FDC2214: INxA, INxB connection method

Hi Kaneko,

The team is out for the labor day holiday. They should be able to take a look at your post when they come back tomorrow.

Thanks,

Mekre

Hi

I found a description about single end and differential connection.
I'm doing paramtere adjustment with FDC2214 EVM.

Could you review my parameter adjustment procedure ?

Condition
measurement range : 150pF-1000pF
Capacitors are connected to for all 4ch as single end.
Use Sensing Solutions EVM GUI

Adjustement procedure

1)Exterenal L, C, and Idrive adjustment.
To achieve Vsensormin=1.2V, Vsensor=max=1.8V, change external L, C and Idrive
As result,
L=100uH
C=5600pF
Idrive=26(747uA)

In GUI, if "Detect Idrive Init" button is pushed, Idrive value is not recommended.
So i adjust Idrive by observig the voltage waveform of INxA by oscilloscope.

Measure target cap = 150pF -> INxA terminal voltage is 1.64V
Measure target cap = 1000pF -> INxA terminal voltage is 1.5V

2)Change CLOCL_DIVIDERS_CHx 13:12 to b10 because of single-end connectoin.

3)Change Input Deglitch Filter's frequecy
Set minimum frequency greater than frequency in Calculated Sensor Data.

Could you consider to answer there questions ?

- Are external L, C and Idrive value not too high or low ?
- Is it appropriate to measure INxA terminal voltage for Vsensormin/max ?
- Is there any other point that I should care ?

Regards,
Kaneko

Hello Yoshihiro,

Regarding your initial inquiry, I would expect the second capacitor configuration to have a more accurate capacitance measurement from the standpoint that both terminals are directly connected to the LC tank.  This will result in the full capacitance being in parallel with the on board 33pF.  As for the first configuration that has one capacitor terminal going to ground, I expect that its parallel capacitance is diminished by the impedance between INxB and GND.  This might further be influenced by how long the lead is between the capacitor ground and the board ground.

Your adjustment procedure seems to be ok.  However, the datasheet suggests adjusting the clock dividers before adjusting Idrive.  Also you might need to adjust settling time and set the conversion time.  Section 10.2.3.2 of the datasheet provides a series of steps for initial register configuration values.

As  the maximum sensor capacitance is 250nF, I think your capacitance value is fine.  Per the values you provided, fsensor = 1/(2*pi*sqrt(LC))=213KHz which is within fsensor min and max.  Therefore, those values seem reasonable. As your terminal voltage is between 1.2 and 1.8V, I think your Idrive value is also ok. Measuring INxA to ground is ok for a pk measurement reading.  Might want to check the probe input impedance as it could load the sensor and influence the measurements.

Hi Patrick,

Thank you for your answer.

1) Is there any limitation for external L/C ?
Current external L=100uH,C=5600pF can be changed. I need to know how to check the validity of the values.
>fsensor = 1/(2*pi*sqrt(LC))=213KHz which is within fsensor min and max
Is it enough to calculate this equation and confirm the fsensor is in min and max?

2) Is there any method to change the sensitivity dynamically ?
In my system, the capacitance is 150pF in steady state, and when an event occurs, the capacitance goes up to a maximum of 1000pF.
The event trigger need to be captured with high sensitivity by detecting the small change in capacitance.
But generally, high sensitivity would limit the measurable range of capacitance.

So, for example, is the following control possible? 
- From 150pF to 500pF, the sensitivity keeps high.
- If the sensitivity remains high, Vsensor_min/max may be deviated from 500pF or more, so change the sensitivity from 500pF to 1000pF to low sensitivity.

500pF is just an example. Need to investigate threshold capacitance for the future.

3) Is it a GUI bug that Idrive value is not recommended even "Detect Idrive Init" button is pushed ? or any setting wrong ? Currently "NaN" is shown.

Regards,

Kaneko

Ok, thank you for your support.

I will close this thread.