Input Impedance - Page 1
Forum section: electronics / beginners

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 25, 2022, 03:45:02 pm
content: Hi guys. I have to work with 50 Ohm input impedance. If I overload the scope input by providing >5V by mistake, will the scope switch automatically to 1 MOhm?Closely related question:How much voltage can a 10X probe handle realistically, and at what frequency range(s)?

author: bdunham7
author_rank: Super Contributor
timestamp: October 25, 2022, 03:46:36 pm
content: 1. It depends.2. It depends.

author: TimFox
author_rank: Super Contributor
timestamp: October 25, 2022, 04:33:24 pm
content: 1. Only if exceeding the power dissipation capability of the 50 ohm termination causes it to blow, which usually results in an open circuit.Not recommended as a procedure.Perhaps if there is a fuse included: one of my -hp- frequency counters includes a "picofuse" in the BNC input connector before the hard-wired 50 ohm termination on the VHF input channel "C".When that fuse blows, the input is disconnected from the counter circuits.2. The specification sheet for a 'scope probe should have derating curves for voltage and frequency, which varies from model to model.

author: tggzzz
author_rank: Super Contributor
timestamp: October 25, 2022, 04:45:55 pm
content: Quote from: balnazzar on October 25, 2022, 03:45:02 pmSome will, e.g. Tek 485, 24x5Some won't, e.g. HP1740Summary: RTFM

author: radiolistener
author_rank: Super Contributor
timestamp: October 25, 2022, 06:24:48 pm
content: Quote from: balnazzar on October 25, 2022, 03:45:02 pmNo, it will damage your oscilloscope

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 25, 2022, 09:34:36 pm
content: A real big signal, lots more than 5 V can damage the front end of some scopes with no protection.Depends on the scope specs. Read them.5 Volts into 50 ohms is 1/2 WattA lot of older scopes input transistors can be burned out. But lots more than 5 volts.This used to be a way for techs to make easy money...buy a "burned" scope for nothing and fix it easily.That is for older scopes with replaceable input transistors/fets.

author: james_s
author_rank: Super Contributor
timestamp: October 25, 2022, 09:37:44 pm
content: Some scopes might, but none of the ones I've worked with will. If you overload the input on 50 ohm it will burn out the terminator inside the scope. This is unfortunately fairly common. Even if you do find a scope that will automatically switch it out, you should never rely on that because most scopes don't.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 25, 2022, 09:58:29 pm
content: Ok, that's one thing to pay attention to, then. Thanks!

author: rstofer
author_rank: Super Contributor
timestamp: October 25, 2022, 10:25:10 pm
content: Can you use 1MOhm input on the scope with an external 'T' and 50 Ohm terminator?

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 26, 2022, 10:50:46 am
content: Quote from: rstofer on October 25, 2022, 10:25:10 pmSee if I understood it right: if I set 1 MOhm on the scope, and use a 50 Ohm external termination between the cable and the oscilloscope input, even if I provide a large voltage by mistake, the scope input won't be fried. Rather, I will just fry whatever resistor there is in the external termination. Right?

author: radiolistener
author_rank: Super Contributor
timestamp: October 26, 2022, 01:56:48 pm
content: Quote from: balnazzar on October 26, 2022, 10:50:46 amyes, external terminator has a little bit worse frequency response, but on the other hand it's better to damage external terminator than oscilloscope input, if you put high power signal by mistake

author: pcprogrammer
author_rank: Super Contributor
timestamp: October 26, 2022, 02:04:06 pm
content: Quote from: balnazzar on October 26, 2022, 10:50:46 amBut still be aware of the maximum input voltage the scope can handle. With a probe on 1x by accident and then measuring a high voltage will also blow up your scope.So read the manual of the scope to find the safe limits.

author: bdunham7
author_rank: Super Contributor
timestamp: October 26, 2022, 03:37:39 pm
content: Quote from: balnazzar on October 26, 2022, 10:50:46 amYes, with the previously stated caveat that 'large' voltage is subject to limitations. In practice the typical differences are so large (5VRMSvs. ~300VRMSor 400VPK) that this isn't all that likely an issue.However, the question that I would ask is why one would use 50R inputs or termination in the first place and depending on the answer to that question, we can discuss if using 50R termination is the proper solution to a particular problem. IMO, it often isn't. And if it is, we can discuss the best way to protect the scope from being burnt.Typically (at least for me) the 50R inputs are used either to display a signal directly from a 50R source or using a probe like the Tek 6156 that is designed for a 50R input. These tests are typically for HF signals where 50R systems offer better performance (say >100-200MHz) and external terminators will diminish that. In some cases you might be looking at something like the LF output of an AWG or something that is a 50R source, but that can be handled in other ways since reflections and such aren't an issue. So here are a few cases and proposed solutions.If your HF source is potentially powerful enough to fry a 50R termination, the proper solution is probably an attenuator, not a terminator.If your HF source is inherently low-level (the most common case) but has the potential to put a significant DC bias on the output in the case of a malfunction, then a high-quality DC block will protect your scope and still allow you to use the superior internal termination.If you are looking at various LF signals of almost any power level, then the approved solution is using a 10X probe and if termination is desired for testing (like the AWG example), terminating it at the source and using a BNC probe adapter.I realize that doesn't cover every conceivable possibility and it requires buying a fairly expensive attenuator and DC block. Precise work with HF is not cheap. An external terminator may not be better than termination at the source followed by a good 10X probe but I haven't done an extensive comparison.

author: radiolistener
author_rank: Super Contributor
timestamp: October 26, 2022, 04:30:12 pm
content: Quote from: bdunham7 on October 26, 2022, 03:37:39 pm50 Ω terminator is used because usually signal is delivered to oscilloscope with 50 Ω coax feed line. And feed line needs to be properly loaded at receiver side in order to avoid reflections...

author: G0HZU
author_rank: Super Contributor
timestamp: October 26, 2022, 06:32:01 pm
content: Quote from: balnazzar on October 26, 2022, 10:50:46 amYou can still damage both the scope and the 50 Ohm terminator if you input RF frequencies. The 1 MOhm input on a typical 100MHz scope input might only be rated to about 15V peak above a few MHz. Some are rated even less.With faster scopes the derating can still be significant. By the time you get to UHF, the 1Meg input of a really fast scope is probably only rated to the same 5Vrms level as the internal 50 Ohm terminator.

author: Bud
author_rank: Super Contributor
timestamp: October 26, 2022, 07:47:17 pm
content: If working on RF projects you want a 50 Ohm capable scope. Many measurements require 50 Ohm termination (RF circuits) but many do not (like power/bias circuits). There is constant switching 50Ohm on and off. It is pain in the rear to do it externally using a T or an inline connector. If using a T, disconnecting the 50 Ohm termination you create an open stub in the T arm which causes reflection at higher frequencies. When using an inline termination you wear out your scope input connector by frequently connecting and disconnecting the fermination. It is easier and safer to switch the internal scope 50 Ohm termination. I am not byuing any scope that does not have 50 Ohm input.

author: balnazzar
author_rank: Frequent Contributor
timestamp: October 26, 2022, 10:25:55 pm
content: They should definitely implement failsafe mechanisms...

author: alm
author_rank: Super Contributor
timestamp: October 26, 2022, 10:34:44 pm
content: Some scopes do, as long as it's within the 1 MOhm input range (including frequency derating), but you should also get used to being careful around expensive equipment. Especially RF equipment does not tend to be so forgiving against abuse. The world isn't all coated in rubber tiles.

author: bdunham7
author_rank: Super Contributor
timestamp: October 26, 2022, 10:54:48 pm
content: Quote from: balnazzar on October 26, 2022, 10:25:55 pmI would like to see it as a feature too, but it isn't as easy to implement as you think, at least if you expect it to protect against all, or almost all, types of overload that might occur without tripping on signals that aren't damaging the termination resistor. Think of over-bandwidth signals, irregular, high-crest-factor waveforms and pulsed power. Even the protected Teks can be damaged and they warn you about that in their manuals. At some point users have to pay attention to what they are doing.

author: MathWizard
author_rank: Super Contributor
timestamp: October 28, 2022, 08:51:18 pm
content: So what power rating is a typical 50ohm input on a scope ? I have it on my sds1204x-e, bu I don't have any extra hardware for all that. I don't need it for now, but it's on my list to get a few BNC parts next year.

author: bdunham7
author_rank: Super Contributor
timestamp: October 28, 2022, 08:53:13 pm
content: Quote from: MathWizard on October 28, 2022, 08:51:18 pmYou have what on your SDS1204X-E? Surely not a 50R input?A typical scope spec would be a maximum of 5VRMS@ 50R, so 1/2 watt.

author: TimFox
author_rank: Super Contributor
timestamp: October 28, 2022, 08:53:53 pm
content: An example:https://www.tek.com/en/support/faqs/what-are-maximum-voltage-limits-tds-oscilloscopes-and-how-do-limits-change-over-frequenThey quote 1/2 W power rating for the physical 50 ohm resistor in that series.

author: EPAIII
author_rank: Super Contributor
timestamp: October 29, 2022, 06:14:17 am
content: But WHY would you ever want to feed the full power of an RF source INTO the scope?If at all possible, that is, if a 50 Ohm termination is needed, why not put that 50 Ohm termination AT THE RF circuit or source and then use a standard, 10X probe into the standard, 1MOhm scope input. The RF source is properly terminated, the probe is used properly, and the scope is safe.The only reason why I can see using a 50 Ohm termination at the scope's location is if you want to test the coax between there and the RF source. And even then, terminating the coax at it's end and then using a 10X probe to the scope is a better way to do it. And many terminators have a test point brought out for just this purpose.I have seen scopes or scope input modules that had 50 Ohm inputs, sometimes dedicated and sometimes switched. But I never saw a real reason for their use. If you have an example of a real need for this, where an external termination and normal (10X) scope probe would not work, I would love to hear about it. With a good explanation as to WHY!Quote from: G0HZU on October 26, 2022, 06:32:01 pm

author: alm
author_rank: Super Contributor
timestamp: October 29, 2022, 06:39:47 am
content: Quote from: EPAIII on October 29, 2022, 06:14:17 amObviously you want to use attenuation if necessary.Quote from: EPAIII on October 29, 2022, 06:14:17 amIf you want better signal integrity than the passive probe can provide. Either because you're right at the limit of the scope, or because you need more bandwidth than a/your passive hi-Z probe can provide. 1 GHz scopes are commonly sold with 500 MHz passive probes and a 50 Ohm mode for if you need more bandwidth than a 1 MOhm input can provide.

author: Gyro
author_rank: Super Contributor
timestamp: October 29, 2022, 09:47:34 am
content: z0 Probes use the 50 Ohm termination to achieve higher frequencies and better waveform integrity. Also for lower capacitive circuit loading than ordinary 1:10 passive probes.https://www.eevblog.com/forum/projects/lo-z-probe/

author: tautech
author_rank: Super Contributor
timestamp: October 29, 2022, 10:13:29 am
content: Quote from: bdunham7 on October 28, 2022, 08:53:13 pmCorrect, only the 2ch 2000X-E’s sport a selectable 50R input.