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selectable gain with analog switch

Discussion in 'The Projects Forum' started by Lupu, Oct 30, 2011.

  1. Lupu

    Thread Starter New Member

    Sep 18, 2009
    1
    0
    Hello,

    I am trying to build a selectable gain circuit with 74HC4066 and a LM324 op-amp.
    (the 4066 is used to shunt two out of three resistors that set the gain of the op-amp )

    But i have trouble with the OFF impedance of the analog switch:
    - [OK] when control pin (pin13) is pulled high the impedance between i/o gate is lower then 100Ω which is what I was expecting from the data sheet
    - [Problem] when pulled low(GND) i get around 10kΩ between the same i/o pins( pin1 & pin2) which is extremely low,
    I was expecting something beyond 1MΩ (although the datasheet doesn't say anything about this)
    so instead of two resistors 5kΩ and >1M in parallel i get 5kΩ and 10kΩ.

    I attached the schematic of the test circuit where i took these measurements.
    Any advise is welcome since i am stuck with this for a couple of days already.
    Thank you
     
  2. Audioguru

    Expert

    Dec 20, 2007
    11,251
    1,350
    The 74HC4066 is Cmos. Then EVERY unused input must be connected to V+ or 0V.

    When an input is used then it must be biased between 0V and V+ but is usually half the supply voltage for audio. Then when a switch is turned off it passes almost no signal.

    The datasheet shows that it has a small capacitance from input to output and when it is turned off then this capacitance couples high frequencies from input to output.
     
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