[電子]如何即時量度變化中的細電容,範圍約2pF-300pF

[匿名]

2009-07-29 5:44 pm

Would any one please tell me how to have real-time measurement (sampling rate: ~50Hz, using a data logger that receives Voltage inputs) on a varying capacitance?

(LCR meter is not suitable in my case, since I wantto have the measurement inputed into the data logger so that I can do some real-time analysis using the computer by programs like Matlab.)

Please notice that the capacitance is very small (varying between about 2pF to 300 pF).

By the way, would it be possible to have the above measurement with DC only? If not, AC is still okay.

更新1:

Thanks for your reply! 那麼如果我只measure 2pF至約50pF, 唔用LCR meter還有可能嗎? 我有儀器(i.e. data logger) 去real-time入voltage去電腦到,亦有software去比我寫program去做analysis,但不知如何可以將個那麼小的capacitance轉變為一個相對的voltage值去給data logger measure

更新2:

Please notice that it would be great for me if the measurement of capacitance can be changed to a form of voltage. Q=CV, but I cannot keep the Q to be constant so easily, since the capacitance is toooooooo small.

更新3:

The time constant is so small (C*R<<0.01seconds) even if I use a CRO to measure the voltage, meaning that the capacitor discharge toooooooo quickly.

更新4:

Thanks for your kind reply again. What if the capacitance range I want to measure become "3.7pF - 4.8pF"?? Will it be easier to measure the varying capacitance??

回答 (1)

2009-07-29 6:25 pm

✔ 最佳答案

這個量程實在很大，沒有相關的儀器是非常困難做到你的要求。

就算用電腦程式協助，也得要有高敏度和寬量程的電路組件配合，才能捕捉到它的變化，捕捉到後也需再經放大、整理、數碼化後才能令電腦工作。

最簡單是用一個可以量度你以上所說範圍的電容錶去讀出。
用 LCR meter 當然係最理想。

2009-07-29 15:32:56 補充：
上面都提到：
就算用電腦程式協助，也得要有高敏度和寬量程的電路組件配合，才能捕捉到它的變化，捕捉到後也需再經放大、整理、數碼化後才能令電腦工作。

非常複雜的做法，用這電容做一個 LC 高頻振蕩器 (100MHz )，當改變它的容量時，電路的振蕩頻率也會跟隨改變，可以從頻率的變化值，計算出這電容的變化情況。因為這個頻率會容易一點可以使用在電腦的連接上。但不要問我點計這條數，因為我唔識。

2009-08-02 09:29:08 補充：
隨非你係製造或生產電容的公司，要分析它。這個電容數值的多少，無需要百份百準確，這麼小的一個電容，與電路連接後，焊錫、室溫、走線都會令整體電路結果有所不同，如果用於高頻調諧，要準確頻率時，這情況下都要做接電調校。用頻率計之類的儀器做調準。現在高精度的產品，如手機之類，電路會通過鎖相方式，以晶振作參考進行自動調整，只要在電路可調整的範圍內，容量 +/- 10%都應該沒問題。

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