Signal generators have an internal impedance of 50 ohms and expect a load of 50 ohms when the 50 ohm setting is selected. The 50 ohm impedance prevents reflections of high frequency (rf) signals as it changes mediums. Ideally you should not be using a signal generator to drive an output, but instead buffer it or use an amplifier. If you use high-z the output voltage is exactly what you set it to (e.g. 5Vpp) but with the 50 ohm setting the applies voltage is twice that with an internal resistance of 50 ohms.
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