FDC1004: Capacitance value given by FDC1004 decreasing over time continously

Hi Akshay,

I'd be happy to help you debug your issue with some additional information. Are you using this sensor with the FDC1004EVM and the Sensing Solutions GUI, or are you using your own system? Also, is the 130 code drop in the average code value of the channel without a target present, or is the code drop in the peak code value with a target present? Are the code drops in discrete 130 code drops, or is there a small code decay over time that sums to 130 codes per hour?

Regards,

Hey Kristin,

We are using our own system. The 130 code drop is average channel  code drop without/with target present. It is small decay over time that sums 130 codes per hour. We also checked the battery voltage level during that period and it dropped from 3.5V to 3.2V, Is it dependent on battery voltage ? As our requirement is very low power consumption circuit we have not added the voltage regulator before FDC1004.

Dear Kristin,

We tested the system with stabilized voltage supply, now it is showing 130 code drop every 4 hours. Challenge is our system gonna run for more than a year, so cumulative addition of this code drop will disturb the entire system. What might be the causes of this cumulative drop? we have added ground plane below FDC1004 thinking that might be a reason but it didnt effect. Any other idea?

Hi Akshay,

Could you provide data showing the decay over time? 130 codes every 4 hours is a very small shift, so it's possible that this is due to temperature change, which will affect the capacitance measurements. It would be helpful to see at least 8-10 hours of data. You can sample it every 1-10 seconds; we don't need to see abrupt changes, just a general trend. 

Regards,

HI Kristin,

After struggling a lot with design, we made few changes in size of the sensor and ground plane and took reading every minute, PFA the reading values for more than 10 hours. We still observe the shift in the code values, which will results into huge shift over the period of a year.

Sensor reading every Minute.xlsx

Hi Akshay,

Are there any sources of noise in your test setup? The FDC1004 (and all traditional capacitive sensors) are sensitive to noise from fluorescent lights and other sources that can affect the charge on the capacitors. I see that the data both increases and decreases, and changes in noise levels could cause that.

It also looks like the output code increases at night and starts to decrease in the morning. It could be that air conditioning is being turned off at night and the FDC1004 is responding to temperature changes. Could you provide 24 hours worth of data so that we can test this theory?

Finally, the capacitance readings seem very small. Could you share your schematic and sensor setup?

Regards,

Hi Akshay,

I haven't heard from you in a while, so I'm assuming you solved your issue. If this is not the case feel free to reply on this thread or post a new question to the forum.

Regards,