These aren't too bad... the 700V ones (way more than I need) are $332=20 for the 25Mhz ones. Little nervous about the 25Mhz rating, since I deal=20 with things like 56VDC square waves where the fall time is in the tens=20 to hundreds of nanoseconds category. But the 'rise time' spec seems to=20 be ok that way. I will say that this whole 'development exercise' has given me a whole=20 new appreciation for how painful it can be to validate a design when=20 your scope is lying to you. Not that I didn't know that before, but=20 sometimes reinforcement by pain is helpful to drill home a lesson=20 learned over and over again. -forrest On 8/19/2010 12:23 AM, Sean Breheny wrote: > Hi Forrest, > > A couple of years ago at work I bought two $500 25MHz differential > scope probes from www.probemaster.com. They were one of the best > purchases I ever made. I often use them now even when ordinary scope > probes would be acceptable because the signals are so much cleaner > (not just because of the narrower bandwidth - I mean that the signals > are a more faithful representation of the actual signal). > > These type of probes are NOT cheap - they are very specialized, rugged > differential amplifiers. You probably cannot just buy a diff amp IC > with these specs. I suspect that, inside the probe, there are some > discrete transistor input stages with matched transistors and > resistors, followed by some high-speed op-amps, all connected in an > instrumentation amplifier topology. > > For home use, I did manage to get an old (1970s vintage) 100MHz Tek > diff probe for about $100. The downside is that it is bulky (has a > separate power supply, probe, and amplifier box). The going rate for > used modern diff probes seems to be about $300. New they are in the > $500 range - this is for 20 to 30MHz BW. The really high end ones (GHz > BW) are thousands of $. > > Sean > > > On Thu, Aug 19, 2010 at 12:00 AM, Forrest W Christian > wrote: >> My most recent project has created a new challenge for me. I use a >> TDS2014 4 Channel DSO on the bench, and to date, just some good garden >> variety 1X/10X scope probes (basically decent quality but not really >> name-brand probes - maybe pomona or tenma or something like that). >> >> This new project involves putting 120us holes in 60VDC. The stuff is >> basically done at this point, but I think I've spent almost more time >> dealing with scope grounding issues than actually solving problems. >> Switching 60VDC off and on that quickly obviously creates some >> interesting inductive possibilities. For instance, earlier today I was >> seeing some stray spikes on one of the signals being monitored. Pulled >> the scope probe out of it's 'grabber clip thingy (yes technical I >> know)', and touched the tip to the grounding clip (you know, the one >> with about 3 inches of wire attached directly to the probe itself). >> Amazingly, the spikes were *still* there. Ended up disconnecting all >> the other grounds on the other probes and referencing only to that one >> to verify that signal was clean. Of course, then all the other signals >> weren't clean anymore - at least not on the scope. >> >> So, I figured to fix this in the future, I'd spend a bit and get some >> differential scope probes. So I glanced on ebay and google for costs >> and availability - and in the process about fell out of my chair in >> shock. Maybe there's a decent one out there at a reasonable cost that >> I'll find, but looks like they're few and far between. >> >> So, before I either just suffer - or go out and buy a few isolation >> amplifier ICs and build my own, I figured I'd ask on here if there are >> either some tricks I'm missing (I.E. maybe it's the scope probes I'm >> using - a $30-50 probe just isn't good enough, OR there is some other >> way to get a good picture of what a signal looks like referenced to >> ground near the chip producing it - when you have 4 possible grounding >> points. >> >> Ideas? >> >> -forrest >> >> >> -- >> http://www.piclist.com PIC/SX FAQ& list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .