Permanent BNC test point for scope, use resistor? - Page 1
Forum section: electronics / projects

author: Apothus
author_rank: Contributor
timestamp: July 30, 2015, 02:55:40 am
content: We have an application where we use a filter to clean the noise from a signal before it is sampled by our control system. The waveform is a 1-10Hz sinusoidal so nothing high speed here.Occasionally (once every few years) there may be a case where we want to hook a scope directly to the input signal to monitor the behavior or if the control system falls over and we want to do some manual control.To make that easier and to reduce the likelihood of someone disconnecting a signal to probe it and then forgetting to reconnect it, or worse. I wanted to add some BNC connectors on the filters housing so technicians could just connect a bnc cable between the filter and the scope.Knowing enough that the probes are more than just a piece of wire I was expecting to connect the BNC terminals to the source signal with a 9Mohm resistor in series, essentially mimicking a 10x probe. There would be no compensation for the capacitance of the cable however considering the low frequency I wondered if this would still be a valid solution?

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2015, 03:58:31 pm
content: For 9 megohm driving a cable plus input, it would take about 1.8 nF to be -3dB at 10 Hz, or 900 pF to be -1dB at 10 Hz, or 180 pF to be -3dB at 100 Hz, etc.Was that 1 to 10 Hz frequency range a typo for khz?What is the impedance driving the filter? If the impedance is much lower than 1 megohm, maybe 10 kohm would suffice for the series resistor, giving only -1% into a 1 megohm CRO input at low frequency.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 30, 2015, 05:14:00 pm
content: For a HF signal 50 Ohms would be the right choice.For this really low frequency signal, something like 1 -10 K would be a good idea to decouple the output. So a 1 M Scope would still read an unchanges signal.

author: DanielS
author_rank: Frequent Contributor
timestamp: July 30, 2015, 05:45:55 pm
content: At such extreme low frequencies, cable impedance, reflections and other effects have negligible effect on signals unless we are talking about kilometric lengths.

author: Apothus
author_rank: Contributor
timestamp: July 31, 2015, 12:39:19 am
content: Quote from: TimFox on July 30, 2015, 03:58:31 pmHow are those capacitances calculated and what is their effect? My assumption had been to replicate standard 10X probe but I am clearly missing something fundamental here about the operation of a scope. If 10k resistors and 50 ohm resistors are applicable for the frequency range why do most probes utilise something as high as 9Mohm?No typo, just really long slow signals

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2015, 12:57:39 am
content: Why not add a standard PCB tespoint with a name and add it to your test documentation that also describes now this test connection is to be used? (eg. 10x scope probe)

author: Apothus
author_rank: Contributor
timestamp: July 31, 2015, 01:09:38 am
content: Quote from: tautech on July 31, 2015, 12:57:39 amThe whole PCB is kept inside a DIN rail housing within the control system. So I would rather not have it as a requirement to open the housing, hence the BNCs.I had wired up the 9Mohm series resistors last night before heading home and testing them this morning shows that they work fine when referenced against standard probe traces. So I probably wont change them for 10k unless there is a compelling reason.I would still like to know why most people recommended much lower values?

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2015, 01:16:08 am
content: Quote from: Apothus on July 31, 2015, 01:09:38 amFine, but you didn't disclose that in the OP.QuoteAs to not affect the waveform so much as a much higher value but still offer some protection against accidental input of whatever.Some might make it more failsafe/idiot proof and add clamping as well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2015, 04:08:04 am
content: This sounds industrial. Is that about right?It should be fine to have the technician attach a standard, straight BNC cable from the test connector to the scope. (When a cable is not in place, you can get dust protection caps, or sealed covers/caps, if you need better environmental properties. The really fancy metal capped connectors aren't terribly cheap, but you only buy them once, after all.) The scope will be set for high impedance input, and the pin driver inside the equipment can be any old op-amp suitable for driving an arbitrary length of cable (which for your bandwidth, can be quite a poor driver indeed).It will probably be a higher priority to keep a low impedance on the cable, to avoid the introduction of noise, rather than feeding it through a very large resistor (which would be possible, if you wanted a 10x reading, but I don't recommend it). You do have the choice of 0 ohm or 50 ohm drive; I wouldn't suggest oddball impedances, and 50 ohms has the advantage that, if it's accurate, and the technician accidentally left the scope on a 50 ohm terminated setting, it will attenuate by exactly half (a 2x probe, in essence), which should be a more obvious error than if the reading were off by a subtle amount (as would be the case for a 0 ohm driver plus unknown cable resistance, into a 50 ohm terminator).In addition to the capability of driving a cable, the circuit should also have EMC filtering and ESD protection, more for its own sake than for anything outside. The bulkhead connector can be grounded to the enclosure, but you need to be careful about ground loops in that case. Preferably, the cable should be grounded to the board which generates the signal, and nothing else. But proximity to other noise sources, or the board itself being a noisy place, can also make this undesirable, so there are many possible routes, some better than others, all dependent on what's around it.If precision in this measurement isn't terribly important, you could just as well skip any filtering. Ideally, the signal should be filtered down to only the bandwidth required for the signal, eliminating possible interference; but this could even be left to the scope itself (typically on Hi Res, averaging, or low bandwidth settings), where it's probably less accurate but the simplest (laziest) option, even in the presence of heavy noise.Tim

author: rs20
author_rank: Super Contributor
timestamp: July 31, 2015, 04:25:23 am
content: OP, why are you so keen on mimicking a 10x probe? Any particular reason you don't want to just do a 1x straight cable (perhaps with buffering, as suggested above)?

author: daqq
author_rank: Super Contributor
timestamp: July 31, 2015, 06:47:33 am
content: There are probe tip adaptors:http://www.keisokuki-land.co.jp/images/131503100_512.jpgIt allows direct insertion of a scope probe into a "socket", with the ground connection intact. If it has to be an outside available connection that does not involve removal of the device, you can mount that in some clever way.Are these digital signals, or analog?By the way, does anyone know where can I find these on farnell? I've looked and found nothing.