How can I make a differential measurement with passive probes?
You can use two passive probes to make a pseudo-differential measurement. To do this, you must use the math function on your oscilloscope. With the probes connected to ch. 1 and ch. 2 respectively, invert ch. 2 and then use the math function to add ch. 1 and ch. 2. When this is done, the ch. 1 probe acts as the non-inverting input, and ch. 2 acts as the inverting input. Connect the probes to your circuit with both grounds connected to the circuit ground, as seen in Fig. 1 below.
Fig. 1: Example of pseudo-differential probing
Note that the oscilloscope channels should be set to the same volts/division and the probes should be matched to maximize the common mode rejection radio. On newer oscilloscopes, the math function can be used to subtract ch.1 from ch. 2, which accomplishes the same thing as the above method.
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