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I have recently made a High Z (1 MΩ) attenuator with R1= R3= 8.75 MΩ, and R2= 243 kΩ. I am using an LCR meter to test the impedance at higher frequencies. At frequencies above 50 kHz, the impedance is much smaller than the 1 MΩ intended value. I am using the mean sum method where Z= sqrt ((R of closed circuit) * (R of open circuit))

Keep in mind, the method I am using for calculating impedance is just a method of estimation. Moreover, I am an amateur; so please keep in mind obvious mistakes are not so obvious for me.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Show us your PCB layout. \$\endgroup\$
    – Hearth
    Commented Oct 11, 2023 at 1:43
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    \$\begingroup\$ The point is, at high frequencies and high resistances, even tiny parasitic capacitances become very significant. For example, at 50 kHz, just 3 pF of stray capacitance has an impedance of 1 megohm, which is low enough to affect your measurements. \$\endgroup\$
    – Dave Tweed
    Commented Oct 11, 2023 at 3:07

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As pointed out by @DaveTweed, capacitors have some impedance ...

One can note the "evolution" of global impedance of attenuator.

enter image description here

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