Making matched 1x probes for a LeCroy DA1855A differential amplifier - Page 1
Forum section: electronics / testgear

author: rx8pilot
author_rank: Super Contributor
timestamp: February 12, 2017, 12:32:14 am
content: Hi all -I have two Preamble/LeCroy DA1855A differential amplifiers. Interesting and unique piece of test equipment that I use mainly for testing power electronics. There are purpose built 'matched' probes for these units that are 1x, 10x, and 100x that are essentially just like any other passive oscilloscope probe but they are matched to optimize the sensitive differential input of the amplifier. Not entirely sure what characteristics are matched or what the tolerances are.The factory probes are rare on the used market and also quite expensive. On top of that, they are cumbersome to use. I am hoping to build my own 1x matched sets that are intended to be soldered into the DUT instead of hand probed. Imagine testing with two of these and 2 other single ended probes - you have 6 probes competing for space on a tiny PCB. That is why I want to construct a solder-in-place version.The problem is that I do not understand what they have done to 'match' these probes sets. I have one XC200 1x 50Mhz probe (pic attached). When I took the plate off, all I can see is the un-populated PCB from the 10x XC100 probe. The construction makes it difficult to disassemble to see what is on the opposite side of the PCB (if anything at all). All I can see with my eyes is a shield that isolates the two channels. I was expecting something that would compensate for any attenuation or variation in capacitance. Maybe the only thing that is matched is the length of the cable?Anyone have an idea how to best figure out what needs to be done, build it, and test it. I have never built any probes before, but this seems to be a reasonably easy place to start. The end goal is that I have (at least) two sets of solder-in-place probes that have reasonable matched characteristics. The necessary bandwidth is anything >25Mhz. The factory model is rated at 50Mhz/-3db while the 10x version is good past 100Mhz.

author: nctnico
author_rank: Super Contributor
timestamp: February 12, 2017, 01:43:24 am
content: I'd go lo-tech and buy some RG316 coax cable with BNCs pre-crimped and cut them to exactly the same length. I think you'll need to connect the shields together at the circuit side and perhaps connect this a grounding point in the circuit. Due to the capacitive coupling between the wire and the shielding an AC current will flow from the test points into ground. Using disposable coax cables (they cost very little on Ebay) makes it easy to cut these cables as short as you can get away with without breaking the bank.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 12, 2017, 02:28:04 am
content: Quote from: nctnico on February 12, 2017, 01:43:24 amWouldn't that be a high-capacitance solution - lower bandwidth? Is there a low capacitance probe cable? Maybe I just get some old Tek 1x probes and cut the ends off? Again - being the inputs on a very sensitive differential amplifier, how do I minimize the error from the probes?

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2017, 11:01:49 am
content: 1x probes are always high capacitance and are essentially a coaxial cable with some refinements to optimize transient response. Nothing special needs to be done to match them.10x and 100x probes have a series resistance which acts as a voltage divider into the 1 megohm input resistance of the differential amplifier inputs. So in order to be matched, they have to be trimmed to match each other for maximum DC common mode rejection. They also have AC frequency compensation which needs to be matched as well.DC matching is done by connecting both probes to the same source and adjusting the DC attenuation of one or both probes for a minimum change in DC level. AC matching is done in the same way but using something like a probe compensation source.

author: SeanB
author_rank: Super Contributor
timestamp: February 12, 2017, 01:51:38 pm
content: Your probe cost comes from making them in a matched pair, and getting this matching accurate within the CMRR limits over the frequency range of the probe.You can easily just buy 2 X10 probe sets ( buy new, and buy around 5 of the cheapest ones initially) and simply calibrate them all on a single probe input of the diff amp ( so you get within the ballpark of matching), and then match them.You put in 2 at a time, set up on that input to best square wave on a test signal with other shorted, put on a decent signal generator set to a 1MHz square wave ( as high as it can deliver with a decent rise time) and adjust the compensation for each probe slightly for best CMRR ( lowest amplitude horrid waveform) and then make up 2 sets of them. Mark the probes as to input they go on, and lock that compensation adjustment so it cannot be accidentally moved, and use some spiral wrap to tie the probe cables together.Remaining probe is for either using if you cannot find a match in the set, or the CMRR is too poor. then it is just another probe.Otherwise you will be making up probe sets using matched resistors in a case with tiny gimmic capacitor plates for higher frequency compensation, which is hard without being able to buy resistive coax.

author: djnz
author_rank: Regular Contributor
timestamp: February 12, 2017, 07:22:58 pm
content: Quote from: SeanB on February 12, 2017, 01:51:38 pmWon't differences in skew show up as phase differences at the diff amp input, ruining CMRR?

author: rx8pilot
author_rank: Super Contributor
timestamp: February 12, 2017, 07:36:46 pm
content: Quote from: djnz on February 12, 2017, 07:22:58 pmThat is what I am trying to sort out. The goal with these probes is to look at tiny signals in the uV and mV ranges at low to modest frequencies. A 10x probe is too much attenuation and the additional bandwidth is not needed. The factory design has no visible adjustments or components that could be be used for any sort of matching. They are also rated at 30pf and 50Mhz which is significantly better than my expectation of just a piece of RGxx 50 Ohm coax.Maybe the matching part is that the cable is sequentially cut off the same spool so it will best match, carefully cut to the exact length, etc. Not sure if they put any sort of special effort into the hand-piece end of the probe.

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2017, 09:51:41 pm
content: The XC200 probe length is only like 2 feet which is much shorter than standard 1x oscilloscope probes lowering its capacitance. High impedance passive oscilloscope probes all use low capacitance cable.If you use 1x probes, then nothing special is needed to match them for best performance compared to 10x probes. A 10x probe requires separate AC and DC attenuation trims and very few (none these days?) include a DC trim.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 12, 2017, 11:30:24 pm
content: True, and since I can easily deal with a short cable - maybe the easiest solution is to get some well made 1x Tek/Agilent probes and just cut them down.Sent from my horrible mobile....

author: rx8pilot
author_rank: Super Contributor
timestamp: February 13, 2017, 12:00:43 am
content: Or...... find a source of low capacitance coax with well terminated BNC connectors.Sent from my horrible mobile....

author: nctnico
author_rank: Super Contributor
timestamp: February 13, 2017, 12:27:28 am
content: Low capacitance coax will also be much thicker and the capacitance won't be very low either. Why is the capacitance a problem?

author: tautech
author_rank: Super Contributor
timestamp: February 13, 2017, 12:40:19 am
content: Quote from: rx8pilot on February 12, 2017, 07:36:46 pmQuite doable IMHO.Skew shouldn't be of anything but minor concern if the cables are from the same spool, terminated in the same manner and length matched.Have a look at this chart for some low capacitance coax:http://www.rfcafe.com/references/electrical/coax-chart.htm

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2017, 01:51:20 am
content: Quote from: nctnico on February 13, 2017, 12:27:28 amSince the probe cable operates as an unterminated transmission line into 1Mohm, its distributed capacitance looks like a lump capacitance at the probe tip. This is what makes attenuating probes desirable; their attenuation factor also divides the unterminated probe's lump cable capacitance down to a reasonable level to support operation above 10s of MHz.

author: nctnico
author_rank: Super Contributor
timestamp: February 13, 2017, 02:10:26 am
content: Quote from: David Hess on February 13, 2017, 01:51:20 amI know but earlier rx8pilot stated he is not looking at high frequencies so I'm wondering if several tens of pf is going to be a problem in the first place.

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2017, 02:52:20 am
content: Quote from: nctnico on February 13, 2017, 02:10:26 amHe says he is testing power electronics so that is likely the case. Real 1x probes have the advantage over simple coaxial cable substitutes of slightly lower capacitance for a given length and equalization to produce a clean transient response at higher frequencies.When I use my 7A13 or 7A22 differential amplifiers, standard 1x probes work fine for maintaining common mode rejection. Incidentally, I have been told that the Preamble DA1855 is effectively the descendant of the Tektronix 7A13 with the same circuit design but a different layout. John Addis and Steve Ditter helped found Preamble after they left Tektronix.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 13, 2017, 06:37:45 am
content: Quote from: tautech on February 13, 2017, 12:40:19 amHandy chart ;-)Pasternack has RG180 BNC cables pre-made that I can just cut in half. They are rather low capacitance and I really only need about 18" length. My signal generator only goes to 20Mhz, but that is enough if it is flat-ish to that frequency I think. Cheap to try for sure.Quote from: David Hess on February 13, 2017, 02:52:20 amInteresting note. On a thread where I was trying (unsuccessfully) to repair a DA1855A, an EEVblog user chimed in as one that worked on them. Quite a sophisticated piece of analog gear - well beyond my understanding. The mechanical packaging makes it extremely difficult to work on, schematics are locked in a dungeon somewhere, and there is a lot of hand-built nuances in it.

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2017, 02:29:39 am
content: Quote from: rx8pilot on February 13, 2017, 06:37:45 amI remember reading a comment that Tektronix had problems with VHF oscillation in the 7A13 which makes sense given how complex the bootstrapped input stage is. The entire thing is shielded and subdivided so it does not surprise me at all that the DA1855A had the same problem.I know that even the 1 MHz 7A22 which is a completely different design has problems with oscillation; if the tiny straight through fuses on the inputs are replaced with spiral wound ones, it oscillates. Replacing the input JFETs with alternatives which should work makes it oscillate.I have read that the DA1855A is practically a duplicate of the 7A13 circuit design with obviously a different layout so if you want to learn about its circuit design details, theservice manual for the Tektronix 7A13which even includes a detailed theory section is worth checking out.

author: djnz
author_rank: Regular Contributor
timestamp: February 16, 2017, 07:08:23 am
content: I wonder if there are any new probes available today with attenuator trimming (DC compensation)...

author: rx8pilot
author_rank: Super Contributor
timestamp: February 16, 2017, 07:14:59 am
content: Quote from: djnz on February 16, 2017, 07:08:23 amI have not seen any. May be one of the rare moments where I really get some use out of my 6.5 digit DMM.

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2017, 12:44:50 pm
content: Quote from: djnz on February 16, 2017, 07:08:23 amIf you find some let me know.The Tektronix P6135A and P6055(A) have DC trims but they were intended for this application. The 100x P6009 environmental probe has a DC trim and I suspect the similar 10x P6008 could have a DC trim added. Tektronix 1000x high voltage probes have DC trims. I think maybe the P6023 (10x 8M input) has a DC trim. Tektronix also made a precision 10x probe for their oscilloscopes that include a voltmeter which samples the BNC signal; this probe has a much tighter tolerance.Some Tektronix 10x probes which have the compensation box on the BNC side could have a DC trim added but what I would try is modifying a pair of the 10x Texas probes available inexpensively on Ebay that have a compensation box on the BNC side.Quote from: rx8pilot on February 16, 2017, 07:14:59 amThat much resolution is not required and the measurement does not even have to be that accurate since it is the difference in probe resistance which matters. The real test is done by measuring the CMRR on the oscilloscope anyway.