The 50ohm Termination: How, Why and When? - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 14, 2012, 09:57:09 pm
content: 1) if i have 2 high speed opamp very close to each other, should i terminate the output of the 1st amp before feeding to the 2nd amp? like in the 1st picture below?2) how long coax cable then termination is required? or i should always terminate it no matter how short it is?3) if i have an opamp output directly feed to (1Mohm || 15pF) dso using BNC connector (no coax cable), should i terminate the opamp output (on the pcb) to avoid dso looks capacitive or displaying odd things on the screen? (2nd picture)4) i read that by using termination, the coax cable will looks purely resistive (the capacitance of the cable and dso can be neglected, at least thats what i understand). can someone please explain in more elaborate (with math formula if necessary) way. why the capacitance become negligible?5) more confusing combination, eg:a) 50ohm series at source, 50ohm terminate at load end (1/2 attenuation)b) 50ohm series and terminate at source, 50ohm terminate at load (1/3 attenuation)c) 50ohm series and terminate at source, 50ohm series and terminate at load (1/5 attenuation)d) 450ohm series at source, 50ohm terminate at load end (1/10 attenuation)e) 950ohm series at source, 50ohm terminate at load end (1/20 attenuation)f) etc...why all the different combination and when best to use which? how come using 450ohm (not coax characteristic) also result in very high freq desirable respond?too much to ask? well i wish i can post more, but this is all for now.

author: olsenn
author_rank: Frequent Contributor
timestamp: April 14, 2012, 10:07:38 pm
content: You don't need to do that, no. The two two 50ohm resistors in the first photo will simply form a voltage divider to half the voltage being fed into the second op-amp. Since there is no gain on part of either op-amp, all the circuit does is provide a buffered output voltage that is at all times equal to one half of the input voltage.It is common practice to add a small resistor (10 ohms or so) to the output of your final op-amp just to protect whatever device you're connecting it to in case of a voltage spike, but this isn't strickly required. One thing though that it typically done, but is missing from this schematic is to place a high quality film capacitor between your signal input and the input pin on the op-amp. You can add to large resistor to ground as well which adds further protection and noise immunity to the circuit since it acts as a high pass filter to get rid of any 60hz hum or whatever.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 14, 2012, 10:14:08 pm
content: Quoteanswer to question 1,2 and 3? the picture is just for example, voltage follower shown, but it can be gain amplifier, inverting, summing etc. the point is to the "mystery of termination". if its on the pcb and long trace, from what i understand, we need termination, but i dont have definitive answer to why/when is that necessary or not. if it is necessary, then i can only understand it is to avoid capacitive effect.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 14, 2012, 11:02:36 pm
content: Quote from: Mechatrommer on April 14, 2012, 09:57:09 pmGenerally no.QuoteIf the cable is around 1/10 of a wavelength for sure.QuotePossibly. This isn't actually acting as a terminator (the resistor doesn't have to match any particular impedance), it is just protecting the op-amp from seeing the capacitive load, which could lead to instability.QuoteThe cable has both inductance and capacitance. They work together to give the cable its impedance. Explaining it with any degree of justice is probably beyond the scope of a forum, check out an electronics textbook or website.

author: Harvs
author_rank: Super Contributor
timestamp: April 14, 2012, 11:50:35 pm
content: Quote+1 Requires quite a bit of paper to explain this one, and lots of pretty pics of waves traveling in the transmission line.To say the line will look purely resistive is possibly a bit missleading. What you will get if the source impedance, transmission line impedance and termination impedance are equal, is zero reflected energy. Whether this matters to you or not typically depends on the length of the transmission line. If the line is much shorter than the wavelength of what you're putting down the transmission line (like the <<1/10 ejeffrey stated), then the reflections will occur quick enough to likely not cause any disturbance to the circuit. If however the line is long enough you'll end up with peaks and troughs in the middle of you're wave.Do some reading on transmission line theory. You don't necessarily need to go too heavily into the maths, it can be explained quite well by the diagrams and words most books have in them.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 20, 2012, 03:50:56 am
content: now, there are replies since my last post i'm not aware of. how come i didnt notified? duh! thanks people for answering, i didnt noticed.Quotei mean not the series resistor, but terminated resistor to ground. from the datasheet, i read the series resistor is a must if the opamp output is to see anykind of capacitive load. coax cable or dso capacitance in this case.so... can i omit the resistor to ground? and feed the 50ohm impedance opamp output directly to dso? so i can get rid of 1/2 attenuation opamp output? will it reduce the measuring BW of the scope? noobs question here.

author: free_electron
author_rank: Super Contributor
timestamp: April 20, 2012, 05:59:02 am
content: To get the full bandwidth out of the scope you will need to terminate it at 50 ohm using its internal 50 ohm mode. You need to check what is the maximum vpp you can send into the connector when terminated at 50 ohm. Most scopes do not like more than 4vpp at that setting. You risk burning the terminator.Read the manual of the scope. Its in there.Now, you must tune the output impedance of your driver as well. The opamps datasheet will show you the output impedance for a given vpp. You will have to put in a series resistance from opamp output into the coax feed. Your drive opamp needs to be set for a gain of 2. You need to tune the series resistance so that, if you feed 1 vpp into the opamp , you get 1 vpp on the scope.The series resistance and the 50 ohm terminator form a 1/2 divider. The opamp gain of 2 cancels that ut.Most simple opamps have an output impedance in the order of a few tens of ohms. That ths3062 is much lower. This value should be listed in the datasheet.If the ths doesn't pull it switch to an ad811 or ad815. Those beasts can definately do it.I seem to recall that the ths30 series can drive 50 ohm termination without problems. ( provided you have the right series resistance... . Driving 50 ohm does not mean ' hook 50 ohm to ground'. It means from the output pun to ground you need tot have 100 ohm !Driving 50 ohm is NOT the same as driving a 50 ohm TERMINATION,I have the impression that you are in way over your head. You are trying to do something that is quite complex and lack a solid understanding of the things you are dealing with. Making an analog system like you are trying to do is not a matter of taking some junk parts and slapping them together. Also , the 'simulator' aproach is wrong. The problem with simulators is that people blindly believe what comes out of them... For kost of the sims the models are wrong. There are too many factors that are not modelled in simple sims like tina amd ltspice. A really good sim like ADS or Eldo costs a lot of money and it takes years to learn how to use it. And you need accurate models.Where o work we characterise devices with all kinds of information including parasitics , interconnnects and more. The output of the sim is very accurate to the real finished product. But it takes several labs and 20 people to build the sim models of the parts we use.Those cheap sims use simplified models. Like in your other question : good luck getting the input bias current info out of the sim. Most opamp models dont specify that. Ltspice has it becaus linear went through the trouble of putting it in , but that info is encrypted. Try the same in multisim and it wont work. Heck , multisim is so stupid that, if you take an opamp , put a gain of 1000 and inject 1 volt it dutyfully says the output is 1000 volt ... With a 5 volt power supply ? Riiiiight ....

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 20, 2012, 08:36:25 am
content: Quotemy scope doesnt have internal 50ohm.Quotewarning: ths3062 is not meant for repetetive signal of more than 900V/us. FWIW.Quotemost opamps can, even if you short it to ground direct. the problem is with capacitive load.Quoteme know not de transmissioun line.Quotesuggestion?

author: ejeffrey
author_rank: Super Contributor
timestamp: April 20, 2012, 09:11:40 am
content: Quote from: Mechatrommer on April 20, 2012, 08:36:25 amNo, most opamps cannot drive 50 ohm loads, at least not with anything like the full voltage swing.

author: free_electron
author_rank: Super Contributor
timestamp: April 20, 2012, 02:24:50 pm
content: Quote from: Mechatrommer on April 20, 2012, 08:36:25 amlet me guess. Wing wang pong scope ? It's not a scope if it doesn't have an input switchable between 1meg and 50 ohm...You can get around it by putting a 50 ohm resistor yourself as close as possible to the bnc. PlugQuotemost opamps can, even if you short it to ground direct. you are kidding right ? That is not driving. That is shorting. Driving a load means being able to do full vpp across that particular load. If you short the output you have no usable output !Quoteyou dont need a transmission line ! You are dealing with 10MHz sinewaves .. Come back if you have edge rates in the nanosecnod range... Then we'll talk transmission lines.Also , the 'simulator' aproach is wrong...suggestion? soldering iron ?I dont know of dave reads this one but here's an idea for a new t-shirtFront shows an old school soldering iron under 45 degree angle. Text above says ' i have a soldering iron ' ( white text ) text below says 'AND I'M NOT AFRAID TO USE IT' (red text )

author: Time
author_rank: Frequent Contributor
timestamp: April 20, 2012, 04:37:30 pm
content: wing pang pong scope? lol, how delightfully offensive.Lots of entry level/hobbyist scopes don't have 50 ohm options. i.e. the Tektronix TDS2000 series which goes for about $2k these days I believe

author: free_electron
author_rank: Super Contributor
timestamp: April 20, 2012, 09:29:28 pm
content: Quotesimply install a 50 ohm fedd through terminator at the end of the bnc cable. problem solved. By tre way, that tektronix is a toy. I can get used scopes for half that price that do 1GHz...

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2012, 12:45:48 am
content: yeah! that a hell of offensive statement!. my original question is... a fast opamp... have a series 50ohm, that is a must all right... now the question is... the 50ohm to gnd after the series 50ohm.. do i need it? because my scope doesnt have internal 50ohm. i dont need "external 50ohm inline", since i'm intending to connect the pcb direct to scope, "external inline terminator" is pointless imho, since i can do it in pcb... now... for that short distance "direct pcb to scope... do i need "50ohm to ground" in pcb? (acting as "internal 50ohm inline" TO GROUND?, and of course after the "a must series 50ohm just after the opamp output")? if not... that series 50ohm combined with 15pf scope internal, will act as low pass right? duh! is my question confusing enough. i guess so since i'm noob, i may come out with nonsensical question.Quoteand then what?... taking some junk parts and slapping them together?ps: "["quote"]" must be terminated with "["/quote"]" (excluding double apostrophe)no! thats not an offense! thats a... friendly reminderneed several cycle for editing

author: vk6zgo
author_rank: Super Contributor
timestamp: April 21, 2012, 06:59:59 am
content: Quote from: free_electron on April 20, 2012, 02:24:50 pmyou are kidding right ? That is not driving. That is shorting. Driving a load means being able to do full vpp across that particular load. If you short the output you have no usable output !Quoteyou dont need a transmission line ! You are dealing with 10MHz sinewaves .. Come back if you have edge rates in the nanosecnod range... Then we'll talk transmission lines.Gee! I'll have to tell the blokes down at the MF AM Broadcast sites,that they don't need a transmission line!Also , the 'simulator' aproach is wrong...suggestion? soldering iron ?I dont know of dave reads this one but here's an idea for a new t-shirtFront shows an old school soldering iron under 45 degree angle. Text above says ' i have a soldering iron ' ( white text ) text below says 'AND I'M NOT AFRAID TO USE IT' (red text )OK,Shafri gets a bit overextended sometimes,but he's a nice bloke,& he is learning by doing!

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2012, 08:45:04 am
content: Quoteyeah i guess so. which one is better? a stable 0V, or an oscillation?Quotefrom time to time, i kept thinking that, i need that "a piece of paper with golden mark on it" to be eligible to participategoing back to school is not an option for me anymore. so what hope do i have? being a lumberjack is another option

author: free_electron
author_rank: Super Contributor
timestamp: April 21, 2012, 12:27:43 pm
content: Pieces of paper with golden stars are overrated. Waaaaaay overrated..@vk6zgo : any decent scope has the capabilit to terminate its input at 50 ohm. The reason is to maintain signal integrity all the way , through the connector to the digitizer.Even the old tektronixes with vacuum tubes already had that.As for you transmission line. You are dealing with a modulated signal at high power.It is not frequency that matters but edge rate. Like i said a 10 MHz sine wave doesn't need transmission lines ( we are talkin stripline or microstrip on a pcb over a distance of an inch here between two opamps) . We'll talk microstrips when your edge rates are much higher. No need to overengineer and seek nonexistent problems. Problems are very well capable of finding you all by themselves. I've done enough boards for signal integrity in my life and gone through enough EMC quals to know what matters and when it matters.Emc is not the same as signal integrity. Neither are power losses. The question is what are you shooting for.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 21, 2012, 01:37:30 pm
content: Quote from: free_electron on April 21, 2012, 12:27:43 pmMy friend,I've used more Oscilloscopes,(mainly HP & Tektronix), than you've had good breakfasts!Noneof them had 50 Ohm terminations included.Yes,all of them could have an external termination added,but none of them had it as part of the normal input circuitry.By the way,what is this digitiser?---Not in old vacuum tube Tektronix 'scopes!"Edge rate",unless I am very much mistaken is a $5 word meaning "rise time".The minimum rise time of a signal is intimately related to the highest frequency content of the signal.So how can you say it is not frequency that matters?OK,granted,for short distances you can get away with things,& as you said,at 10MHz,an inch or so doesn't matter!As far as modulation & high power,that doesn't really matter,what does matter,as you commented,is the length of the transmission line.

author: jahonen
author_rank: Super Contributor
timestamp: April 21, 2012, 03:24:15 pm
content: I think that is easier to avoid that frequency thinking altogether when dealing with digital signals and just think about how much physical space will the signal edge take on a transmission line. This works whether we have widely separated fast edges or very high repetition rate signal. It works even with single transitions where the frequency and harmonics thinking breaks down.So for example if we have a 1 ns edge and put that onto a stripline structure where signal velocity is approximately about half of that of free space, 1.5e8 m/s. So 1 ns edge will take 1 ns * 1.5e8 m/s = 15 cm length of transmission line (yes, you can put several of those edges simultaneously on the line if the line is long enough and/or edge spacing is small enough!). And now if edge length starts to be 10%…20% of total trace length, then one should start worrying about reflections.For RF things are a bit different. Although frequencies are lower for that 10 MHz system, but the system is more sensitive to reflections. Digital system may well operate happily with just 10 dB return loss (or SWR of1.74~1.9 or |S11| = -10 dB) but that is probably too little for RF power transmission. Imagine a 1 kW transmitter which gets900W100W return power due to mismatch.Regards,Janne

author: free_electron
author_rank: Super Contributor
timestamp: April 21, 2012, 03:38:43 pm
content: Quotewanna take a bet on that ? besides its not how many , its what you do with themjust looking around here there's at least 7 sitting around and i have used all of them in the last couple of months , and they're all different.The tek7405 only has 50 ohm input... same for the lecroy 7200 . 50 ohm only input. Agilent 90000 has APC7 connectors ( called sexless connectors since the are surface mating. there is no male or female. twist connector on using a torque wrench... its that precise. then again... it does 20 Ghz bandwidth ...The scopes in my hobbylab at home ( i got a few there too :p ) : all switchable from 1meg to 50 ohm and back, includes my 20 years old agilent 54645d.My first scope i bought was a german made hameg 604. full analog. 50 ohm switchabel inputs. followed by a philips pm3360. guess what : 50 ohm switchable inputs. i haven't even seen or used a scope that does not have a 50 ohm termination option.Quotea digitzer is a circuit found in any non-stone age dinosaur technology scopeQuotesome peole call it that. depends on britisch english vs american english. tomatoooo tomato . poteeeeto potaaato.QuoteThat is a correct definition. Unfortunatley it is commonly quoted as'the rise time of a signal is related to the frequency' ( note the omission of minium and maximum ) and then it becomes a blatant lie !

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2012, 06:34:50 pm
content: so... with all this very "informative" posts (i mean it), the actual point of 50ohm terminator is... to avoid reflection. right? all i have to worry is that... if the trace length is more or less than 1/10 of wavelength? so say, if assuming the signal from my opamp to the next passive/active element inside the scope is less than 1/10 wavelength, then... i dont need the 50ohm teminator within my pcb (which will be connected directly to the scope, 1Mohm 15pF impedance) right? only when the the trace/coax length is more than 1/10 wl, then only then i will need to terminate the line, right?

author: vk6zgo
author_rank: Super Contributor
timestamp: April 22, 2012, 02:26:20 am
content: Quote from: free_electron on April 21, 2012, 03:38:43 pmYou made a sweeping generalisation:"It's not a scope if it doesn't have an input switchable between 1meg and 50 ohm..."& I disagreed,so it does no good to enumerate the 'scopes at your work,etc,your original comment is incorrect.Ask a few people working in general Electronics to show you their Oscilloscopes.

author: steve_w
author_rank: Regular Contributor
timestamp: April 22, 2012, 05:41:55 am
content: I'll show you mine if you show me your's ;-)I don't really think we are really helping Mechatrommer, he just wants to know about electronics why not stick to answering the questions he asks?BTW Mech good questions I am learning from the answers as well.

author: jahonen
author_rank: Super Contributor
timestamp: April 22, 2012, 06:35:37 am
content: Quote from: free_electron on April 21, 2012, 03:38:43 pm54645D at my work does not have 50 ohm inputs, and neither Agilent spec sheethttp://cp.literature.agilent.com/litweb/pdf/5968-2610EN.pdfmentions one?Regards,Janne

author: vk6zgo
author_rank: Super Contributor
timestamp: April 22, 2012, 07:02:29 am
content: Quote from: steve_w on April 22, 2012, 05:41:55 amSteve,I think you are right!free_electron & I are carrying on a similar argument two threads down,maybe we should take it there.What do you think about that idea,free_electron?Or will we just take a break?

author: Tube_Dude
author_rank: Regular Contributor
timestamp: April 22, 2012, 07:51:25 am
content: Quote from: free_electron on April 21, 2012, 03:38:43 pmHiThe Hameg 604 doesn't have switchable 50 Ohms inputs.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2012, 08:22:24 am
content: hillarous but quite informative. dso brand/model which has or hasnt 50ohm internal termination. i dont really mind, i treat it as "general knowledge"

author: jahonen
author_rank: Super Contributor
timestamp: April 22, 2012, 09:11:19 am
content: Back to original topic, why not try to simulate if the transmission line needs termination or not? In attached simulation I have made the line length to 1 meter which can be adjusted my modifying the "llen" parameter (100e-2 = 1). er is relative permittivity (3 is reasonable guess for a microstrip), and c is the speed of light in vacuum.Regards,Janne

author: vk6zgo
author_rank: Super Contributor
timestamp: April 23, 2012, 03:34:07 am
content: Quote from: jahonen on April 21, 2012, 03:24:15 pmSorry Janne,but you had a bit of a "slip of the pen" here!10dB return loss equates to just 100W return power,and an SWR of approx 1.9:1.Not marvellous,but it wouldn't cook the Tx!900 W "reflected power" is just 0.4576 dB return loss.I used to do this stuff for a living,but I thought Senility might have caught up with me,so I did my calculations with my antique HP22S,& verified them with these charts.http://www.jampro.com/uploads/tech_docs_pdf/VSWRChart.pdfhttp://www.minicircuits.com/app/DG03-111.pdf

author: jahonen
author_rank: Super Contributor
timestamp: April 23, 2012, 08:00:41 am
content: Quote from: vk6zgo on April 23, 2012, 03:34:07 amDamn, my last minute edit was wrong after all, my first thought was actually 100 W, like you saidRegards,Janne

author: vk6zgo
author_rank: Super Contributor
timestamp: April 24, 2012, 07:17:56 am
content: Before free_electron & I had our little disagreement,he,& others, had made the points that:(1) Two Op Amps operating at 10MHz.& around one inch apart,one driving the other, do not need to be terminated in 50 Ohms(or any other specific value of resistance).(2)Until the distance between them reaches approx 1/10 wavelength,( in this case 1/10 of the wavelength of a 10MHz signal is 3 metres in free space,a bit less in a transmission line) ,the connections do not constitute a transmission line.(3)The output impedance of an Op Amp in many cases is very low,so that if a 50 Ohm resistor is connected in line with the output,the apparent output impedance to any circuit driven from that point becomes so close to 50 Ohms that the difference may be ignored.I think that (3) is the "sticking point" for Mechatrommer,in that it is difficult to visualise an amplifier with nearly zero Ohms output impedance.I had the same problem,many years ago,when I was first introduced to Video Amplifiers,with their 75 Ohm output impedance.Here is a link to a site with practical schematics of Video Amplifiers which use the technique in (3).http://cds.linear.com/docs/Application%20Note/an57fa.pdfThese are 75 Ohm devices,but the concept is the same.

author: free_electron
author_rank: Super Contributor
timestamp: April 24, 2012, 01:58:20 pm
content: Oh, it's not a disagreement. It's a matter of perspective. I Can't remember ever using a scope that did not have that capability (and yes the 54645 does not have it. I remembered that wrong. I'm not using that very often. My main staple in the hobbylab is the MSO7104 and the 54832D)And vk6zgo can't remember ever using one that has.So it's proven once more that you can't make assumptions on what is 'the norm' based on what you do or use.Anyway. One comment on opamp output impedance.A typical opamp will have between 100 ohm and 30 ohm. For a high current opamp it can be lower.How do you measure it ?Put opamp in unity gain,Connect input of the opamp to ground ( this is assuming dual supplies. I don't like the usage of 'ground' . The output should be parked at 1/2 the power supply is a better definition.)From the output of the opamp, connect a resistor (100 ohm) to a signal generator. Yes from the output. Apply 1 volt pp. measure voltage before and after the resistor.You have just made a voltage divider. You know two voltages. Go do the math.Oh, and note that, as frequency increases, the phase will start changing... And that the impedance is not constant either...

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 24, 2012, 05:34:27 pm
content: Quotei'll prefer "ground" as a meaning for 0V, and "reference" as a meaning for "other potentials", including 0V, if you likeand as ack: opamp output impedance is not constant and changing based on frequency. the stated output impedance in datasheet is only for prefered freq, usually near or is DC. that will make life more miserable

author: Unknown
author_rank: Unknown
timestamp: April 24, 2012, 06:47:16 pm
content: Quote from: vk6zgo on April 24, 2012, 07:17:56 amI think you may have a problem with senility after all, this should obviously be 30 meter in free space.

author: free_electron
author_rank: Super Contributor
timestamp: April 24, 2012, 06:51:18 pm
content: it's not senility. neuron misfires , commonly known as brainfarts.Why does the robot take the long way around ? They're afraid of short circuits...

author: gregariz
author_rank: Frequent Contributor
timestamp: April 24, 2012, 08:23:58 pm
content: Quote from: Mechatrommer on April 14, 2012, 09:57:09 pmWith regard to 1. its worth noting that typically its quite rare to be able to have full control of the input and output impedance of any amplifier whilst maintaining all other specs you may need ie gains, noise etc. This is because they are often competing requirements. They don't teach this very well in school but when you cascade circuits you have the choice of either impedance matching between sections as you are thinking or adding another section to the circuit to make up any gain loss due to mismatches. Typically designers would simply add another stage, particularly back in the days of transistor design when matching transformers were expensive. the same holds true today however matching sections will likely cost more than another opamp. Its not uncommon to have variable output and input impedances in amplifiers. Ways around this include using a buffer circuit such as an emitter follower or implementing negative feedback.With regard to all the other points its worth noting that any transmission line only operates as a transmission line (reasonably lossless) when it is terminated at each end by the characteristic resistance, typically 50 or 75 ohms. As soon as the resistance is taken away from this value or goes cap or inductive you will get a standing wave on the outside of the coax and your coax will behave like a complex impedance transformer. You will typically notice this on your scope if when you grab the coax you disturb the impedance of the cable and your measurements change. Placing a 50 ohm termination on a T connector at the input of your scope fixes that end. You'll need to impedance match the other end if your active output impedance of your circuit is not 50 ohms.hope that helps a bit

author: vk6zgo
author_rank: Super Contributor
timestamp: April 25, 2012, 02:07:54 am
content: Quote from: free_electron on April 24, 2012, 01:58:20 pmWell caught,I should have referred to the Op Amp used in a particular circuit,not the output impedance specified in the spec sheet.The effective output impedance does reduce to a much lower value,or the Video Amplifiers in the link would not work.The method of measuring output impedance works with any amplifier,though the first time I was confronted by it in the old Perth Tech School back in the '60s it freaked me out a bit!

author: vk6zgo
author_rank: Super Contributor
timestamp: April 25, 2012, 02:12:37 am
content: Quote from: alm on April 24, 2012, 06:47:16 pmI think it effects reading comprehension too!What is 30/10 ?

author: free_electron
author_rank: Super Contributor
timestamp: April 25, 2012, 02:17:55 am
content: national semiconductor has a nice app note on measuring output impedance. don't rmember the exact number. hang on ....this is a good one on phase margin and how to measure it:http://www.national.com/assets/en/appnotes/AppBrief108.pdfcan;t find the 'opamp measurement cookbook. it was appnote 23 or something like that. a big fat appnote with all kinds of practical circuitry...dang it , where's those blue books when you need them ... ( i still have the whole collection of blue national semiconductor appnote books on the shelf at home... )

author: vk6zgo
author_rank: Super Contributor
timestamp: April 25, 2012, 03:01:22 am
content: Shame the Engineers in the dear old People's Republic who designed the Transmitters we had at my last work didn't read this App Note.They used an LM358 in a diff amp configuration to control the level of RF drive to a 1kW RF amp.Due to a capacitive load,at certain power settings,it would "take off"at 38kHz,Amplitude Modulating the drive signal to 100%+This really upset the switchmode power supplies!We eventually fixed it "brute force" with a 220 Ohm resistor in line with the 358's output.Luckily,the Transmitters were used in an ISM application so we didn't put this crud to air!

author: Unknown
author_rank: Unknown
timestamp: April 25, 2012, 06:32:33 pm
content: Quote from: vk6zgo on April 25, 2012, 02:12:37 amCould you speak up please? My hearing aid is broken because a damaged coaxial cable is causing an impedance mismatch at high audio frequencies.You're of course completely correct.