Input to single ended probe designs - Page 1
Forum section: electronics / projects

author: beardedhustler
author_rank: Newbie
timestamp: June 07, 2021, 06:47:04 am
content: Originally posted this in another thread (https://www.eevblog.com/forum/testgear/gt-1-ghz-diy-differential-probes/) but as it was pointed out there it's probably better to make an own post since it's not entirely the same topic.At the company where I work, we have a Signatone probing station with (passive) "micropositioners" (probes) that we use to probe on bonding pads on various chips and read data with some reader attached. Coaxial cables from probes to reader are ~1m, so signal integrity isn't always great when the frequency increases, thus the need to add an active stage/probe. For now we've mostly dealt with various SPI flash chips, eMMC chips in 1-bit bus width mode, up to ~36MHz clk speed (I got enough probes that I can go for the 4-bit bus width mode which supports clk speeds up to 200MHz if I want, but it's not always beneficial to spend time and energy trying to place six probes within the small area where the bonding pads are located and at the same time not break any bonding wire between the pads and the controller, etc, so usually I stick to using a 1-bit bus width..).Due to the possibility of using the HS200 mode for eMMCs I think a 1GHz BW is an ok goal to aim for here, and since eMMCs use single ended signals I don't have the need for a differential probe (for now - that might change later on if we decide to try probing UFS chips, but that's another story - we'll cross that bridge when we get to it...). Another important thing to mention is that the goal here is to make the pcb that we can custom fit on the micropositioners in some way to utilize what we already have and thus buying a general purpose probe isn't an option for this now.I've started on a few different single-stage designs trying out various op-amps from TI (LMH3401, LHM6702, OPA855/OPA858) and getting a 3dB-BW > 1GHz seems quite straight forward with the op-amps mentioned (some trade offs here and there for the various devices..) but I'm a bit unsure on the pitfalls when it comes to designing a active probe. I know I should have as high input impedance as possible / as low input capacitance as possible and that both these will be limited by the devices themselves.It's been a few years since I did design and back then it was mainly high power RF PAs with different things to consider during the design process. I've only used TI's spice simulator for now, and although it's a nice tool I guess it has its limitations. I tried importing some of the spice models they offer on their web page into Keysight ADS, and although I get almost identical results for similar simulations I'm not sure of the validity of the models when used in different simulation software / other simulations, e.g. S-parameter simulations, which for some of the devices result in S11>0dB for a wide frequency range (and in my experience from the RF PA design days that's a big no-no).Anything else (besides input impedance/capacitance) to consider other than e.g. trying to impedance match output from op-amp to 50Ohm (coax cable) to maximize power transfer to the reader?I've only tested TI op-amps for now since I found the availability of their simulation software and spice models handy, but I'm open to try op-amps from other vendors (Analog Devices, others) if someone should have suggestions and there exist models / software to use them in. Also, should anyone have suggestions to some literature for the topic I'd greatly appreciate that. I tried looking through my old text books from the university days but didn't find much beside general "op-amp"-theory and other types of circuits we made in the lab back then. Doesn't matter if it's theoretical literature or more practical oriented - I'm used to both.

author: David Hess
author_rank: Super Contributor
timestamp: June 07, 2021, 06:29:30 pm
content: Quote from: beardedhustler on June 07, 2021, 06:47:04 amFeedback amplifiers can work in less critical applications but recover very poorly from overload, which is a common problem with active probes anyway. I was thinking of using the LTC6268 configured as a follower for this sort of thing.Low capacitance is important for high speed however high impedance is usually not because the source impedance is unlikely to be high; 25 to 150 ohms is common. A moderately high resistive input from a resistive divider of 1000s to 10,000s of ohms may be acceptable, and "medium impedance" active probes using bipolar input transistors used to be available.Consider going the simple route with an RF JFET or MOSFET configured as a source follower followed by complementary pair of emitter followers driving the 50 ohms transmission line. The most difficult part about this now is finding a fast PNP transistor, which makes me wonder if a JFET source follower in front of a fast current feedback amplifier to drive the transmission line would work well enough.QuoteIt is not necessary to match the output impedance to the transmission line if it is terminated at the other end. Matching the source impedance actually halves the voltage and quarters the power. In a double terminated system, the source sees twice the impedance of the transmission line but without the source termination, it sees the impedance of the transmission line itself at all frequencies and requires twice the output current to drive it.Most active probes use double termination to minimize reflections anyway of course. The disadvantage of this is that it doubles the amount of gain required.

author: Marco
author_rank: Super Contributor
timestamp: June 07, 2021, 10:57:37 pm
content: There's a couple versions of the AC coupled dual gate MOSFET active probe out there for sale.

author: beardedhustler
author_rank: Newbie
timestamp: June 08, 2021, 08:08:08 am
content: First of all, thanks a lot for the input, greatly appreciated!Quote from: David Hess on June 07, 2021, 06:29:30 pmSo you'd recommend staying away from the voltage FB devices if going down the op-amp route? To begin with I was considering a design built around a MOSFET or another RF transistor, but I found it a bit harder to search for (more precisely: filtering on digikey, etc) parts compared to. e.g. op-amps, where at least TI have made it very easy on their own web page.Quote from: David Hess on June 07, 2021, 06:29:30 pmYou're probably right here - so good thing I asked. I was convinced I'd need a very high input impedance to not load the DUT. The two schematics I've spent the most time on (not much though; they're very simple...) are both based on current feedback devices (LMH6702- 1.6pF input capacitance on the non-inverting input according to the data sheet /LMH6703- 0.8pF input capacitance on the non-inverting input according to the data sheet) in a follower configuration, but in my simulations the input impedance was quickly decreasing from about ~700Ohm @ ~200MHz to ~100Ohm @ ~1GHz (in the region up to 50MHz it's ~7-8kOhm), and thus I was thinking this maybe wasn't enough, and hence my post here.Quote from: David Hess on June 07, 2021, 06:29:30 pmMakes sense. I'm probably still used to always think impedance matching is key on the output stage. Again, thanks for the input. I'll finish one design with the LMH670x devices to see how it goes but also try making a different design based on a RF JFET or something as well.Quote from: Marco on June 07, 2021, 10:57:37 pmYah, I know. It might come down to that in the end, however, since I want to "integrate" the probe into the micropositioner we already have at our lab due to the fine precision tuning of XYZ it offers, buying a probe to retro-fit isn't the first option. But it's definitely not out of the question should I fail to design something that does the job for us, but then I'll have to find one that is made for probing ICs/bonding pads, which I guess isn't the first thing people are thinking of when speaking of active probes. Signatone (and others, e.g. Agilent/Keysight) have this for sale (e.g.https://www.signatone.com/products/accessories/pico.aspandhttps://www.keysight.com/zz/en/product/N2884A/infiniimax-differential-fine-wire-probing-tips.html), but they cost a *fortune*.It's basically something like the pico probes from Signatone I want to mimic. I will of course not manage to make something with similar specs wrt. input capacitance etc, but I hope I can end up with something where I can use the probe tips we already have and make it an active system to improve the signal integrity when we do read outs / etc.

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2021, 05:36:06 pm
content: Quote from: beardedhustler on June 08, 2021, 08:08:08 amIn a design for general purpose work, I would avoid shunt feedback because it recovers from overload very poorly. I would also avoid it at higher performance levels because it is very difficult to get a clean high frequency response unless bandwidth is limited. But in your application, neither of those may apply so the simplicity of becomes an advantage and I think you can get away with it. It only needs to be good enough.QuoteIt needs low capacitance, but not necessarily high resistance, so a low impedance resistive divider could be acceptable at the input to increase the input voltage range and decrease the input capacitance. This brings up whether a much simpler low-z probe would be sufficient with a 450 or 4950 ohm resistor directly drives a 50 ohms transmission line to make a x10 or x100 probe.QuoteIt comes up a lot in early oscilloscope design because they were pushing what was possible with the parts they had. Double termination would mean twice as much gain is needed so either more stages or lower bandwidth. Of course to take advantage of this, the output amplifier needs to be able to drive a real 50 ohm load and not 100 ohms from a series termination at the source. But if the output voltage range is limited, then this is not a problem with modern integrated parts.The old Tektronix P6203 is the 1 GHz 10x bipolar active probe that I was thinking of and the type of active probe that I think would be easiest to duplicate. Its input capacitance is 2 picofarads and input resistance is 10 kilohms, which is a much lower input resistance than an FET probe. I suspect it used a 10 kilohm 5x resistive attenuator, followed by a bipolar buffer to drive a doubled terminated 50 ohm output, yielding 10x attenuation.This could be duplicated with with a 5x compensated resistive attenuator followed by a voltage feedback voltage follower, current feedback voltage follower, or a diamond buffer. Of these, a current feedback voltage follower is probably the best option because the value of the feedback resistor between the output and inverting input can be tailored to give a cleaner high frequency response at the expense of bandwidth, but getting 1 GHz performance will be difficult. 500 MHz might be possible, or maybe TI has some better parts for this.