Active probe? - Page 1
Forum section: electronics / general

author: awallin
author_rank: Frequent Contributor
timestamp: January 02, 2020, 07:28:43 pm
content: Are there any good ~1 GHz active probe open designs out there?would simply a high-speed FET opamp work? comments, ideas?The +/-2.5V supplies would be nice to generate from 5V USB - any ideas for a small low-noise dc-2-dc converter for that? (followed by low-noise LDOs)

author: David Hess
author_rank: Super Contributor
timestamp: January 02, 2020, 10:50:46 pm
content: An FET operational amplifier will work however transient response will be compromised by feedback to the inputs even if configured as a voltage follower. I think you would actually get better performance using a medium input resistance and current feedback amplifier or even better, an operational transconductance amplifier like the OPA860 however package and circuit layout limitations will limit ultimate performance.I think a better way is the classic JFET source follower input stage preceded by a x5 compensated divider and then bipolar emitter follower to drive a double terminated 50 ohm input yielding a total x10 attenuation. Without any gain stage, this makes best use of the parasitic elements in printed circuit board construction.There are some more specialized amplifier ICs which someone else may suggest but I am not familiar with them.

author: RCinFLA
author_rank: Contributor
timestamp: February 15, 2020, 09:21:04 pm
content: The main objective of an active scope probe is to reduce the 11 pF to 15 pF shunt capacitance of a normal divide by 10 passive scope probe. Secondary objective would be to avoid the 10x signal loss. In order to get the capacitance down you have to have the amplifier at the probe tip end of the scope cable.Expensive active probes have only couple tenths of pF shunt capacitance. This is less than most common chip resistor parasitic shunt capacitance.There are some low cost FET followers on ebay but they are capacitively coupled.For response to D.C. you pretty much have to use op amp. For an op amp solution look for the highest slew rate op amps. Keep the feedback resistors values low to prevent effects of op amp neg input node stray capacitance and layout needs to avoid nearby ground stray capacitance at the op amp - input node. Too high feedback resistor with too much neg input node capacitance can make the op amp unstable.The op amp output drive current must support the low value feedback resistor in parallel with 50 ohm load impedance output to scope. Scope input needs 50 ohm termination so cable is terminated properly. If scope does not have 50 ohm input option a BNC-T with 50 ohm terminator load will do.

author: nctnico
author_rank: Super Contributor
timestamp: February 16, 2020, 03:54:23 pm
content: The parasitics will likely kill any hope of getting some kind of bandwidth. The schematic in the first post isn't going to work at all. It starts with an 100k / 15pf RC filter.There is a long thread about making your own (differential) 1GHz probes:https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/which ended up with a neat product

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2020, 04:53:46 pm
content: Quote from: RCinFLA on February 15, 2020, 09:21:04 pmJFETs work just fine down to DC and a JFET source follower has many advantages over a series feedback amplifier. Up to 500 MHz, the common active probe design in the past used two JFETs and two RF bipolar transistors.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 16, 2020, 06:28:37 pm
content: Quote from: nctnico on February 16, 2020, 03:54:23 pmwhere do you get the 15pF figure? BAV99 is 2pF (maybe 4pF for both) OPA858 is negligible. but yeah, talking about copycatting from bad schematics (pin 2 BAV99 should go to +ve power rail)

author: PartialDischarge
author_rank: Super Contributor
timestamp: February 16, 2020, 07:14:27 pm
content: These old JFET probes (from Tektronix, HP, Philips...) not only have a low bias input but also a very low input C, which is what makes them really useful in HF measurements at mostly any point in the circuit.However I'm amazed at the high input C of the hyperexpensive tektronix IsoVu (starting at 41dB$ according to the X-chapters), 20pf at 1MOhm with no input divider and 0.5V range, there goes all the bandwidth

author: nctnico
author_rank: Super Contributor
timestamp: February 16, 2020, 07:45:59 pm
content: Quote from: Mechatrommer on February 16, 2020, 06:28:37 pmThe pads of the board and other capacitances add up quickly so 15pf is kind of a worst case number. And no, the BAV99 are drawn correctly if it was for a low impedance clamp circuit. Dumping excess current into supply rails isn't a good idea. But in this circuit the uneven leakage current through the diodes could introduce an assymetric loading of the input resistor. Dual diodes are often 2 dies in one package which in theory should come from the same wafer but there are no guarantees. With such a high input series resistor you don't really need an external clamping diode. The ESD diodes in the opamp are sufficient.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: February 18, 2020, 08:04:58 pm
content: What about this thing:https://github.com/dkramnik/Eagle-Public/tree/master/instrumentation-active-probe-elektor?I think it's been around for ages and the ones sold on ebay for 20€ (based on this circuit) provide decent results.

author: Warhawk
author_rank: Frequent Contributor
timestamp: February 21, 2020, 05:18:26 pm
content: What you need is thishttp://welecw2000a.sourceforge.net/docs/Hardware/Aktiver_Tastkopf_mit_OPA659.pdfThe author is active here on the forum too.I built three pieces and now I am doing actially a power supply with this IChttp://www.ti.com/product/LM27762I'll piggy back it to the board and supply it from USB.Close enough.Best regards, Jiri

author: SaabFAN
author_rank: Frequent Contributor
timestamp: February 23, 2020, 10:07:14 am
content: Quote from: Warhawk on February 21, 2020, 05:18:26 pmI would suggest using one or two LiPo batteries and recharge them via USB.Much one less cable that is being dragged along the bench when trying to measure something

author: Noy
author_rank: Frequent Contributor
timestamp: June 12, 2020, 12:26:40 pm
content: I'm also interested in one or two. I know the branadic design. Maybe we should respin it.Maybe with USB supply and a newer /better Opamp? I'm not sure...I need only 500MHz for my Rigol MSO5000 i want to compensate the lower impedance of the passive probes at the higher frequencies.

author: TimFox
author_rank: Super Contributor
timestamp: June 12, 2020, 06:53:49 pm
content: Note that the dual JFET in the Tektronix design achieves nominal zero offset voltage due to the matched FETs and the equal source resistors on the input follower.

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2020, 10:05:28 pm
content: Quote from: TimFox on June 12, 2020, 06:53:49 pmIt does but grading single JFETs into pairs is almost as good and Tektronix often did exactly this. Graded pairs are less desirable in a production environment however.

author: teomondo
author_rank: Newbie
timestamp: August 30, 2020, 08:36:09 am
content: Dear ntcnicoIs DIP1400 1.4GHz differential probe currently avaliable?I have an Operating manual dated 2019 any new version avaliable?What about the price of DIP1400 1.4GHz differential probe and how I can buy it.Thanks for your answerRoberto

author: Noy
author_rank: Frequent Contributor
timestamp: January 28, 2022, 11:39:45 am
content: Pretty awesome new device for a DIY active probe:Ti BUF802https://www.ti.com/lit/ds/symlink/buf802.pdf?ts=1643347428244&ref_url=https%253A%252F%252Fwww.google.com%252F

author: JohnG
author_rank: Frequent Contributor
timestamp: January 31, 2022, 04:05:23 pm
content: Good luck getting any TI parts these days. They made some great parts, but it doesn't matter if they are unobtainium.John

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2022, 12:50:26 am
content: The Texas Instruments BUF802 looks ideal for a simple as possible design. It can be improved by adding a divider to the input to lower its relatively high input capacitance which will otherwise limit performance. Typically a x5 input divider would be used so that with the x2 attenuation of a double terminated output, x10 attenuation would result. Many x10 active probes do exactly this.

author: KE5FX
author_rank: Super Contributor
timestamp: February 01, 2022, 02:56:23 am
content: No shortage of BUF802s at DigiKey or TI's direct-sales outlet.However, you really want a differential probe.Buy one from nctnico.

author: JohnG
author_rank: Frequent Contributor
timestamp: February 01, 2022, 03:18:13 am
content: Quote from: KE5FX on February 01, 2022, 02:56:23 amYou are correct, sir! To be fair, a number of TI parts are hard to get these days, but apparently not this one.QuoteI am a happy customer.John

author: Noy
author_rank: Frequent Contributor
timestamp: February 18, 2022, 02:12:05 pm
content: I got 5 samples from TI (TI direkt store) for the BUF802 within 2 days from Singapur so no problems for now..Now i have to try to find "hobbytime" to do the schematic for a single ended active probe..

author: profdc9
author_rank: Frequent Contributor
timestamp: February 27, 2022, 09:25:13 am
content: I designed a simple probe using three 2SC3356 transistorshttp://www.github.com/profdc9/https://github.com/profdc9/RFUtilityKnifeYou could also use a BFR93 rather than 2SC3356 (I used this because it was available at LCSC cheap, it is a 7 GHz transistor).

author: cdev
author_rank: Super Contributor
timestamp: May 02, 2022, 07:59:57 pm
content: Quote from: awallin on January 02, 2020, 07:28:43 pmI have an FET RF probe, it works well. It uses what must be a few dollars in parts.. It does not load the circuit, so the scope can see RF better.Basically thats what it does. I got mine on ebay...It works well..

author: jonpaul
author_rank: Super Contributor
timestamp: May 02, 2022, 08:06:35 pm
content: no reason for itUse a Zo probe like Tektronix P6156https://w140.com/tekwiki/wiki/P6156See Tektronix Circuits Concept book Oscilloscope Probe CircuitsJon

author: Gyro
author_rank: Super Contributor
timestamp: May 02, 2022, 08:24:31 pm
content: A forum search for 'z0 probe' (note z zero) will yield a lot of threads.Edit: Actually, so will 'Zo probe' (although the z0 ones are more relevant).

author: Brenton7
author_rank: Newbie
timestamp: January 01, 2025, 09:00:35 am
content: I have a Tektronix P6202A 500Mhz active probe that uses this exact circuit. However when I attempted to use it I discovered it was U/S. Must have been like this when I received years back. Tracked the problem down to the input FET chip. The half that connects to the -7 Volt was short. I have attempted to replace this chip with two MMBF4416LTI UHF Jfets but getting the bottom FET to conduct seems to be a problem. This fet was specified having a IDSS of 10mA but analysing the circuit seems in indicate higher current f around 15mA. the circuit uses + and - 7 volt supplies with only 27Ohm source resistors. Not sure whether the fet characteristics are vastly different or what the specs were for the original. Working on this front end in micro surgery with the SOT-23-3 devices making changes almost impossible.Any ideas?

author: haastyle
author_rank: Regular Contributor
timestamp: January 02, 2025, 11:10:49 pm
content: This design works amazingly well:https://jmw.name/projects/active-probe/Files are here:https://github.com/drandyhaas/oshw-active-probeI've tested it extensively and it performs as advertised. Flat within 1 dB from DC to 2 GHz!I plan to also sell them alongside the Haasoscope Pro 2 GHz open-source oscilloscope on CrowdSupply:https://www.crowdsupply.com/andy-haas/haasoscope-pro(I made a 3D printed case for it - looks much better now than on the webpage.)

author: ozkarah
author_rank: Regular Contributor
timestamp: August 07, 2025, 01:12:48 pm
content: Quote from: haastyle on January 02, 2025, 11:10:49 pmWhat supply voltage did you applied and what about the accuracy at lower frequencies like 1 MHz?

author: haastyle
author_rank: Regular Contributor
timestamp: August 07, 2025, 01:20:27 pm
content: "What supply voltage did you applied and what about the accuracy at lower frequencies like 1 MHz?"There is 12 V supplied, which then gets regulated in two steps for noise reduction down to +-6 V for the two key op-amps.Or if you mean what the input signal voltage accepted is, it is +-30 V. And it's a 10x probe, so +-3 V is output.Check out the design. There are two op-amps, one for low freq and another for high, which are then matched, for good accuracy at low freq as well as good bandwidth.

author: haastyle
author_rank: Regular Contributor
timestamp: August 07, 2025, 01:23:46 pm
content: FYI - these have now shipped. You can get one from CrowdSupply or Mouser.https://www.crowdsupply.com/andy-haas/haasoscope-pro/updates/good-news-products-have-shipped-to-our-distributor