Differential scope probe - Linear Technology design / schematic. Thoughts ? - Page 1
Forum section: electronics / projects

author: Freelander
author_rank: Regular Contributor
timestamp: December 10, 2017, 08:16:23 pm
content: Hello all,I came across this neat little circuit for a differential scope probe. It is a sample design by LT.I have just ordered the parts as samples (free) from them (via my account). - so nothing to lose apart from time.Any comments on the design / schematic - good or bad ? -What kind of actual probes would be used with this ? - do I need anything special ? - also, I presume the input is via 50 ohm coax (unterminated ?)As it ends up as effectively 1:1 it would be limited in range by the scope's vertical input limit on X1. also, selecting x1 on my hantek 5102 limits bandwidth to around 6Mhz I believe, although I cannot see that I will be measuring anything near that.I just want a simple to use probe that I do not have to worry about scope grounding and loops etc. It would be good fun to make something as well.Is there any way that the unit could be adapted for a X10 use (for single phase mains / smps) ? (I believe the amps are operated at at least a gain of 10 for stability - so presumably any divider type X10 option would be external)Look forward to your comments and observations.Link to article, schematic and AMP pdf.http://www.linear.com/solutions/6058

author: rhb
author_rank: Super Contributor
timestamp: December 10, 2017, 09:10:37 pm
content: Did you find Gerbers for the board? It looks like an excellent project.As for your question about probes, the pin header in the photo would work very nicely for reading waveforms across SMD resistors and capacitors. Another possibility would be a pair of tweezers of the type you press to open with insulated pins attached.

author: Freelander
author_rank: Regular Contributor
timestamp: December 10, 2017, 09:15:55 pm
content: Quote from: rhb on December 10, 2017, 09:10:37 pmHi, no board details unfortunately. I was just going to 'uglyboard' / deadbug itThere are the LT spice details there for modeling however I have never used LT spice. I found some reference to a guy going to build one on an 'audio' site but the thread died out. I think he did a board however the board layout would be important so no idea if his layout was any god as it was never finished. Uglyboard/dead bug is usually ok

author: cat87
author_rank: Regular Contributor
timestamp: December 10, 2017, 09:27:20 pm
content: The design is pretty simple, but like you said, kind of limited. to get it to where you could put mains on it... not really. Yes, you could add attenuators to the front of it, but some redesign would be needed.At first glance, the design is OK, but because the signal amplitude is so small, I'd thik og upping that capacitor on the output AC coupling... C6 I thik it was.1uF is a bit on the small size and it might affect you signal quality... I'd try and experiment with values ranging from 100uF to 470uF and see where I get. of course, too much capacitance is also a problem.Also, because you're using the op-amp in differential mode, your CMRR will drop quite a bit. Have a look on the datasheet regarding this... something like 60-80dB would be ideal for this kind of set-up, and anything north of 100dB is welcome.

author: Freelander
author_rank: Regular Contributor
timestamp: December 10, 2017, 09:55:33 pm
content: Quote from: cat87 on December 10, 2017, 09:27:20 pmHi Cat,What is your reasoning behind recommending a cap value change please ?There is an LTspice circuit listed in the link to the LT page, if you can use the program you can check the theoretical operation . As I said, unfortunately I have never used LTSpice. Anyone that has a good handle on LTspice and can check things over -t aht would be great !

author: nctnico
author_rank: Super Contributor
timestamp: December 10, 2017, 09:58:07 pm
content: Quote from: Freelander on December 10, 2017, 08:16:23 pmI don't see a reason to build it. If you've read the text the end result is pretty crappy. A much better and usefull idea is to use a DC coupled differential opamp to create a probe with can deal with a ground offset of a few Volts. Add a 10x amplification and you have an excellent solution for getting weak signals into your oscilloscope.

author: Freelander
author_rank: Regular Contributor
timestamp: December 10, 2017, 10:14:50 pm
content: Quote from: nctnico on December 10, 2017, 09:58:07 pmWhere do you get 'pretty crappy' from ?sheesh....Also, I am not interested in a 10X AMPLIFICATION , ATTENUATION, is what is needed.

author: cat87
author_rank: Regular Contributor
timestamp: December 11, 2017, 07:26:36 am
content: QuoteWell, basically, electrolytic caps are crap when it comes to small signals. Because of the nature of the dielectric, they have non-liniarities in them, which manifest themselves more, the smaller you signal is.Seeing how your full scale is 2.5V, that means the average signal amplitude will suffer because of the cap distortions.So basically, the bigger the value of the cap, the less impact it will have on a small amplitude signal... i.e. a 100uF cap will distort much less that a 1uF one.But like I said, this needs some experimentation

author: Freelander
author_rank: Regular Contributor
timestamp: December 11, 2017, 09:19:11 am
content: Quote from: cat87 on December 11, 2017, 07:26:36 amHi Cat87.That would be an interesting point, but that isnotanelectrolyticcapacitor

author: cat87
author_rank: Regular Contributor
timestamp: December 11, 2017, 12:49:16 pm
content: Fair point. Looked like one when I gleamed over itBut really, who makes non-polarized caps look like polarized onesAnyway, the point I mentioned is still viable, were that cap an electrolytic one.

author: Gyro
author_rank: Super Contributor
timestamp: December 11, 2017, 03:06:33 pm
content: That's a nice looking opamp! I hadn't realized that you could get such a high bandwidth with such low input currents these days.Ideal for a Hi-Z differential probe, or single-ended active probe come to that.The first thing that comes to mind is that for what you're looking for, you don't need so much gain, you could probably ditch the second device. You'll need to look very carefully at other differential probe projects to get a idea how to implement a high voltage, frequency compensated, input attenuator but the high input impedance of the opamp will certainly help when using higher resistance input divider chains.

author: Freelander
author_rank: Regular Contributor
timestamp: December 11, 2017, 03:52:04 pm
content: Quote from: Gyro on December 11, 2017, 03:06:33 pmHi Chris,Thanks for the input.If you are registered with LT and have a business email then you can get the opamps as samples FOC.Thinking out loud ( - I know - dangerous lol).................... the following comments...The 'gain' / second stage issue . ? - I would have thought the second stage was necessary as to balance the input divider to give a 1:1 in/out - ie - 0 gain overall for low level stuff.The higher voltage limit is effectively set by the input dividers and op amp limits (removing 'safety' out of the equation for the purpose of the exercise). Would not removing the second stage effectively give you an attenuation at its current working levels. This would not be desirable me thinks.The use of a high voltage compensated input attenuator / divider would be very nice. I wonder how the circuit of the 'Sapphire' probe unit could be adapted, the cheapy that is badged for everyone and his dog - (PDF included) - the reverse engineering has a few bugs (not my work!) - the caps across the 4 x 1meg input resistors are 6.8 PUFF (pF) - not nan....This is the Sapphire 9001 through hole version. (I have the maps for the SMT version too)I also have the full alignment details for that unit.Could the input divider from the sapphire be 'adapted' to add to the LT unit ?Attached is the rev engineered Sapphire unit.. (again input series caps are 6.8pF - not as on the schematic) -(also there are other items on the schematic that were placed as 'place holders' only. - the opamp for example.Would be interested in your thoughts...Also a link to alignment data for the Sapphire unit -http://www.sapphire.com.tw/nsi9001um.htmMust admit - the ridiculous prices they charge for the sapphire units are a total rip ! - stick a bloody LeCroy or what ever it is called sticker on it and rip someone a new one. (even with its pretty poor build quality) But, thats an aside..If anyone wants the SMT sapphire schematic then let me know. - alignment is the same afaik.

author: cat87
author_rank: Regular Contributor
timestamp: December 11, 2017, 05:42:34 pm
content: Well, not exactly the Sapphire one, but this should also be of some use.http://www.diyaudio.com/forums/equipment-and-tools/248505-differential-probe-reverese-engineered.htmlUnlike the LT design you might notice these are actually done with a JFET buffer... that's used mainly to get a higher CMRR than using only op-amps.

author: Freelander
author_rank: Regular Contributor
timestamp: December 11, 2017, 05:47:39 pm
content: Quote from: cat87 on December 11, 2017, 05:42:34 pmHi Cat,If you check the file I added in the post above, it IS the one from the link you gave......And yes, this IS the sapphire unit rev engineered.I think you might be having a bad day - or old age like me haha.

author: cat87
author_rank: Regular Contributor
timestamp: December 11, 2017, 05:55:18 pm
content: I think it's the one with the bad day. I don't have to worry about the "old age" stuff... at least not yet

author: Freelander
author_rank: Regular Contributor
timestamp: December 11, 2017, 05:57:04 pm
content: Quote from: cat87 on December 11, 2017, 05:55:18 pmNice one.

author: Freelander
author_rank: Regular Contributor
timestamp: December 11, 2017, 06:12:44 pm
content: It would be very interesting to compare the CMRR of the sapphire and the LT unit in actual use. I would be very surprised if there was any testable 'real world' difference at all and probably a far better spec from the LT at higher frequencies - (if used in a finished design). the Sapphire is speccing at around 85 and 66 at 50 hz and 20Khz respectively whereas the LT seem to be indicating a much flatter value across the range and a FAR higher bandwidth.I have no idea how good LTspice is to get any meaningful simulations but the file for LTspice is there if anyone knows how to effectively use it. I realise that simulated and real-world are chalk and cheese in a lot of cases but LT seem to be good (afaik) for their spice models of their own components. They obviously also believed the design they published was a really good way of showing off these amps and their abilities.All good fun.

author: Gyro
author_rank: Super Contributor
timestamp: December 11, 2017, 07:54:15 pm
content: Quote from: Freelander on December 11, 2017, 03:52:04 pmSorry, I probably misread your OP, it looked as if you were wanting a x10 probe rather than a X1.P.S. I see that Farnell stock both the 6268 and 69 for not too scandalous money, I'm a bit surprised RS don't though.

author: Freelander
author_rank: Regular Contributor
timestamp: December 11, 2017, 08:10:24 pm
content: Quote from: Gyro on December 11, 2017, 07:54:15 pmRs never surprises me these days. You were reading my post correctly Gyro, I was probably not clear really.I would like to ADD attenuation, (effectively making an X10 'probe'(s) - but as a PRE stage for higher voltage. At the moment, the input voltage range would not really be high enough.(I hope that makes sense)Ideally I would like it switchable for X10 and X1 -ie 10:1 attenuation and non.Get an account logged at Linear and get the samples for free. You will need a personal business email address though. A 'free' or common email supplier will not suffice and be rejected. Same on AD as well. Some really nice stuff and usually delivered in a few days. Linear allow up to 4 different items with up to 2 of each per order and a minimum of 2 weeks between orders.Have used them for years. PM me if you like. ?

author: Gyro
author_rank: Super Contributor
timestamp: December 11, 2017, 08:26:03 pm
content: Ok, I'm with you now. Switchable X1/X10 sounds like a nice idea, but in the context of a high voltage, high frequency probe I suspect it might cause a few issues in practice (parasitics and safety). Maybe as an add-on front end adapter though...I'm glad and smug to say that I am beyond the days of business email (although I have used my AD account occasionally for hard to source stuff). These days, instant gratification (well next day anyway) tends to outweigh sample requests and justifications. Thanks though.

author: Freelander
author_rank: Regular Contributor
timestamp: December 11, 2017, 09:08:19 pm
content: Quote from: Gyro on December 11, 2017, 08:26:03 pmHey Chris, I retired at 47 and never looked back.. but I keep the addresses as they are very very useful.So I am also 'Smug' that 'work' is a distant memory..............