DC resistance of a 1x oscilloscope probe - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: October 26, 2014, 12:51:48 am
content: I was trying to measure a 5V DC voltage from a power supply with my scope today using a 1x probe. In order to measure a little bit of load on the supply, I set my scope termination to 50 ohms and was expecting to read 5 volts. Instead, the reading I got was 1.76 V. Having been burned before by the resistance of a 10x probe, I decided to measure the resistance of my 1x probe. The resistance measurement of the probe was 100 ohms, when I measured from alligator clip to tip while shorting the BNC side of the probe.I was expecting for the resistance of the probe wiring to add up to milliohms and for the probe to completely stay out of the measurement equation as a result. As it turns out the probe wiring (and internal circuitry) accounts for 100 ohms. Is this normal? If it is, why does the probe have such a large resistance, skewing measurements made with it when termination is added into the equation?

author: kg4arn
author_rank: Unknown
timestamp: October 26, 2014, 01:06:12 am
content: The 100 ohms sounds reasonable. The probe is built with resistance wire to dampen reflections.Also it is designed to be used with the 1Meg input to the scope, not the 50ohm.This article may be helpful:https://www.dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdf

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 26, 2014, 01:20:48 am
content: KG4ARN is right. You motivated me to check a couple of probes I own, for comparison. A no-name ebay probe is 100.6 ohms, while a Rigol probe is 228.1 ohms. Both measurements were performed by shorting the ground lead to the tip, and measuring the total round-trip resistance across the BNC plug while in 1X mode.The 1x probe is designed for use with the high impedance scope input, where the distributed resistance is required in order to dampen reflections.

author: CosPhi
author_rank: Regular Contributor
timestamp: October 26, 2014, 01:47:30 am
content: Dave mad a video about it X1 probes.EEVblog #453 - Mysteries of x1 Oscilloscope Probes Revealed

author: SharpEars
author_rank: Unknown
timestamp: October 28, 2014, 12:41:14 am
content: Quote from: CosPhi on October 26, 2014, 01:47:30 amWell, I watched the entire video and it's even more "clear as mud." How the heck do you get a good 1x probe that doesn't add resistance into the circuit??For example, how in the world could I possibly use the 50 ohms of termination in my oscilloscope with any probe?Plus, it seems that if 10x probes are the only way to go as the video would have you believe, then how in the heck do I use the 500 uV setting of my scope, which is only available for 1x probes, to measure low level power supply/LDO ripple noise.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: October 28, 2014, 12:49:55 am
content: a differential amplifier, with a properly balance i/o....is what you need to see at or below those noise levels....also remember that bandwidtch is limited with such setups....you will never find a passive device or piece of wire that doesn't load an input (capacitance, inductance etc...). The key is matching impedance i/o properly and removing the loading and noise with a balanced connection and proper amplifier...Again it's not an issue of the differential or active probe NOT having resistance, it's that the resistance is nullified with gain from a pre-amplifier....which you then match to the input amplifier of your device....and believe me doing this the "right" way ain't cheap.....you need to make sure your entire signal path is operating at unity gain....I won't even get into phase offset and doing this with multiple (time-correlated) channels....With the precision calibrations necessary, I am sure you can imagine why this is so costly to do the "right" way....

author: w2aew
author_rank: Super Contributor
timestamp: October 28, 2014, 12:52:36 am
content: Quote from: SharpEars on October 28, 2014, 12:41:14 am1X probes are ONLY to be used with the 1Mohm input impedance setting. These are fine for measuring the ripply on your power supply. The 100 ohms or so series resistance is negligible in series with the 1Mohm input impedance. Use 10X probes with the 1Mohm input for larger and higher BW signals, and where the 100pF or so of loading from the 1X probe is too much for the circuit. The 50ohm termination is NOT designed to be used with a 1X probe - period. It is designed to be directly connected via 50ohm coax, or to be used with Z0 low-capacitance probes - such as those with a 450 or 950ohm resistor in series at the tip (10X/20X low-Z probes).

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 28, 2014, 01:02:46 am
content: Quote from: SharpEars on October 28, 2014, 12:41:14 am1) you use a DMM, because your scope's 50 ohm input is not a DC dummy load2) 50 ohms is not for probes, it's for direct coax connect.

author: ludzinc
author_rank: Unknown
timestamp: October 28, 2014, 01:46:13 am
content: Quote from: SharpEars on October 28, 2014, 12:41:14 amIn a 50 ohm system, with coax.BTW - what's the power rating of your 50 ohm internal load? You're trying to dump 1/2 watt into your scope! The x1 cable's 100R resistance probably saved you there.

author: SharpEars
author_rank: Unknown
timestamp: October 28, 2014, 01:55:46 am
content: Quote from: ludzinc on October 28, 2014, 01:46:13 amI am fairly certain that the internal 50 ohm load can handle half a watt. I mean this isn't a cheap knock off scope for crying out loud, it's a Rigol. I don't know the exact load limit, but I think it can handle half a watt. It's not like I dumped 60 volts into it. If they were not smart enough to at least put in a 1 watt resistor (or hopefully much larger), well I don't know what to say...Update: Just checked the specs - the 50 ohm impedance is selectable up to 1V per division. So, even with a 1X probe, I think my scope is pretty safe...

author: kc9qvl
author_rank: Regular Contributor
timestamp: October 28, 2014, 02:24:30 am
content: Take a look at some of Alan w2aew's video's on scopes. Most 1x/10x on 1x settings are good to about 5 mhz. So measuring ripple on a linear or switching power supply not a problem./www.youtube.com/watch?v=SX4HGNWBe5M&list=UUiqd3GLTluk2s_IBt7p_LjAwww.youtube.com/watch?v=067W7h1BhxE&list=UUiqd3GLTluk2s_IBt7p_LjA][url=https://www.youtube.com/watch?v=067W7h1BhxE#ws]www.youtube.com/watch?v=067W7h1BhxE&list=UUiqd3GLTluk2s_IBt7p_LjA

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 28, 2014, 05:51:21 am
content: Quote from: SharpEars on October 28, 2014, 12:41:14 amA good 1X probe MUST use lossy cable which will add resistance to the circuit in order to work. Dave Ford explains the issue here:https://www.dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdfBriefly, you need that lossy coax because there's a huge impedance mismatch where the 50 ohm coax meets the 1Mohm scope input. That impedance mismatch causes reflections that would ring back and forth on the coax transmission line, distorting the measured results. The distributed resistance of the lossy line dampens those reflections.QuoteWell, if you're using the 50 ohm termination in your oscilloscope, you no longer have the impedance mismatch issue described in Dave Ford's paper I linked above. So for that case, you should use conventional 50 ohm coax, and NOT a normal scope probe with its lossy coax.Quote10x passive probes aren't ALWAYS the way to go, but they're the cheapest way to get the rated bandwidth out of your scope.For the lowest possible signal levels, you can use a normal 1X probe with the normal 1Mohm input impedance. The bandwidth will be limited, but should be enough to measure power supply ripple. The limited bandwidth is the price you pay for the ability to see such low voltage signals. If you can't pay the price of reduced bandwidth, but still need to see the lowest voltage signals, an alternative might be to use an amplified probe of some sort, but it will be much more expensive than a 1x passive probe.

author: rf-loop
author_rank: Super Contributor
timestamp: October 28, 2014, 06:37:00 am
content: Also this is nice to read (old but basics have not changed).Tektronix OLD "famous" probe circuits by Joe Webber.These first pages very low level basic fundamentals do not change every year.Read least around 20 first pages.Tektronix old passive probes use typically these R coaxials, not Z0 coaxials.(also there is later nice explanation about Z0 probes what are easy to DIY and very good in some circumstances for high frequencies)BTW, many older expensive professional scopes have protecting circuit for 50 ohm inputs. Just turn off 50 ohm if it detects too high voltage.These many chinese oscilloscopes do not even give any specifications about max voltage/power for 50 ohm inputs. Very sad or perhaps they think "who cares" because after smoke there can buy new toy.

author: coppice
author_rank: Super Contributor
timestamp: October 28, 2014, 08:19:14 am
content: Quote from: rf-loop on October 28, 2014, 06:37:00 amMost older high end oscilloscopes gave no information about their 50 ohm terminator rating or protection for the terminator. Many engineers only ever use an external terminator because of bad experiences with the internal ones.

author: tggzzz
author_rank: Super Contributor
timestamp: October 28, 2014, 09:24:46 am
content: Quote from: AG6QR on October 28, 2014, 05:51:21 amTrue, false.Try finding a *10 high impedance passive probe to get >500MHz bandwidth! Especially without excessively loading your circuit. Even 100MHz is problematic with the typical 15cm ground lead!The cheapest way to get 1GHz-6GHz is to use a *10 or *20 low impedance Z0 probe. Fortunately a 1GHz probe is easy to build yourself, as others have noted.

author: rf-loop
author_rank: Super Contributor
timestamp: October 28, 2014, 10:19:53 am
content: Quote from: coppice on October 28, 2014, 08:19:14 amThis tell lot of about average level engineers level of experience and quality of education.Yes I still remember when engineer destroy expensive things just after someone tell that oh you are using too high signal level and hurry set it more low. What he did... he quickly turn level to 0dBm --> go to cashier and never enter back to this lab.Quotesome tiny random examplesI do not know if "most, but in professional use Tektronix oscilloscopes have been nearly as "standard".On my old Tektronix analog scope front panel, just below input BNC, read very clearly: 50? ? 5VrmsAnd in manual. more detailed.(but this was not "high-end" or "state of art" scope. only just usual and very common 2400 series work horse.)Then also with old digital "work horse" Tek 2440, manual tell about 50 ohm input: Maximum input voltage 5Vrms; 0.5W sec for any one-second interval for instantaneous voltages from 5V to 50V.So, it means that in this case maximum acceptedpeak poweris 50W (max 10ms pulse) because there is also voltage limit, not only power limit. One second average need stay 0.5W or less.And lot of other examples.

author: nfmax
author_rank: Super Contributor
timestamp: October 28, 2014, 10:31:52 am
content: It can happen to the best of us. Many, many years ago my PhD supervisor fried the fixed 50-ohm input of a Tektronix 7000 series 500 MHz plug-in trying to troubleshoot a laboratory furnace. It didn't stop him from subsequently being elected FRS though.

author: coppice
author_rank: Super Contributor
timestamp: October 28, 2014, 11:06:10 am
content: Quote from: rf-loop on October 28, 2014, 10:19:53 amYou might be more right than I realised. I've scanned through the front panels of some classic scopes on the web. Most early 70s ones have no 50 ohm termination option. In the late 70s and early 80s they had an unlabelled 50 ohm option. They must have learned their lesson, as later scopes seem to be labelled about their limitations. I thought the duration of unlabelled 50 ohm inputs was much longer than it seems to have been.One thing that's really dumb about the 50 ohm option on a lot of scopes is its far too easy to slip into that position when selecting among other options on the same control. I seen a few nasty bangs occur that way.

author: tggzzz
author_rank: Super Contributor
timestamp: October 28, 2014, 11:16:42 am
content: Quote from: coppice on October 28, 2014, 11:06:10 amMy 1970s HP1740 has a lever switch which slides to select AC ground and DC, but you have to lift-and-slide to make it 50ohms.Good attention to detail from a great company, now calledAgilentKeysight.

author: rf-loop
author_rank: Super Contributor
timestamp: October 28, 2014, 11:34:49 am
content: Quote from: coppice on October 28, 2014, 11:06:10 amYes, this is true.I remember some HP old 17xx analog. It was nearly impossible to turn switch to 50ohm position accidentally. (it was just mechanically protected. It need turn and pull over threshold before it can turn to 50ohm position.)Some these old scopes have also protection circuits what shut off 50ohm if input level is too high.Edit: oh, tggzzz tell about HP also when I was writing slowly my msg.

author: TonyStewart
author_rank: Contributor
timestamp: October 28, 2014, 11:37:48 am
content: FET buffered Tek probes were blown by young engineers repaired by our female lab tech on a monthly basis in the 80's. All you had to do is touch the probe with more than 25V of ESD to blow them, but were only 0.5pF and high bandwidth. Same with differential Tek. FET- Buffered probes. They had convenient tiny shorting plugs that got conveniently lost.But as most know, x1 probes must be clearly labelled with impedance, AC and DC power rating or read the fine manual.- most good DSO's have 50 Ohm option built-in for low power.- if you are probing a voltage source (low Z) for noise a 50 Ohm cable NEEDs 50 on input and load , thus 100 Ohm resistance.

author: SharpEars
author_rank: Unknown
timestamp: October 28, 2014, 05:04:30 pm
content: Quote from: EEVblog on October 28, 2014, 08:02:39 amYes Dave, I watched the video in its entirety, hence my followup questions, such as how to measure power supply ripple at the 500 uV setting which requires the use of a 1x probe, while loading the supply with a 50 ohm load and getting correct measurement results...

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2014, 06:23:23 pm
content: Quote from: SharpEars on October 28, 2014, 12:41:14 amYou would use the 50 ohm termination inside the oscilloscope with a low-z probe which is essentially a 50 ohm coaxial cable with a 450 ohm resistor in series on the end making a x10 probe. Low-z probes are available but expensive because of low demand.QuoteThere is no problem here. You do not use the 50 ohm termination with a x1 high impedance passive probe and the series resistance of the x1 probe has an insignificant effect because of the 1 Mohm input resistance of the oscilloscope.

author: Marco
author_rank: Super Contributor
timestamp: October 28, 2014, 06:43:38 pm
content: Just to point out the obvious ... you should use AC coupling when using the 1x probe to measure power supply ripple (and again, don't use 50 Ohm mode).

author: tggzzz
author_rank: Super Contributor
timestamp: October 28, 2014, 07:45:55 pm
content: Quote from: rf-loop on October 28, 2014, 11:34:49 amSynchrotonicity. Great minds think alike.

author: TonyStewart
author_rank: Contributor
timestamp: October 29, 2014, 01:56:51 pm
content: Quote from: SharpEars on October 28, 2014, 01:55:46 amIn 30yrs as a Test Engineer, I never had to read an equipment manual unless it was for programming syntax or calibration of the Anritsu VNA to <0.25 dBBut i made an exception here and was surprised to not find any AC limit on 50 Ohms , but for DC, I found a limit.±1.25 V, 50 ? that should read Omega not OMG or ?I guess experience and common sense dictates using equipment properly by learning how to break them.

author: SharpEars
author_rank: Unknown
timestamp: October 29, 2014, 05:38:28 pm
content: Quote from: TonyStewart on October 29, 2014, 01:56:51 pmLOL, so even if my scope had such a ridiculously low limit, the 100 ohm probe would still only place about 5v/3, or 1.66 volts across it. I think it would live...

author: coppice
author_rank: Super Contributor
timestamp: October 29, 2014, 11:59:56 pm
content: Quote from: TonyStewart on October 29, 2014, 01:56:51 pmIf you experiences are like most labs, the manuals were locked away somewhere the day the equipment arrived, and nobody in the lab even knows they exist.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 30, 2014, 12:20:16 am
content: Quote from: SharpEars on October 28, 2014, 01:55:46 amNot the point---- the 50 Ohm termination is not there to act as a load for a DC supply.Its only function is to supply impedance matching if you are looking at a signal at a point which is expecting such a termination.For instance,you may be looking at a Signal Generator with a 50 Ohm output impedance,with the two connected with normal 50 Ohm coax.If you set the SG to give 1 volt output,this will only display correctly if the 'scope's 50 Ohm termination is on.There is nothing wrong with putting a load on your DC supply,but this must be connected directly across that supply,not at the 'scope end of the probe lead.The x1 probe can then be connected across the load.