Arduino oscilloscope voltage divider design
Figure 1. Typical oscilloscope probe.


  Problem is oscilloscope only needs one input to measure voltage ...


No, your hunch is correct. All voltage readings require a reference point so a ground reference is required as well. This is typically the ground of the circuit under examination. 

A typical probe uses a coaxial or screened cable with the probe tip connected to the centre conductor and an optional ground clip connected to the screen. (That's the little crocodile clip in Figure 1.) If the clip is not used then a separate ground wire has to be run out from the scope.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. Typical 'scope connection setup.

Be very aware that you have no electrical isolation when using your Arduino. If you have a USB connection and you make a mistake with your ground connection you could damage the Arduino or the device it is connected to.


  But technically voltage meter has infinite resistance, so how the voltage meter's voltage divider works?








simulate this circuit

Figure 3. A potential divider for one channel.

The potential divider is external to the ADC. It doesn't use the ADC's internal resistance as part of the divider.