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| Mechatrommer:
yes some of us who are interested in building diy differential probe know that link very well. its like a chapter in the net that we must read in order to pass "build my own probe" degree. some members here too successfully built their own probe and try to sell it, check out nctnico differential probe, i'm also still finding time to populate my own differential probe, pcb ready few weeks ago, parts also ready. but i kind dont like the design you choose because its AC coupled it cannot read DC voltage. i'm not sure what you dont understand with my earlier comment or something i wrote wrong? EDIT: ok now i look closely the link and you followed that exactly. this is why we dont follow someone's circuit blindly. with gained knowledge we know something is wrong. he tied the + and - input of 2nd opamp, that could work, the THS3202 may handle it, but it may also create extra distortion on the 1st opamp who knows? if we familiar with brand name circuit, or some advice from more knowledgable people here, thats not how its done. he also didnt put the 50 ohm output impedance, that will create reflection along the 50 ohm line from and to the 0.01 ohm output impedance of THS3202, he may be happy with the reflected result, just a little bit of ringing, the sense of increased risetime or whatever. you may spot the spike on his actual (not simulated) freq respond plot @ 60MHz or something in the 2nd report, it maybe it, but i cant be sure this is dark art if you dont have proper tool such as VNA. those spike may move around depending on cable length and where its terminated. he also missed the CMRR report, thats one part in differential probe spec, but it maybe not matter for that application, even LeCroy GHz differential probe didnt report their CMRR. anyway ymmv, happy building ;) EDIT again: you can check in THS3202 datasheet itself again how its done if there is transmission line involved, 75ohm transmission line will need 75ohm on the opamp output. 50ohm transmission line needs 50 ohm resistor on the opamp output, 1Kohm characteristic impedance line/trace needs 1Kohm etc etc, thats the real signal integrity. |
| FriedMule:
--- Quote from: Mechatrommer on September 17, 2019, 07:14:49 pm ---yes some of us who are interested in building diy differential probe know that link very well. its like a chapter in the net that we must read in order to pass "build my own probe" degree. some members here too successfully built their own probe and try to sell it, check out nctnico differential probe, i'm also still finding time to populate my own differential probe, pcb ready few weeks ago, parts also ready. but i kind dont like the design you choose because its AC coupled it cannot read DC voltage. i'm not sure what you dont understand with my earlier comment or something i wrote wrong? EDIT: ok now i look closely the link and you followed that exactly. this is why we dont follow someone's circuit blindly. with gained knowledge we know something is wrong. he tied the + and - input of 2nd opamp, that could work, the THS3202 may handle it, but it may also create extra distortion on the 1st opamp who knows? if we familiar with brand name circuit, or some advice from more knowledgable people here, thats not how its done. he also didnt put the 50 ohm output impedance, that will create reflection along the 50 ohm line from and to the 0.01 ohm output impedance of THS3202, he may be happy with the reflected result, just a little bit of ringing, the sense of increased risetime or whatever. you may spot the spike on his actual (not simulated) freq respond plot @ 60MHz or something in the 2nd report, it maybe it, but i cant be sure this is dark art if you dont have proper tool such as VNA. those spike may move around depending on cable length and where its terminated. he also missed the CMRR report, thats one part in differential probe spec, but it maybe not matter for that application, even LeCroy GHz differential probe didnt report their CMRR. anyway ymmv, happy building ;) EDIT again: you can check in THS3202 datasheet itself again how its done if there is transmission line involved, 75ohm transmission line will need 75ohm on the opamp output. 50ohm transmission line needs 50 ohm resistor on the opamp output, 1Kohm characteristic impedance line/trace needs 1Kohm etc etc, thats the real signal integrity. --- End quote --- Fantastic comment! What I am trying to do, is as you say, to build a differential probe. I do that for two reasons: 1) To be able to measure DC circuit isolated from ground 2) Save the insane ammout of money they normally costs I have no nee for high voltage or something like that. My hope was to build a circuit I can put between my scope and my probe so I do not have to invest in a separate probes. I know that I can use two probes to do the measurement but that solution has many disadvantages beside making my 4 ch scope to a 2 ch scope. :-) What I need is a schematic that works and help to draw the PCB. Hope I am not asking for to much here. |
| Mechatrommer:
since you are building GHz range diff probe (low voltage maybe up to 30V input?), you may follow this thread, you may find usefull schematics there... https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/?topicscreen nctnico came up with his product there selling at $150 iirc if maybe you are interested, but never make the circuit open, so i'm just guessing. maybe you can PM him to publish his circuit here >:D i for so long want to build my own high voltage version maybe up to 500V input, maybe few MHz up to 100MHz should be fine, but CMRR demand is difficult as requirement need to read transistor Vbe or Vgs, so it never realized, the existing solutions are too bulky and oldish style (using 9V battery? duh) and too expensive for my taste. so still dreaming something that never realized... https://www.eevblog.com/forum/projects/oshw-diy-1kv-100mhz-differential-probe-(dilemma-vs-hope)/?topicscreen i have the pcb and parts ready for both my HV (ver2) and GHz diff probe (trying to immitate nctnico's topology et al in the thread i linked, i never try GHz diff opamp before so this is just a matter of luck), there is another GHz active probe (not differential) and another mini type passive hi-Z probe that i'm trying to build/explore. the problem is when project list get expanding, the time is shrinking into peanut. so, good luck to us... |
| FriedMule:
--- Quote from: Mechatrommer on September 17, 2019, 09:57:24 pm ---since you are building GHz range diff probe (low voltage maybe up to 30V input?), you may follow this thread, you may find usefull schematics there... https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/?topicscreen nctnico came up with his product there selling at $150 iirc if maybe you are interested, but never make the circuit open, so i'm just guessing. maybe you can PM him to publish his circuit here >:D i for so long want to build my own high voltage version maybe up to 500V input, maybe few MHz up to 100MHz should be fine, but CMRR demand is difficult as requirement need to read transistor Vbe or Vgs, so it never realized, the existing solutions are too bulky and oldish style (using 9V battery? duh) and too expensive for my taste. so still dreaming something that never realized... https://www.eevblog.com/forum/projects/oshw-diy-1kv-100mhz-differential-probe-(dilemma-vs-hope)/?topicscreen i have the pcb and parts ready for both my HV (ver2) and GHz diff probe (trying to immitate nctnico's topology et al in the thread i linked, i never try GHz diff opamp before so this is just a matter of luck), there is another GHz active probe (not differential) and another mini type passive hi-Z probe that i'm trying to build/explore. the problem is when project list get expanding, the time is shrinking into peanut. so, good luck to us... --- End quote --- 30V is fine, higher <70V would be better. <500MHz is what I need but saw the <1GHz so that's why I did choose that. I hope that my needs is not so demanding, I hope to be able to build it for fairly cheap, the probe shall also function as an sort of "blow the probe before your scope" :-) |
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