FDC1004: Length of CIN traces and layout verification

John N

Part Number: FDC1004

Hi, I had a couple of questions. I was wondering if someone could check and verify my design/ layout? Want to make sure the SHLD connections as well as the traces in parallel with the CIN traces make sense. Also, my next question is about trace lengths connected to the CINn inputs. Does it need to be matched? For example, the length of my CIN1 trace from the FDC1004 pad to a connector is 35.63mm vs for CIN3 the trace length is only 16.9mm. Will this present issues when the measurement is taken? I plan to do a differential measurement. Any feedback would be much appreciated.

Thanks!SENSOR_BOARD.zip

over 1 year ago

0 John Miller - Sensing over 1 year ago

TI__Mastermind 40295 points

John,

Thanks for your post and your interest in our products.

If you haven't done done so already, please take a look at our app note Capacitive Sensing: Ins and Outs of Active Shielding.
It provides some guidance on best practices for layout when you are using the FDC1004 active shields.

Another app note that might help with respect to the different trace lengths on your input is How to Calibrate FDC1004 for Liquid Level Sensing Applications.
It is oriented towards liquid-level sensing, but the principles can apply to other applications as well.

If that doesn't help, please reply via this thread and we can move to next steps.

Regards,
John

0 John N over 1 year ago in reply to John Miller - Sensing

Prodigy 20 points

Hi John,

Yes, I looked at both app notes and followed these guidelines when it comes to the layout. I saw another member post asking for a review of layout so thought I would do the same. Just wanted some feedback. I did not see anything regarding trace lengths in the app note however. Some more clarification on this would be nice.

Thanks,

John

0 John Miller - Sensing over 1 year ago in reply to John N

TI__Mastermind 40295 points

John,

The main effect of trace lengths is if they cause too much parasitic capacitance which cause an offset that can compromise your final measurements and the measurement range.
If the parasitics are small, the desired measurement range is not too large, and you can perform a calibration step, they shouldn't cause any problems.

We will review your layout, focusing on the routing and shielding of the input traces, ad provide an update to this thread b COB next Tuesday, Feb. 6th.

Regards,
John

0 John Miller - Sensing over 1 year ago in reply to John Miller - Sensing

TI__Mastermind 40295 points

John,

Your layout looks fine. With the way you have positioned the SHLDs and INx traces, I suspect any parasitics from the longer traces will have very little, if any, effect.
You should be good-to-go.

Please let me know if you have any questions.

Regards,

John

Sensors

Sensors forum

FDC1004: Length of CIN traces and layout verification