It was no isolation measurement, just voltage.I doubt a DVM is the correct instrument to measure isolation.
They're not your zeners they're the ESD protection diodes inside the AD8130 that result in the "Input Voltage (Any Input) −VS − 0.3 V to +VS + 0.3 V" absolute maximum rating (with the diodes having a 0.3V drop)
Note that the datasheet doesn't explicitly diagram them but I assure you they're there. And even if they're not its even worse: the inputs would have NO protection. With the diodes the you can assume the input pins will survive 5-20mA or or so.
Hi,
It was no isolation measurement, just voltage.
...and hopefully it helped to see the problem of your circuit
Again it doesn't matter whether there are clamping diodes or not inside the AD8130.
The solution is what both myself and Klauss have said....:
"Thus you (sadly) need a (high ohmic) relation to your GND."
Not only does this add impedance to the problem path but a proper differential circuit is ideally designed to be completely symmetrical. Just copy your selectable high impedance input circuit twice.
Ok and from my perspective you understand everything except the conclusion. I try repeating the conclusion and you call me a broken record. So I try a different angle.
The diodes illustrate the relationship you've set up between grounds which is the important point. If you can see the diodes conduct when U1 pin 8 exceeds the rail what stops you from seeing that if they're not there U1 pin 8 will just burn up from exceeding its specification?
Anyway the way to understand all this is to simulate. Can my circuit withstand large voltages between input GND and scope ground? Add a voltage source and see. This picture shows that and the symetric divider network I've been describing. A DPDT switch could select the 2k or the 10k nodes to send to the amplifier for 2 range choices. In this case V9 (common mode) is 10Khz while V8 (differential) is 1khz. If the circuit is working we'll only see the differential signal at 1khz. And it does:
That type of symetric divider network where you divide the (-) equally to the (+) is all you need.
The C you're asking about would go in parallel to R3/R8 to act like a high pass filter. Google "scope probe compensation".
You probably don't know the exact differntial voltage you'll encounter either but that didn't stop you from picking something. Your probe will have a common mode limit too. Most important: know what it is. See this probe specifies "-Up to +1400V (DC +Peak AC) Differential and Common Mode". Every differential probe has a common mode limit, usually about the same as the differential one.
https://probemaster.com/4231-differential-probe-1-20-200-25-mhz-1400v/
OK, all clear now. The missing link in my case was that "isolated" sometimes imply "earth"-lyish connection from a engineering perspective.
For 3000Vp I recommend to go close to full 7.5Vp (useful common mode input range of Opamp)
This means up to 2500 Ohms ... for 1MOhms input resistors.
This still is a ratio of 400:1.
C stray should be more like 10p probably. You might only need a 1pF or less across the 1megs.
What are the 12 ohm resistors, that's tiny?
Be careful with resistor selection: Resistors have voltage limits as well as power limits. 2-3kV requires very special resistors or should be implemented with many resistors in series (1206 resistors will be rated for 100-500V or so).
Precision is also very important, not for gain accuracy which is easy to calibrate but for common mode rejection (simulate one divider leg being 10% off, set your differential voltage source to 0 and add a common mode source to see for yourself). This is one reason differential probes are expensive: because precision high voltage resistors aren't cheap.
Farnell has 1% 2512 SMD 3KV resistors at about €1,5 and MELF costs about the same.
I talked about "common mode", but you simulated "differential mode"
According datasheet differential mode input voltage range is +/-2.5V.
Thus distortion is expectable.
1% is pretty poor. Simulate the common mode rejection with a 1% mismatch.
If you're on budget and just building a couple maybe you can match them yourself...
MIN value makes no sense in my eyes, because it makes performance worse.
I recommend to optimize in direction "better performance" --> Higher resistance value.
Difficult.OK, if I do like this then: Keep R2/R3 at 1M and switch between 1K/10K/100K for R6/R9 values, then at 20MHz C3/C4 has to switch between values 0.016p/0.16p/1.6p - is that practically feasible to trim caps to such precise values?
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