Swend
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Differential probe is a rather general topic. Beside bandwidth, you want to specify input impedance, common mode range and common rejection ratio before you start any actual design work.
The original design posted in the blog would be useless for most differential probe applications that I know. Either you have a rather special application in mind, or you don't yet realize the limitations of the design.
O.K., if you can live with AD8130 common range, this is surely an option. I have used it in a number of design, particularly high MHz current sense. +/- 10V is only achieved with high supply voltage, of course. AD8129 has very unpleasant supply current with input overload, you should consider it if overload can happen in your application.
First, since your probe ground is basically tied directly to U1 (only 470R) your common mode voltage is limited to the 9V supply rails. Is that acceptable? I expect differential probes to have common mode ratings equal or greater to their differential ratings.
And I'd like to see your AD8130 input pins have common mode clamps in addition to your differential clamps (zener type or diodes to the rails).
reference your amplifier output to scope ground for some additional ground common mode rejection.
An alternative scheme could use a 5V to +/-9V isolated DC-DC instead of your charge pump.
Finally I'll recommend having linear regulated power. This chip has charge pumps and regulators.
https://www.analog.com/en/products/ltc3265.html
Doesn't work this way, a double pole switch is needed. Reconsider!
And having different grounds is the point of a differential probe...
Your circuit is sending the "Scope Probe" ground (BNC pin 2) directly to U2 (through 470ohm).
If your scope probe ground is NOT referenced to U2's ground then you have big problems.
$7 isn't bad considering the cost and space of discrete linear regulators. If you don't add linear regulators at least add some additional filtering after the capacitive switcher and try to keep that in its own area of the board.
Again consider a real scenario:
1) The ground symbol on your schematic is connected to earth through the oscilloscope -> Call it 0V
2) You connect your "Scope probe ground" to something that's 50V
3) Result: R6 and/or U2 blows up.
Point being you need to have high impedance dividers on both inputs to your amplifier since either input can be at a high voltage in relation to your amplifier's ground.
If you don't add linear regulators at least add some additional filtering after the capacitive switcher and try to keep that in its own area of the board.
Yes I see that scenario easily happening to someone that is not considerate. But that requires that the scope is earthed (as in the planet earth) and that "something" is earthed too. Obviously if you are careless you can always blow your equipment, or worse - incorrect measurement results. For that purpose I have adapted the old adage "measure twice, cut once" to electronics: "Think twice, measure once".
The AD8130 is U1 btw.
If the 'something' isn't earthed then you don't need the differential probe. You can connect your earthed scope probe anywhere you want on an isolated circuit.
I'm curious where you think you need a differential probe.
So your scope isn't earthed. It doesn't matter.
The scope ground (and your differential probe circuit ground) is at potential A
Circuit B is at potential B
Circuit C is at potential C
If A, B and C could be the same potential you could use your scope as-is right now.
Since they're not you need a differential probe that can tolerate the common mode potential differences between A, B and C.
But your circuit as drawn can't tolerate more than ~9V between it's ground and the circuit its measuring.
Now I'm confused, A,B and C are all floating that means they have no common reference, if they have no common reference there can be no common mode potential differences between A, B and C, am I right or wrong?
And if A, B and C could be the same potential I could NOT use my scope as-is right now if I wanted to measure three differential signals (I could perhaps measure two (with four probes) if I make the scope subtract one from the other. But my scope can't make two subtractions at once, and the result of the subtraction can not be saved.
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