FDC2214: Active Shield for two plate remote sensing

Dan1

Part Number: FDC2214
Other Parts Discussed in Thread: FDC1004

Hi,

I am working on a project where I need to detect if a part is dry or submerged in water.

My plan is to use the FDC2214 with the sensor plates moulded behind roughly 8mm of urethane and 2mm of some currently undecided plastic.

We are waiting on the Dev Kit to start testing, but I suspect that we will need to add an active shield.

This E2E thread gave me a great start but I’m a little confused.
e2e.ti.com/.../2984719
There are a lot of E2E threads about the active shield but I haven’t found any that answer these questions in a way I understand.

This article on driven shields is what’s confusing me. I think It’s using the approach where one of the sensor plates is grounded, which isn’t the case with the FDC2214.
https://www.electronicdesign.com/technologies/analog/article/21799482/driven-shield-enables-largearea-capacitive-sensor

1)
This is my interpretation of the circuit mentioned in the aforementioned E2E thread.
e.g. INxA and INxB each connect to a sensor plate. One of them also connects to the positive pin on an op amp with negative feedback and with the connection point being behind the LC tank circuit.
Is this circuit what I need in order to boost the capacitive input?

This is the approach I was going for:

(image from here: http://www.ti.com/lit/an/snoa935a/snoa935a.pdf )

2)
Am I correct to assume that I will still need the supporting resistors as discussed in the driven shield article?
R3 and R4 (and probably R5) prevent oscillation.
R6 helps the op amp drive the capacitive load.

Linked again:
https://www.electronicdesign.com/technologies/analog/article/21799482/driven-shield-enables-largearea-capacitive-sensor

Thank you for your help.

Kind regards,
Dan

over 5 years ago

0 Kristin Jones93 over 5 years ago

TI__Genius 17440 points

Hi Dan,

I'd be happy to help you with active shielding for the FDC2214, but can you clarify why you cannot use the FDC1004 for your application? The FDC1004 includes active shield drivers, which makes it much easier to use if you already know that your application will require active shielding.

Best Regards,

0 Dan1 over 5 years ago in reply to Kristin Jones93

Intellectual 865 points

Thanks Kristin,

The excitation frequency of the FDC1004 is 25kHz, a frequency we’d like to avoid. FDC2*1* is the only CDC on the market (that I have found) with a controllable excitation frequency which may make it the only choice for this project.

I’m not very experienced with CDCs but I’ve been reading that an active shield will boost the sensitivity of the sensor which may help us penetrate the 8mm+2mm of urethane and plastic. If we can increase the sensitivity enough, it’s also our hope that we can detect the difference between saltwater and freshwater. This product doesn’t need to know the exact salinity level, but knowing which it’s submerged in is incredibly valuable.
The absolute requirement is knowing whether the product is submerged or dry.

We have a rough 40mm x 83mm to work with for at least one of the sensors which is a much larger area than the sensors on the dev kits. That should help a little.

3)
I’ve recently seen on another E2E thread ( e2e.ti.com/.../473144 ) that the FDC2214 is designed for single ended sensors, not differential. I clearly had the wrong idea when I named this thread “Active Shield for two plate remote sensing”. Will a singled ended design reduce sensitivity?

I'm still confused about how to design the shield and the layout of the sensors. Which of the following configurations is the FDC2214 intended for?
Legend: SENSOR - GROUND - SHIELD - FR4

Sensor, ground and shield:

Sensor and shield only:

Kind regards,

0 Dan1 over 5 years ago in reply to Dan1

Intellectual 865 points

I didn't word this question as well as I would have liked, and now I've learned enough to make this thread redundant.

I've started again with a new question.

Kind regards,

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FDC2214: Active Shield for two plate remote sensing