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Why is the Hi-Z (>10GΩ) impedance mode on DC only available for low voltages?
TimFox:
A concrete way of thinking of this is to remember that the input amplifier (with the low bias current and other leakage currents discussed by bdunham7) is connected almost directly to the input in High-Z mode, with no input voltage divider in the path.
You can't safely apply greater than, say, 20 V to an integrated-circuit op amp with normal supply voltages.
David Hess:
Some old oscilloscopes supported ultra high impedance inputs, and had the same limitations. The Tektronix 7A13 vertical amplifier has a switch to disconnect the internal 1 megohm termination allowing "infinite" input resistance at 50mV/division and below. The Tektronix 7A22 also has provisions to disconnect its 1 megohm termination.
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