Active Reduction of Apparent Cable Capacitance - Operational Amplifier Selection - Page 1
Forum section: electronics / projects

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 03:20:59 am
content: So I previously posted about extending oscilloscope probes. After some additional information gained from some of the responses I decided to read and educate myself further on the topic. In that process I came across a publication that was recently published in regards to reducing the capacitance in cable.The link to the publication could be found here.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10574904/I am posting this to hear the opinions of others who are a lot more knowledgeable in this particular topic. I also thought it might be interesting to others as well.The researchers used a Texas Instruments LM6172-2 as the final choice for an operational amplifier.Could there have been a better choice for an operational amplifier that anybody knows of?I am doing this to hear opinions on this particular publication. Looking forward to the education.Thank you to all that contribute.

author: coppercone2
author_rank: Super Contributor
timestamp: December 04, 2023, 03:38:32 am
content: What are you looking to improve?It says that part is applicable to DSL. That is a good sign of robustness if they expect it to connect directly to cables. Generally that might mean a part is beefy if its meant to hook up to cables that go into other equipment. I say its a good choice? Its also been in production for a while. I read something about molten salt and the fact that the part is in production for a while sounds pretty good when you are dealing with molten salt. I assume this might be related to a nuclear reactor. That might be enough time to expose some kind of flaw in the part that makes it unsuitable for critical applications. Not sure you want the latest miniaturized new semiconductor process ultra low quiescent current portable coin cell device ready op-amp that is whipped up recently for that application. Alot of the new stuff is meant to be lower standby power because of the portable electronics craze. I think I will take that over the new 'internet of tomatoes' parts from AD (yes, I did not make that up) for whatever a molten salt pipe is connected to. I am sure there are some more spiffy op amps but they might be intended for bluetooth communications with green tech tomato plant management systems on a 'too cool for the board room' form factor PCB gimmick.Not to say you don't need protection for a part connected to cables, but its a good start. But that is getting heavy into cable protection/telecom stuff. But having a good part behind all that stuff is still a good bet.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 03:50:47 am
content: Quote from: coppercone2 on December 04, 2023, 03:38:32 amThank you for the response. I am not really looking to improve anything, nor do I think I would be capable of improving anything. I'm starting this conversation to hopefully learn more in the process.One of the things I was wondering is up to what BW could you push this to? The paper kept it fairly modest.For me the application would be the capability of extending oscilloscope probes.I am trying to further my knowledge of operational amplifiers and I was wondering if there might be a better operational amplifier that could be used in that particular circuit for that particular application.

author: coppercone2
author_rank: Super Contributor
timestamp: December 04, 2023, 03:52:07 am
content: hmm if you want to keep the VFB op amp then like 1 GHzhttps://www.analog.com/en/products/ad8045.htmli have some hunch that it might get interesting with the triaxial cable at that frequency though

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 04:10:24 am
content: Quote from: coppercone2 on December 04, 2023, 03:52:07 amYour guess is probably a lot more educated than mine. lolThanks for sharing the link. What was your thought process in recommending it?I will read the datasheet in the morning.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: December 04, 2023, 04:26:50 am
content: Hi,Using triaxial cables and driving the inner shield is a common technique instrumentation. It really only works well at low frequencies. What are low frequencies? Frequencies below those where you have to consider the cable as a transmission line.Jay_Diddy_B

author: coppercone2
author_rank: Super Contributor
timestamp: December 04, 2023, 06:11:56 am
content: I just looked for the highest frequency available VFB amplifier. Whatever magic you come up with, that might be a realistic upper limit for using the same method as the paper.i have no idea if it will work. it does not seem to drive alot of capacitance. and i only considered the coax case, the triax case is ??

author: ejeffrey
author_rank: Super Contributor
timestamp: December 04, 2023, 06:27:57 am
content: The bandwidth is ultimately limited by at least the cable round trip time. This only works if the voltage at the source and the amplifier are approximately equal. For a 1 meter cable the round trip time is about 10 and so you need to stay well below 100 MHz.It's also a bit of a trick. It doesn't actually improve the ideal SNR. Remember, the input noise of an amplifier is independent of the feedback network. This guard is just part of the feedback network. What it does is act as an automatic gain equalizer to compensate for the RC rolloff at high frequency, but the noise gain increases proportionately.It's still beneficial since generally flat response is desirable, and without it the high frequency gain might be too low to get over the noise of the second stage amplifier. So I that way it can improve your overall system SNR. But there are limits, if you try to increase the bandwidth, eventually the noise will get too high.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 04, 2023, 08:28:11 am
content: If a really fast probe with a longer cable is needed, it may be easier to use / build an active probe. So have the first amplifier (actually only a buffer or maybe a gain of 2) directly at the proble head. The cable can than be normal coax plus an extra power cable.

author: jonpaul
author_rank: Super Contributor
timestamp: December 04, 2023, 04:45:16 pm
content: The referenced paper is nothing new.Analog and instrumant designers have used driven guards in bridges, analyzers, meters since 1930s.The amp is uaually a X1 to drive the guard and the freq resp needs to be good for low phase shift at the measured freq what ever the band of use will be.Emitter and cathode followers were the per-integrated circuit solution.Guarded capacitance is especially used in shielded transformers.Bon chanceJon

author: ConKbot
author_rank: Super Contributor
timestamp: December 04, 2023, 04:56:46 pm
content: What are you trying to probe? Rather than the headaches of extending probe cables(which are not simple low loss transmission lines), can you put signal conditioning circuitry close to the signals of interest and just use a 50ohm line and 50ohm termination to have the signals play nice? The opamp doesn't need to drive 50 ohms if you let there be some attention. I.e. buffer opamp drives a 200 ohm serirs resistor into the transmission line, 50 ohms termination gives you a 5x probe, and the opamp only sees 250 ohms of load.A variation on this is just using a 450 ohm feed resistor and no opamp, of the circuit can be loaded with 500 ohms without problem.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 04, 2023, 06:20:01 pm
content: Bootstrapping a shield to neutralise its capacitance is nothing new. In essence you feed the signal onto the shield via a buffer amplifier. Then you need a secondary shield to carry the return signal current and divert interference to ground. Easy right, get some triax and and an opamp, job done?Well it can go horribly wrong.The source impedance, cable capacitance, inductance and stray capacitance at the input node can add a peak in the frequency response and the sysytem either rings or oscillates. In practice you want the buffer operating a tad shy of unity. Excesive neutralisation leeds to peakyness.See the most excellent paper attached.

author: jonpaul
author_rank: Super Contributor
timestamp: December 04, 2023, 07:51:25 pm
content: Terry : Fine IEEE paper from Argentina, 2010.I think a simple emitter follower of x1 discrete FET buffer will do the same.Jon

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 08:02:05 pm
content: Quote from: Jay_Diddy_B on December 04, 2023, 04:26:50 amSo in the regards to this paper, what would be the upper limit in your opinion when it comes to frequency and usage as an oscilloscope probe application?

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 08:05:11 pm
content: Quote from: Kleinstein on December 04, 2023, 08:28:11 amI just recently read about active probes and the circuitry involved in making one. I understand everything you are saying except "plus an extra power cable". Could you please elaborate on that, I am trying to learn.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 08:10:03 pm
content: Quote from: jonpaul on December 04, 2023, 04:45:16 pmBased on what you are saying, as well as the 2 books that I read on probe design (which I believe were from the late 60s) it is possible to make an active probe that could be extended to a reasonable length of 12 feet.That is what I deduced from your post and from my reading. Although how to proceed about that I am completely unaware of at this point. At this moment I am just reading and asking questions, this is the way that I learn.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 08:17:18 pm
content: Quote from: ConKbot on December 04, 2023, 04:56:46 pmI am primarily posting this post along with some of the others as a thought process to help me digest and learn certain topics. As far as me personally, the reason I am looking for extension capabilities is to be able to get to certain places that are not as accessible, because some of those places are not accessible I am trying to position the test equipment sometimes a distance away so that it does not accidentally get damaged or get in the way. For example you are 6 feet away from your oscilloscope and you want to measure a particular sensor.Because I am in the process of learning I really don't know if you're suggestion would be applicable or not. I am also in the process of trying to increase my knowledge on operational amplifiers so when I start playing with that maybe I can experiment with some of your suggestions.Thank you.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 08:19:58 pm
content: Quote from: Terry Bites on December 04, 2023, 06:20:01 pmThanks for the paper. I will read it within the next day or so. I don't quite understand everything you wrote. Let me read the paper and then maybe I will have some questions where I can gain some clarification.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 08:20:50 pm
content: Quote from: jonpaul on December 04, 2023, 07:51:25 pmFor somebody who is not on your level could you please elaborate if possible.Thank you.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 04, 2023, 08:21:41 pm
content: Quote from: JJ_023 on December 04, 2023, 08:05:11 pmAnd active probe has the amplifier right at the probe tip, so it needs power as well as a the coax for the signal to send to the scope. If you look at many oscilloscopes, they have a row of contacts under or near the BNC connector. Those are for powering active probes, and also for identifying them (to correct for the gain/attenuation).QuoteAn active probe will normally drive with a 50 ohm source impedance and it can be extended essentially as far as you want.

author: JJ_023
author_rank: Frequent Contributor
timestamp: December 04, 2023, 08:46:23 pm
content: Quote from: ejeffrey on December 04, 2023, 08:21:41 pmThank you for the explanation. I understood everything you wrote.

author: bdunham7
author_rank: Super Contributor
timestamp: December 04, 2023, 10:32:40 pm
content: Quote from: ejeffrey on December 04, 2023, 08:21:41 pmI'm not sure that works as well as you might think. Active probes and the like are intended to drive a 50R scope (or SA) input directly and might not deal as well with the capacitance of a long cable.

author: xavier60
author_rank: Super Contributor
timestamp: December 07, 2023, 11:03:25 pm
content: Quote from: bdunham7 on December 04, 2023, 10:32:40 pmIf something is designed to drive a 50Ω load, any length of 50Ω cable shouldn't matter except for some loss. The cable's measured capacitance doesn't matter when the cable is used as a matched transmission line.