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FDC2214EVM: fdc2214evm vs. fdc1004evm

Alex Semeniuk
Intellectual 300 points

Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC1004EVM, , FDC2114, FDC1004, FDC2214

Hello,

I am using fdc1004evm and fdc2214evm for distance measurements and wanted to double check if I'm doing it correctly, since I have some noise issues)

Background:

Due to the nature of my applications, I need to use 1.5 m long coax cables to connect my 2''x2'' copper sensor plates to EVMs. 

The target is a 4''x6'' copper plate, grounded to the EVM's ground. EVM is connected to desktop computer/laptop (plugged in) via USB cable.

For measurements I use the default (longest) integration times and lowest count rate. 

Under these settings, the  effective measurement range is 3 cm. 

Problems:

1). fdc2214evm seems to be super sensitive to noise (even more than 1004EVM!)

If the shielding of my coax cables is floating or connected to IN0B, I pick up some stray interference, which causes my signal to drift away.

(Although in the link below it says that the best settings is connecting IN0A and IN0B)

e2e.ti.com/.../618777

The situation improves, if cable shielding is connected to the ground. 

However, even in that case I still have interference pick up through the supplied USB cable!

Indeed, when i touch or at least bring my hand close to usb cable, the signal drifts away. 

It should be noted that only channels connected to sensors with long cables are susceptible to this.

The signal from EVM board sensors is not affected by wiggling usb cable. 

At the same time, when FDC1004evm is connected to computer with USB cable, I do not notice such effect.

Would you please advise if I'm doing it all correct and how to improve noise immunity?

2).  With FDC1004evm I need to have the shielding of my coax cable connected to shielding of EVM board (not floating or grounded), otherwise it gets saturated with the cable (1.5 m).

It does not really matter what is select in gui settings for the active shielding - the result don't change much if it is disabled, set to follow the channel or used with CAPDAC,

as long as I coax shielding is connected to gui shielding. 

Is it how it is supposed to be? 

3). I noticed that they my noise gets higher when I bring my hand close to the EVM (both 2214 and 1004).

Also, currently my setup is sitting on the metal cart and I can see a signal drift on both EVM if I touch the cart.

Would you please advice how to shield EMV boards themselves? 

4). Finally, a more general question. I need to detect position changes of a conducting target (human).

Do capacitive sensors outperform inductive ones (TIDA family) in these applications?

If yes, would you please kindly suggest an appropriate EVM?!)

Thank you in advance for your help)

Sincerely,

Alex

 

 

  • 0
    TI__Genius 17440 points

    Hi Alex,

    I've answered your questions below in the same order you asked them.

    1. The FDC2114's resonance-based architecture is actually significantly more noise immune than the FDC1004's switched-capacitor-based architecture. The LC tank is inherently a narrow band-pass filter that rejects most noise, except frequencies very close to the LC tank's resonant frequency. You can read more about both architectures in this blog post. 

    I believe you are seeing noise issues because your sensor is so far from the EVM. The distance between the sensor and the LC tank should be fairly small (even 10cm can be a large distance). The INA and INB traces should be as short as possible. The long cable will add significant parasitic capacitances to the system, and as the cable moves these capacitances will shift. Even if the cable is not moving, the length makes the system much more susceptible to lower frequency EMI. When INB is connected to the shielding of the cable, the system will respond more significantly to environmental shifts. It will be very challenging to use the 1.5m cable. If you connect ground to the cable's shield instead of INB, you will lose a lot of sensitivity in your sensor.  

    We generally notice USB cable sensitivity when the system is using a virtual ground. If you plug your laptop into earth ground this issue should go away. We refer to this effect as ground shifting, which you can read more about in this app note. 

    2. You will definitely need to connect a SHLD pin from the FDC1004 to the cable's shielding. Otherwise the signal from the sensor will not be effectively shielded. The SHLD signal and the CINx signal should be at the same voltage so that there is (ideally) no capacitance between them. This way parasitic capacitances affect the SHLD signal and not the signal you want to measure. I would recommend reading section 8 of the datasheet for more information about the shield, CAPDAC, and other features.

    3. I believe this is the same issue as the USB cable issue. Try plugging your laptop into earth ground.

    4. A human is not nearly conductive enough for inductive sensors, which require very conductive materials (like copper, aluminum, or stainless steel) for their targets.

    Regards, 

  • 0 in reply to Kristin Jones93
    Intellectual 300 points

    Hello Kristin,

    Thank you for the prompt reply!))))

    Few follow-up questions: 

    1. I've checked both EVM's performance on three computer.

    One of them was a desktop and two others are laptops. 

    All machines were plugged in and I still see that ground shifting effect.

    Can it be something else but ground? or how else to check for the ground issues? 

    2). GUI does not work correctly, when laptops are on battery. 

    The power/performance settings on laptops were identical for plugged in and battery modes.

    Despite this once laptops are plugged off, gui starts acting up. Sometimes it just doesn't start.

    If I unplug laptop during data acquisition, it displays data points with huge delays and buttons become inactive (i cant even stop data acquisition).

    Luckily, measurements should be performed with laptop plugged in, which fixes everything)

    Did you see something similar?

    Kind regards,

    Alex

     

      

  • 0 in reply to Alex Semeniuk
    TI__Genius 17440 points
    Hi Alex,

    1) Sometimes this issue can still occur if the USB cables are especially long. I noticed that you mentioned you have the FDC1004 connected via a USB cable as well, which is unusual because you can plug the EVM directly into a USB port. Are you using a USB extension cable? Try shortening the cable and see if that helps. I just tested an FDC2214EVM with the USB cable that comes with it and did not see any cable sensitivity. The devices are very sensitive, so if you touch the USB cable near the EVM, you may slightly move the EVM and cause a change in capacitance.

    In addition, the FDC1004 is driven with DC voltages, unlike the FDC2214 which is driven with two half-rectified sine waves. The ground shifting issue will only occur in the FDC2x1x devices because of the way they are driven.

    2) It's odd that the power cable affects it, but the GUI freezing is a known issue. Unfortunately it does not occur on all computers, so we haven't been able to solve the bug. I believe it just affects the graphical display and not the data acquisition, so if you use the data logging feature you should be able to work around it. I'm sorry this is an issue, but hopefully plugging in your laptop continues to solve it.

    Regards,