FDC1004: Question

user5929126

Part Number: FDC1004

If the target 1 to be measured (capacitance known) and the capacitor 2 (capacitance known) are connected in series, can the FDC 1004 measure the capacitance correctly?

(Example)

When target 1 is 150 pF and capacitor 2 is 100 pF
Is it recognized as 60 pF of synthetic capacity as reading on the IC ?

Imagewise, the potential between target 1 and capacitor 2 can not be determined.
I feel that I can not measure correctly with short time charge transfer.

Best Regards,

Koji Kuboyama

over 6 years ago

0 Clancy Soehren over 6 years ago

TI__Mastermind 23810 points

Yes, that should work; however, make sure that the CAPDAC setting is correct. The CAPDAC introduces an offset to the capacitance measurement since the FDC1004 only supports +-15pF of dynamic range.

0 user5929126 over 6 years ago in reply to Clancy Soehren

Prodigy 150 points

Thank you for answering.
It is an additional question.
If offset setting is performed with the measurement target and capacitor connected in series, the linearity between the offset setting value and the measured value can not be seen.

Is this a characteristic of the IC?

Or is it the effect of series connection?

0 Clancy Soehren over 6 years ago in reply to user5929126

TI__Mastermind 23810 points

It is possible that if the capacitors are different types or if they are physically spaced further away from each other, that there could be some non-linear behavior. We haven't done as much testing with series capacitive connections, so you could easily test it by repeating the same experiment with a 60pF capacitor by itself.

0 user5929126 over 6 years ago in reply to Clancy Soehren

Prodigy 150 points

Thank you.
One more point, if the capacitors are not connected in series, does the offset setting have linearity? Or can there be non-linear behavior with or without series connection? Or, in normal use, is it linear behavior?

0 Clancy Soehren over 6 years ago in reply to user5929126

TI__Mastermind 23810 points

The CAPDAC setting should be fairly linear, but it isn't perfect. There will also be some small gain and offset error of the CAPDAC as you sweep through the possible settings.

0 user5929126 over 6 years ago in reply to Clancy Soehren

Prodigy 150 points

In short, the use of capacitors in series is irrelevant to the linearity of the offset, right?
If only gain setting and offset error are affected...

0 Clancy Soehren over 6 years ago in reply to user5929126

TI__Mastermind 23810 points

That is correct!

0 user5929126 over 6 years ago in reply to Clancy Soehren

Prodigy 150 points

I tested by changing the value of the offset. As the offset setting value increased, the offset error increased. Is this a characteristic of the IC?

0 Clancy Soehren over 6 years ago in reply to user5929126

TI__Mastermind 23810 points

Can you share your data? When I ran a quick test here, I did not see that behavior. Here is my data:

Offset (pF)	Capacitance (pF)
-5	-4.40
-10	-9.40
-15	-14.40
0	0.60
5	5.60
10	10.60
15	15.60

0 user5929126 over 6 years ago in reply to Clancy Soehren

Prodigy 150 points

It is my measurement data.

Capacity to measurement point[pF]	Offset[pF]	Capacitance[pF]
1.859	14	15
1.859	13	14.096
1.859	8	9.407
1.859	4	5.659
1.859	2	3.783
1.859	1	2.846
1.859	0.5	2.377
1.859	0	1.859
1.859	-0.5	1.390
1.859	-1	0.922
1.859	-1.8	0.171
1.859	-3.125	-1.071
1.859	-6.25	-4.001
1.859	-9.375	-6.913
1.859	-12.5	-9.862
1.859	-15.625	-12.696
1.859	-18.75	-15.000
1.859	-21.875	-15.000

0 Clancy Soehren over 6 years ago in reply to user5929126

TI__Mastermind 23810 points

Your data looks pretty different from mine. If you disregard the over +-15pF measurements (which is out of range), the data has a consistent slope, meaning that you could apply a gain correction factor.

However, I don't understand why you are seeing the gain error in the first place.

0 user5929126 over 6 years ago in reply to Clancy Soehren

Prodigy 150 points

Currently, measurement tests are being conducted, and the test conditions are as follows:

1.The capacity of the measurement object is about 150 pF.

2.Because it is out of the measurement range, adjustment to the offset range by series connection of capacitors.
3.Adjustment within the measurement range by setting the offset.

From the above, it is determined that the linearity of the offset is important.

Does the linearity of the offset be affected by the set gain?

0 Clancy Soehren over 6 years ago in reply to user5929126

TI__Mastermind 23810 points

It looks like the figure I attached in my last post did not load correctly. My point is that your data has a systematic gain error that could be calibrated out. It might be interesting to see how the data looks for a single offset setting with varying capacitance.

However, my data shows very little error and does not match your data. Do you see the same results when using a single capacitor (instead of the series capacitors that you are currently using)?

0 user5929126 over 6 years ago in reply to Clancy Soehren

Prodigy 150 points

It is a measurement result of a single capacitor.

Connection capacity[pF]	Capacity to measurement point[pF]	Offset[pF]	Capacitance [pF]
10.31	1.934	8	15.000
10.31	1.934	5	15.000
10.31	1.934	2	13.470
10.31	1.934	1	12.532
10.31	1.934	0.8	12.344
10.31	1.934	0.5	12.064
10.31	1.934	0.2	11.783
10.31	1.934	0.1	11.688
10.31	1.934	0	11.595
10.31	1.934	-0.1	11.501
10.31	1.934	-0.2	11.409
10.31	1.934	-0.5	11.126
10.31	1.934	-0.8	10.844
10.31	1.934	-1	10.657
10.31	1.934	-2	9.721
10.31	1.934	-3.125	8.665
10.31	1.934	-5	6.908
10.31	1.934	-6.25	5.734
10.31	1.934	-9.375	2.833
10.31	1.934	-10	2.220
10.31	1.934	-12.5	-0.126
10.31	1.934	-15.625	-2.958
10.31	1.934	-16	-3.405
10.31	1.934	-18.75	-5.748
10.31	1.934	-21.875	-8.766
10.31	1.934	-25	-11.541
10.31	1.934	-28.125	-15.000

0 Clancy Soehren over 6 years ago in reply to user5929126

TI__Mastermind 23810 points

Thanks for the additional data set. It looks like you are still seeing error here as well. Let me run a few more tests and I will get back to you by the end of the week.

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FDC1004: Question