There is still a fairly serious flaw in our little voltmeter setup: its Vin input is not at ground level. It’s connected via Z1 and Z2 to that 2.4V or so virtual ground we just created, and when Vin is not connected to anything, it will read as 2.4V instead of 0V.
More importantly, this means that when we connect this to a circuit-under-test, it is likely to influence that circuit by injecting a bit of current.
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