FDC1004EVM: FDC1004: fdc1004qevm

RK,

The CAPDAC acts like a negative offset on the capacitive sensors.
In other words, the output data should follow the relationship DATA = (CINn - CAPDAC).
The CAPDAC has five bits of resolution and a max value of 96.9pF, so a single increment of the CAPDAC should give 96.9/31 = 3.125pF.

I tried some quick experiments with an old EVM and the Sensing Solutions GUI, and the results seemed to track expected behavior (it always pays to check!). 
Without changing anything on the input, increasing the CAPDAC/Code causes the reported CAPDAC/Capacitance to increase in 3.125pF increments as expected, but the Measured Sensor Data/Capacitance stays constant (~0.65pF). So it looks like the GUI has an additional calculation step that accounts for the CAPDAC, something like DATA = (CINn - CAPDAC) + CAPDAC. Not sure why this is.

Does this align with what you are seeing? 

Regards,
John

Measured value doesn't make any sense with the CAPDAC. Here are the different setting on channel 2. Value with CAPDAC disabled is 9.825pF

Value with CAPDAC enable with code =0, Measured data is 11.08pf

Code= 1,2, or 3

Channel Measurement Settings

I don't know what is happening. Any help to understand this or additional settings needed will be greatly appreciated.

RK,

Some parts of the images you posted were cut off, so not all of the data is readable.
Would you please re-size & re-post the images so all of your data can be viewed?

Regards,
John

It is not cut off. slide it to the right side

Thanks RK,

Could you give a little more description/background about the scenarios and problems with the data?

regards,
John

We are planning to have FDC1004 on our control board. The capacitive sensor is connected to FDC1004 through a cable. we want to use the solution for liquid sensing. We want to detect the change in capacitance when it touches the water. We want to simulate it with your EVM. we want to compensate the cable capacitance with the CAPDAC. But looks like CAPDAC is not working properly with the EVM. 

RK,

What is meant by "...when it touches the water."
Is the sensor/electrode immersed in the water?

Regards,
John

yes

RK,

Is the sensor insulated or does the conductive surface make electrical contact with the water?

Regards,
John

it is insulated

RK,

Would you please step thru the scenarios behind the three graphics you posted.
What are the scenarios, similarities & differences that led to the three results?

Regards,

John

Your EVM uses CIN1,CIN4. CIN2 and CIN3 are not used. We soldered a 10pf cap on CIN2 in your EVM. All the graphics are from your EVM. In graphic 1 CAPDAC is disabled and measured value is 9.825pf. which is fine. In the 2nd graphic CAPADAC is selected code is set to 0, measured value is 11.08pf. In the 3rd graphic CAPDAC is selected and code is set to 2, the measured value is still 11.078pf. Even for the code values of 1, 3 the measured value is 11.07 pf. No change in the measured value. We have not connected our system. We are using your EVM only and trying to figure out how the things work!

Hi, Any help?

RK,

So far, I have not been able to reproduce your findings using my EVM.
I will try some additional experiments and will update this thread by COB on Wednesday.

Regards,
John

RK,

When you see the confusing readings, do the selections on the Configuration page stay consistent from measurement-to-measurement?

I may have an EVM that has been hacked one too many times, but I'm seeing the boxes Enable continuous multi-channel measurements and Enable behave intermittently. Unplugging the EVM and closing & re-starting the GUI seems to fix the problem.

Do you see that in your set-up?

Regards,

John

There is lot of inconsistency. I have to restart the GUI many times. Bottom line is CAPDAC is not working properly on the EVM. I gave up. Thanks for your help.