Electronics > Beginners
DIY Differential probe again again - please help :-)
<< < (4/9) > >>
ogden:
One author actually finished, B
MagicSmoker:
Option B is just a standard instrumentation amplifier (IA) circuit made out of 3 individual op-amps. You can buy an IA as a single IC but it won't have a very high gain*bandwidth product - maybe 10MHz at unity gain. Using individual high speed (and looks like current-feedback here) op-amps makes it easier - though not easy - to achieve an arbitrarily high bandwidth.

Note that the voltage divider at each input of a high bandwidth differential probe needs to be precise both at DC, which is easy - just choose the correct resistor values - and across the entire frequency range, which is deceptively difficult to pull off, especially if you need >2:1 attenuation and >10MHz bandwidth.

You want something like 500MHz bandwidth which implies the probe having a -3dB bandwidth that is at least Pi times higher (and ideally closer to 10x). This is just enormously difficult to do and commercially available active differential probes with >2GHz bandwidth have 5 digit prices, and I don't mean in Yen or Yuan. You need to set your sights much lower or else buy a probe outright, or, perhaps, copy a successful design *exactly*.



FriedMule:

--- Quote from: MagicSmoker on October 06, 2019, 12:27:16 pm ---Option B is just a standard instrumentation amplifier (IA) circuit made out of 3 individual op-amps. You can buy an IA as a single IC but it won't have a very high gain*bandwidth product - maybe 10MHz at unity gain. Using individual high speed (and looks like current-feedback here) op-amps makes it easier - though not easy - to achieve an arbitrarily high bandwidth.

Note that the voltage divider at each input of a high bandwidth differential probe needs to be precise both at DC, which is easy - just choose the correct resistor values - and across the entire frequency range, which is deceptively difficult to pull off, especially if you need >2:1 attenuation and >10MHz bandwidth.

You want something like 500MHz bandwidth which implies the probe having a -3dB bandwidth that is at least Pi times higher (and ideally closer to 10x). This is just enormously difficult to do and commercially available active differential probes with >2GHz bandwidth have 5 digit prices, and I don't mean in Yen or Yuan. You need to set your sights much lower or else buy a probe outright, or, perhaps, copy a successful design *exactly*.

--- End quote ---
Okay yes it makes a bit harder, I have tried to find sub 1GHz probe designs and I have also found some, but I do not know how good they are. If you look at the design I have chosen here, it should work fine up to about 800MHz but only in super low voltage. The design you can buy are for the most for high voltage and yes costs 4 times my scope.
I have read every comment in this tread: https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/
Found other probes like this: https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/

But my "wall" is simply that I do not have enough knowledge to build my own, so I have to, as you say, copy a design.
But untill now, I have not found a design that your great guy's says do work.
MagicSmoker:

--- Quote from: FriedMule on October 06, 2019, 01:05:34 pm ---Okay yes it makes a bit harder, I have tried to find sub 1GHz probe designs and I have also found some, but I do not know how good they are. If you look at the design I have chosen here, it should work fine up to about 800MHz but only in super low voltage. The design you can buy are for the most for high voltage and yes costs 4 times my scope.
--- End quote ---

I (and ogden) have already told you this is an enormously difficult task, and your only hope - hell, my only hope! - would be to copy someone else's design exactly. I mean, the same components, board layout, physical construction, every last detail.

This is not the sort of problem a beginner is likely to run into, nor even the sort of problem any but RF and high-speed digital design engineers will ever have to contend with, frankly, so I have to ask why you think you need a >500MHz differential probe? Earlier you wrote this:


--- Quote from: FriedMule on October 05, 2019, 10:39:28 am ---...If that had been a 500MHz would that have been enough? I mean, if all I want is to be able to measure where there is different grounds....

--- End quote ---

But that isn't an answer. Suffice it to say that there are precious few instances where an RF or high speed digital signal is not supplied by an isolated power supply, so there isn't a safety requirement for needing a differential or isolated probe. There are plenty of examples of high speed differential digital signal standards such as Ethernet, LVDS, HDMI, SCSI, etc., but, again, not the sort of thing the typical EE will ever work on at the signal level, much less a beginner or hobbyist.

So what, exactly, do you intend to probe with an oscilloscope that has convinced you you need a really expensive differential probe good for (much greater than) 500MHz?
BravoV:
A 500MHz differential probe capable of < 65Volt ... cheap ... DIY & relatively easy to build ... just nope.

Suggesting to change plan.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod