How to measure differential signal with high voltage differential probe

How to use high-voltage differential probe to measure differential signal ：

1、 Differential measurement

A better solution for floating measurement is to use a differential probe with high common mode rejection ratio , Because there is no grounding problem at both inputs , The differential operation of two input signals is completed in the front-end amplifier of the probe , The signal transmitted to the oscilloscope channel is the differential voltage , The oscilloscope can realize safe floating ground measurement without removing the grounding terminal of the three wire plug .

2、 Oscilloscope floating measurement

At present, the common wrong floating measurement method is the oscilloscope floating measurement method , By cutting off the standard three ends AC The method of grounding the socket or using an AC isolation transformer , Cut off the connection between the neutral line and the ground line . Float the oscilloscope from the protective ground , To reduce the impact of the ground loop . This method is actually not feasible , Because in the wiring of the building, the neutral line may have been connected to the ground line somewhere , Is an unsafe measurement method ;

Besides , It violates industrial health and safety regulations , And the measured results are also poor . And there will be a large parasitic capacitance when the oscilloscope floats on the ground , Floating measurement will be damaged by oscillation , The measured waveform is seriously distorted , There will be an example demonstration later . To make a long story short , Floating measurement by oscilloscope is easy to damage the tested device ; Damage oscilloscope ; Bring potential harm to people ; The measurement error is large .

3、 Use two probes to measure , Then use the mathematical operation function of oscilloscope to calculate

Two single ended measurements using two probes , This is a common method , It is also an undesirable method for differential measurement . Measure the signal to the ground （ Single ended ） And use the mathematical operation function of the oscilloscope （ passageway A Signal minus channel B）, The differential signal can be measured . When the signal is low frequency signal , The signal amplitude is large enough , Be able to exceed any noise concerns , This can be done . The combination of two single ended measurements has several potential problems .

One problem is that there are two separate long signal paths along each probe until each oscilloscope channel . Any delay difference between the two channels will cause the two signals to time shift . On high-speed signals , This offset will cause significant amplitude and timing errors in the calculated differential signal .

Another problem is that they do not provide sufficient common mode noise suppression . In actual circuits , There are many common mode noise sources , for instance , Noise caused by nearby clock lines on two signal lines , Noise from external sources such as fluorescence . With the increase of frequency , Single ended CMMR（ Common mode rejection ratio ） The performance of will degrade rapidly . If common mode interference is preserved , This will cause the signal noise to be much larger than the actual noise .