestions that may work better with the FDC2214?

2)  If I were to eliminate the active shielding requirement, how would you wire this sensor?  Single-ended or differential connection? 

3)  Which copper regions would you connect this scheme to?  Would the differential connection go to copper regions A and B?  Ground connection to C?

4)  Should I eliminate any of the three copper regions?

In the meantime, I will alter my amplifier circuit.  It is identical to the one you posted in the old thread, except your circuit is using the dual supplies.  I will probably have to order a few components to build this prototype.  Considering that and the upcoming holiday, it may be a week or so before I accomplish this.  Please keep this thread open, and I will post my results.

Thank you. 

Hello Kristin,

Thank you for the response. 

I suppose that my hesitation with inductive sensing is past issues I have had with EMI.  I design products that are used in challenging environments.  However, I have now read the SNOA962 application note on EMI considerations, and I will give the LDC series a try. I have ordered 4 LDC eval kits and several sensor coil boards.  I will evaluate these, and if I have any more questions I will restart another thread.

Thanks again!