FDC2214: Measuring a large Number of Inputs in parallel

Sam G

Part Number: FDC2214
Other Parts Discussed in Thread: FDC2114EVM,

Tool/software:

I am trying to measure the capacitance of a large number of elements in parallel. The capacitance of theses elements will change with applied voltage but should stay in the range of 10-50pF. I do not care so much about the absolute capacitance, I am more concerned with weather or not it is consistent from element to element. I want to measure at least 16 elements in parallel. The simplest way I see to do this is to multiplex the sensor inputs. For example something along the lines of adding a 1:4 analog CMOS switch to each sensor input. This would allow me to multiplex a total of 16 measurements to one FDC2214. I would like to use the FDC2214EVM or the FDC2114EVM. However the FDC2214 only offers 4 channels, and the datasheet says "The FDC does not support hot-swapping of the sensors. Do not hot-swap sensors, for example by using external multiplexers.". What would happen if I did do something similar to what is described above? Is there a work around that would allow me to measure 16+ elements in parallel with only one evaluation module? Or is there a solution that would allow me to design my own board using only one FDC2214 or 2114 to measure 16+ capacitances in the range of ~10-50pF?

2 months ago

0 Eddie LaCost 2 months ago

TI__Mastermind 44825 points

Hello Sam,

The note regarding hot swapping is referencing activating the switch while the channel is active. If you can manage the timing so that the switching occurs in between measurements, a mux could be implemented. You can also find some good information in the thread below.

FDC2214: FDC2214

0 Sam G 2 months ago in reply to Eddie LaCost

Prodigy 10 points

Supposing I wanted to use the evaluation module, there would not be any way to synchronize the timing of the msp430 to the control pins of the mux would there? Meaning if I wanted to multiplex the input such that the channel switching on the mux occurs during a time that the channel of the FDC2214 is inactive, I could not do that with the evaluation module could I?

0 Eddie LaCost 2 months ago in reply to Sam G

TI__Mastermind 44825 points

No, there is not a method from the EVM that would enable control signals for the mux. You could likely break off the FDC + sensors section of the PCB(see image below), then connect this to an Arduino or other microcontroller of your choice where you can program it to control the mux. There is an Arduino library available already.

https://docs.arduino.cc/libraries/fdc2214/

0 Sam G 2 months ago in reply to Eddie LaCost

Prodigy 10 points

Okay, Thank you. I do have one more question. At some point going forward I would like to read the absolute capacitance, and as mentioned before the capacitance will change with applied voltage. This effect is not significant below ~0.9Vpk. Is there something I could do using external components (e.g replace the inductor with a small signal transformer) to drop the DUT voltage lower than the 1.2Vpk specified by the data sheet?

+1 Eddie LaCost 2 months ago in reply to Sam G

TI__Mastermind 44825 points

Sam,

You can lower the voltage on the sensor by adjusting the idrive configuration. Reducing the drive to less than 1.2Vpk will decrease the SNR, so a reduced resolution will be observed. It will not damage the FDC device to run it lower though.

0 Sam G 2 months ago in reply to Eddie LaCost

Prodigy 10 points

Just to be clear, what would be the risk to hot swapping? Would the chip be damaged or would it cause incorrect readings? What exactly are the consequences?

0 Alicia Rosenberger 2 months ago in reply to Sam G

TI__Genius 14580 points

Hi Sam,

Eddie is currently out of office today, but he should be back on Monday to help you with your question.

Best,

~Alicia

+1 Eddie LaCost 1 month ago in reply to Sam G

TI__Mastermind 44825 points

Sam,

I would not expect damage to the IC, but could cause incorrect or incomplete readings.

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FDC2214: Measuring a large Number of Inputs in parallel