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  1. #1
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    Using MOSFET as a switch in analog circuit (Analog/Digital ground issue)

    Hello,

    I stucked during my project looking for proper solution. From simplified schematic below, L1 and L2 are my antenna (L1 = L2 and L_antenna = L1 + L2), and I want to switch tuning capacitance with my two N-MOSFETs driven by digital logic. Digital logic is supplied from same antenna circuit with full-bridge rectifier, however now the issues appears. I can't directly connect antenna virtual ground to digital ground in that layout. I'd like to avoid using two half-wave rectifier due to lower voltage amplitude in that solution. Do you have any clue how can I drive MOSFETs with that kind of layout or how to solve it other way?

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  2. #2
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    Re: Using MOSFET as a switch in analog circuit (Analog/Digital ground issue)

    Hi,

    Depending on the current drawn by the logic side, if it's in the low milliamp range maybe you could use an op amp based precision rectifier/full-wave rectifier instead of the diode only bridge rectifier.

    You can cut "defensive" horseshoe shapes around the MOSFET pins that concern you to sort of isolate noisy return paths from the main ground (and therefore antenna ground), if that's what you mean, or make a horrendous slit across almost all of the middle of the board or wherever is appropriate to "separate" both sections, ...but nowadays I don't think analog and digital grounds are really separated that much.

    I imagine if one looked hard enough, one may find op amp circuits that create a virtual ground, but not so sure about this last suggestion being factual.



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  3. #3
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    Re: Using MOSFET as a switch in analog circuit (Analog/Digital ground issue)

    I suppose a misunderstanding. The rectifier is obviously providing the power supply of a RFID tag. No other supply available, OP rectifier isn't an option.

    Capacitive coupling of AC1 and AC2 can be a solution.


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  4. #4
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    Re: Using MOSFET as a switch in analog circuit (Analog/Digital ground issue)

    A common thing is to use a very resistive gate drive
    network and let the DC level ride with the FET
    capacitances, making a neat little RF switch. The
    common mode needs to be at ground if you want
    to (say) control with 0 - 2.5V signal and a NMOS
    switch. So you capacitor-block somewhere.

    Check out RF SOI CMOS switches. Hope you're
    doing this on SOI, or at really low amplitudes
    where all of those other nasty diodes don't play.



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