Good design for a DIY active probe? - Page 1
Forum section: electronics / projects

author: Marco
author_rank: Super Contributor
timestamp: January 17, 2013, 11:35:28 am
content: I've looked around the web, but they all seem a bit lacking. There is one which uses an opamp at the input, but that's just adding way way more input capacitance than necessary. The poor man's 1 GHz active probe is AC coupled and will be highly non-linear because of the small source resistor. The ancient one from Troubleshooting Analog Circuits still seems the best one over all. I can find no schematic at all for one with active DC offset correction, although that's a bit of luxury I could forego.Anyone know of a better circuit? (Preferably with PCB layout.) I found these :http://books.google.nl/books?id=3kY4-HYLqh0C&lpg=PP1&pg=PA16#v=onepage&q&f=falsehttp://elektrotanya.com/files/forum/2009/10/e04a036.pdfhttp://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdfhttp://oliverbetz.de/hit/hit_e.htm

author: free_electron
author_rank: Super Contributor
timestamp: January 17, 2013, 03:03:51 pm
content: What are you looking for bandwidth wise ?Just pick up a used 1151 or 1152 probe from agilent on ebay. You can find these for 100$. Build a supply to feed it the 3 or four voltages it needs and off you go. There was a topic here on the forum where someone did that. I posted pinouts and schematics there how to make the supply.You are never going to beat the hybrids in those beasts....

author: Marco
author_rank: Super Contributor
timestamp: January 17, 2013, 06:09:38 pm
content: 1141 and 1152 you mean? Neither are up for anywhere near that at the moment. Tektronix P6201 looks more interesting actually, having BNC and inline 50 Ohm terminators build in for high impedance scopes (like my cheapo Owon DSO). Kinda hesitant at buying one off ebay from China though.Beating them isn't necessary, couple 100 MHz bandwidth and mV level noise is acceptable.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 17, 2013, 06:16:40 pm
content: There are also Tektronix 6202 and 6045, the latter comes with power supply, see this ->http://www.ebay.com/itm/Tektronix-P6045-F-E-T-probe-package-uknown-instrument-/321057431273?pt=LH_DefaultDomain_0&hash=item4ac0839ee9Both have higher bandwidth than your scope.

author: muvideo
author_rank: Frequent Contributor
timestamp: January 17, 2013, 09:04:47 pm
content: Marco, less than one month ago I made thesame search and found the same references.I needed only the lowest tip capacitance I couldafford and decent BW, I needed to spot onlythe timing of the event, so didnt bother withlinearity. I liked Bob Peases' one but I built thepoorman's one, only because I had the fewcomponents needed at hand, now I dont remember mymosfet partnumber. Few minutes and it worked finefor my task, it's so simple to build, but I did nottried to characterize it, I would not know from whereto startFabio.

author: Marco
author_rank: Super Contributor
timestamp: January 18, 2013, 01:51:25 pm
content: Mechatrommer : I want to mess around with FPGAs so anything over 5 volt ... less than a couple pF will not add significant load at the I/O speed I'm interested in.Muvideo : did you build it dead bug style on some copper board?

author: muvideo
author_rank: Frequent Contributor
timestamp: January 18, 2013, 03:55:11 pm
content: Quote from: Marco on January 18, 2013, 01:51:25 pmPerfboard, I used an old 0.6pF rod ceramic as input capacitor.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 18, 2013, 07:38:03 pm
content: This is another example of non-vendor specific active probe ->http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300848617979&ssPageName=ADME:B:SS:US:1123Many old Tektonix active probes were equipped with mains power supply, either combined with amplifier box or standalone.

author: nctnico
author_rank: Super Contributor
timestamp: January 19, 2013, 07:54:54 pm
content: Why not build a passive divider probe? I have build two and they work like a charm. I just can't find the website right now. Something with EMI RFI Guy but that doesn't Google properly.

author: Unknown
author_rank: Unknown
timestamp: January 19, 2013, 08:01:04 pm
content: http://www.emcesd.com/1ghzprob.htm

author: nctnico
author_rank: Super Contributor
timestamp: January 19, 2013, 08:51:39 pm
content: @Alm: Thats it exactly! Thanks for taking the trouble to find the link.

author: Marco
author_rank: Super Contributor
timestamp: January 20, 2013, 03:22:40 pm
content: Also an option, though with 1K impedance you'd have to be a bit more thoughtful of when to use it than an active probe.Someone made a good point in another thread though, the change in termination impedance when you parallel a traditional 1M scope input with 50 Ohm is by no means negligible at 100+ MHz. With the 15 pF my Owon supposedly has, termination impedance will already have dropped by nearly 10% at 100 MHz ... not a problem for a more expensive scope with a proper 50 Ohm input of course.

author: nctnico
author_rank: Super Contributor
timestamp: January 20, 2013, 04:38:59 pm
content: 1k Ohm may sound low but the input range of an active probe usually isn't much beyond +/- 15V. In some cases even much less. It depends on what you are measuring but you'll mostly find 50 or 75 Ohm signals in RF systems or 33 to 75 Ohm series terminated signals in digital systems. Compared to those impedances 1k Ohm is huge.

author: Unknown
author_rank: Unknown
timestamp: January 20, 2013, 04:55:19 pm
content: 1K impedance may seem low at DC, but what's the impedance of the so called high-impedance probes at high frequencies? A 100 MHz 10x high-Z probe might have an input spec of 10 Mohm // 12 pF. At 100 MHz, this will have an impedance of 133 ohm (I didn't even bother to take the 10 Mohm resistor into account). A commercial 20x resistive probe might have an input impedance of 1 kohm // 1.5 pF, or about 500 ohm at 100 MHz. Probe datasheets (for good probes at least, not the $5 eBay specials) will show an input impedance vs frequency graph. Note that the vertical scale is logarithmic. The input resistance is only relevant up to a few hundred kHz or so, after that it's the capacitance that matters.A 500 MHz 10x high-Z probe will have an input capacitance of about 8 pF, at 500 MHz this probe will have an input impedance of 40 ohm. The 1 kohm // 1.5 pF probe will be about 175 ohm. There are also resistive probes with only 0.4 pF of input capacitance, which perform even better. A low-end commercial active probe might be 1 Mohm // 1 pF, or 320 ohm at 500 MHz. Not that much higher than the supposedly low impedance probe.DIY designs will be worse, but I still expect for example the EMCESD design to beat a high-Z probe at input capacitance.

author: muvideo
author_rank: Frequent Contributor
timestamp: January 20, 2013, 08:14:09 pm
content: Hello, dont know if I understand all of the arguments exsposed before,but sometimes all is needed is not huge BW but simply a low tip capacitance,decent BW, division ratio not too high for decent vertical resolution onlow amplitude signals.For example I built the mosfet active probe because I wantedto check the sampling window of the internal ADC of a microcontroller.The internal S/H of the ADC is modeled as 8pF capacitor with a 1kresistor in series and is not discharged between conversions.The procedure was to charge the SH capacitance with a slowconversion on a steady known DC, and then start a conversionon a pin where there was a resistor, the probe measured the pulseon this load resistor. The pin, pad and pcb have their owncapacitance.In this case if the probe capacitance is low enough and theresistive loading is high enough I can easily see sampler's "gate"opening (this is easy), but also closing.The 10:1 probe with it's capacitance was uneffective.The active probe was showing the pulse nicely and I couldchoose the right loading resistor to discharge the pin, a goodcompromise was 100kOhm.Could a test like this be done better with a passive probe?Thanks,Fabio.

author: nctnico
author_rank: Super Contributor
timestamp: January 20, 2013, 08:42:54 pm
content: An active probe with a low input capacitance is always better than a passive divider probe because a passive divider probe has an attenuation of 20 times or more (like 40 times) so you'll see more noise from the scope itself.

author: Unknown
author_rank: Unknown
timestamp: January 20, 2013, 08:55:42 pm
content: I would expect the high DC loading of resistive probes to be an issue here.The main issue with resistive divider probes is the low DC input impedance. It requires test points with a fairly low output impedance. This is not usually an issue for fast digital circuits, which is why Howard Johnson advocates resistive divider probes. 10x resistive probes are also common, but DC resistance is down to 500 ohm. High frequency impedance is similar to the 20x probes, though. Many FET probes are also 10x.An issue with active probes is that the input impedance varies over many orders of magnitude over its bandwidth. This can introduce distortion, depending on what the source looks like. A resistive source in combination with a capacitive probe will form a low-pass filter, slowing down rise times. The input impedance of resistive probes is, as the name implies, much more resisitive and constant over its bandwidth.

author: signality
author_rank: Newbie
timestamp: July 23, 2014, 01:10:43 pm
content: Way too late to be of any use for the original post but in case anyone else is looking for the same thing ...Here's a schematic and PCB design for a DIY version of Bob Peases's original jfet input high impedance active scope probe adapter:http://easyeda.com/project_view_Bob-Pease-s-High-Impedance-Active-Scope-Probe-PCB-Design_g2QNnljhf.htmwith a simulation:http://easyeda.com/project_view_Bob-Pease-s-High-Impedance-Active-Scope-Probe-simulations_gsmEyQ8qj.htmThere's also the outline of an idea for a similar (but maybe lower frequency) probe using an opamp:http://easyeda.com/project_view_High-impedance-probe-using-opamp_gmtnFzRL3.htmEnjoy.Quote from: Marco on January 17, 2013, 11:35:28 am

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 23, 2014, 02:56:57 pm
content: Quote from: Marco on January 17, 2013, 11:35:28 amTektronix P6032.Oh wait, I'll bet you wanted it solid state?https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif

author: branadic
author_rank: Super Contributor
timestamp: July 24, 2014, 01:07:26 pm
content: Please refer to:http://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdfand the additional thread:http://www.mikrocontroller.net/topic/188227

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2014, 09:50:40 pm
content: The service manuals for the old Tektronix P6202A (500MHz 10M 2.0pF) and P6201 (900 MHz 1M 1.5pF) active probes have schematics which show what is involved in adding offset to the simple designs Bob Pease and Jim Williams wrote about.