FDC1004EVM: Setting CHA and CHB for differential measurement

Guoxing Gu

Part Number: FDC1004EVM
Other Parts Discussed in Thread: FDC1004

I am testing a FDC1004EVM with a parallel plate sensor, the sensor's two plates (plate 1 and plate 2) are 5 mm apart, no shielding on any testing cable. I obtained 53 pF when Plate 1 to Pin J4.7 (CIN3) and Plate 2 to Pin J4.10 (GND).

I like to know how to set CHB When Plate 1 to Pin J4.7 (CIN3) and Plate 2 to Pin J4.4 (CIN2) for a differential measurement? there are three options for CHB when CHA is CIN2: Disabled, CAPDAC, CIN3. I tried the three options and respective capacitance values are listed below

CHA CHB Capacitance (pF)

CIN2 Disabled 15.99999

CIN3 CIN2 3.017

CIN2 CAPDAC 18.543

CIN3 CIN2 3.390

CIN2 CIN3 -3.059

CIN3 CIN2 3.149

Thanks,

over 6 years ago

0 Kristin Jones93 over 6 years ago

TI__Genius 17440 points

Hello,

The FDC1004's input range is only +/- 15pF. When you have CHA=CIN2 and CHB=Disabled, the FDC1004 is measuring the capacitance from CIN2 to GND. It is larger than 15pF, which is why it outputs a saturated value.

The CAPDAC allows you to offset the +/- 15pF input range. This can compensate for large stable capacitances like long cables leading to the sensors. This is why when CHA=CIN2 and CHB=CAPDAC you see a capacitance reading larger than the full scale input range.

When CHA=CINx and CHB=CINy, this is a differential capacitance measurement between the two channels. This is how you should configure your measurements.

Best Regards,

0 Guoxing Gu over 6 years ago in reply to Kristin Jones93

Intellectual 590 points

Hi Kristin,

Thank you for your answering my questions.

For the CHB setting for a differential measurement, I am still not clear and would like your further clarification. Maybe I did not make my question clear.

The data streaming does not start if all four channels are not selected(enabled) even if for only one differential channel testing. So, I had all 4 channels selected, and connected Plate 1 on CIN3 and plate 2 on CIN2.

For CHA and CHB setting on the second line, I had CHA=CIN2 and CHB=CIN3.

The real question is whether I also need to set up and how to setup CHA and CHB on the third line, which is measurement 3 where to enable CIN3 for the data streaming to work. There are three options for CHB, disabled, CAPDAC and CIN2. The differential measurement is on the second line. I observed differential value change (3.017, 3.390 and -3.059) on the second line when CHB changed from disabled, CAPDAC to CIN2 on the third line. So I am confused.

Regards,

0 Kristin Jones93 over 6 years ago in reply to Guoxing Gu

TI__Genius 17440 points

Hello Gu,

Thanks for explaining your question further. That is odd behavior. Can you clarify if you saw this behavior on the Configuration tab of the GUI or on the data streaming tab? Can you share some screenshots of the behavior you are seeing?

Best Regards,

0 Guoxing Gu over 6 years ago

Intellectual 590 points

Hi Kristin,

I am not sure if you can get this email. I like to send you a few screenshots, but I did not find any way to upload a file at the forum.

I saw it both on the Configuration tab and on the data streaming tab as I logged data to file.

Attached are a few screenshots in a file.

Best Regards,

On 2019-05-16 12:56 p.m., Kristin Jones93 wrote:

A Message from the TI E2E™ Community

Kristin Jones93 replied to FDC1004EVM: Setting CHA and CHB for differential measurement.

Hello Gu,

Thanks for explaining your question further. That is odd behavior. Can you clarify if you saw this behavior on the Configuration tab of the GUI or on the data streaming tab? Can you share some screenshots of the behavior you are seeing?

Best Regards,

Kristin Jones

Sensor Signal Conditioning

Applications Engineer

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0 Kristin Jones93 over 6 years ago in reply to Guoxing Gu

TI__Genius 17440 points

Hello Gu,

Thank you for providing the screenshots. I'm able to view them without issue. Please give me some time to look over them and find a possible cause for this behavior. I'll have an update for you early next week.

Best Regards,

0 Kristin Jones93 over 6 years ago in reply to Kristin Jones93

TI__Genius 17440 points

Hello Gu,

My apologies for the delayed response. After looking over the data you sent, I believe the problem is how you have configured the measurements and the physical sensors. The FDC1004 measures the capacitance between the sensor and ground when it is configured to make single-ended measurements. When you have a single-ended measurement configured, but the sensors are configured in a differential manner (the parallel plates are connected across channels instead of grounding one side), this can cause problems for the device. Your measurements look normal when the device configuration matches the physical setup (either both are single-ended or both are differential). Also please note that the FDC1004 is designed to be used for capacitive sensing applications (like proximity detection or capacitive touch buttons) and is not designed to replace more accurate capacitance measurements that could be made with and LCR meter or similar equipment.

Best Regards,

0 Guoxing Gu over 6 years ago in reply to Kristin Jones93

Intellectual 590 points

Thanks.

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FDC1004EVM: Setting CHA and CHB for differential measurement