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| 2 opamp as bridge output. |
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| Zero999:
--- Quote from: SiliconWizard on December 18, 2018, 06:08:03 pm ---Use a differential probe or show the difference using two probes and a math channel on your scope. --- End quote --- Yes, that's the correct way to do it. --- Quote ---Connect the scope's ground to your circuit ground and certainly not to any of the opamps output except if your circuit is completely isolated... --- End quote --- Even if the circuit is isolated from the oscilloscope's ground, it's unlikely to be sufficiently isolated at AC, unless the frequency and output impedance is really low and capable of driving a relatively large capacitance. The circuit will be capacitively coupled to the oscilloscope's ground, to some extent, even if the circuit and/or the oscilloscope are battery powered. The amount of capacitive coupling will depend on the physical size of the circuit and battery. If the op-amp's output is connected to the oscilloscope's ground it will see a large capacitance a few hundred pF or more which can pick up mains hum and cause oscillation. The situation will be worse if the circuit and oscilloscope are both powered from the mains, even if the circuit's PSU is floating. |
| GigaJoe:
Ok ... great ... so I need to have a diff probe ... to use a scope in multimeter - mode ... I thought to use Smart APC with enough battery to feed the scope for many hours, but it seems not an option .... (or it does ? ) |
| Nerull:
Two probes + a math function doesn't work quite a well as a proper diff probe, but it typically gets the job done. |
| GigaJoe:
true, especially on my cheap Siglent, it not a solid resultant line, but something ladder like white dashes, especially if signals close to eliminate each other. |
| KrudyZ:
As others have said, you already have the two traces on the screen and all you need to do is turn on the math function to compute the difference. In addition you also should use DC coupling, not AC coupling as in your screen shots, to be able to compute the true differential voltage. Most scopes, including yours, have (earth) grounded probe connectors which happily short out your DUT if connected to the wrong nodes. |
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