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Original question:

FDC2214: Question about actively driven shield configuration

FDC2214: FDC2214

Mr. BLACK
Prodigy 20 points
Part Number: FDC2214

Hi. 

I am using fdc2214 chip to measure capacitance. I have designed a circuit that includes buffer opamp configuration for active shielding. My opamp is mcp654 which has 50MHz GBW and 30V/us slew rate.

C = 33pF and L = 22uH are the components that are used for the LC tank circuit. My resonance frequency is 3.27MHz. When i check the frequency of the resonance circuit alone i see 3.27MHz. Then i used two channels of the oscilloscope to see the active shield and the LC resonance signal at the same time. Again i see 3.27MHz as the figure given below.

 

But when i try to observe buffer output (active shield) alone i see a signal at 4.39 MHz as given below. I don't get it.

They should always be the same. What do you think is the problem? How can i solve it?

Thanks in advance.

  • 0
    TI__Mastermind 40295 points

    How do the waveforms compare between the op amp input and the op amp output?

    If they look different, one easy thing to try would be to add a little series resistance between your op amp output and your shield.
    The shield is usually a large capacitance, and some op amps have a hard time driving highly reactive loads.
    If that is the root cause you may have to experiment with the resistance value.

    Regards,
    John

  • 0
    Prodigy 20 points

    There is no shield (capacitor plate) at the moment. I just used the LC resonance signal and fed it to the input of the buffer. But the frequency of the input and output is different. When i check them both at the same time they have the same frequency but when i measure only the shield signal it is different than the input. I don't get it. Hope you can help.

  • 0 in reply to Mr. BLACK
    TI__Mastermind 40295 points

    How are you probing the resonance signal and the input to the buffer?

    John