Detecting minor voltage drops on a 50V source

Thanks for answering, I'm trying to detect a voltage drop on a Telephone line when a 10MOhm load is placed across (parallel) the line. I've measured the Voltage of

line before and after the "load", with a Fluke 197 and found it was about 10uV. It interests me very much if using a precision high impedance comparator may detect this drop. Any other ideas will be welcome of course :) TIA. Best regards, Nina

nina.p20 multiposted to sci.electronics.basics and sci.electronics.design

Each time you post, you post INDIVIDUALLY to multiple groups.

This is called Multi-posting and is frowned on. . . If you would put the name of EVERY group in which you would like your question to appear on the To: line (the Groups: line) the FIRST time you post, you won't have to post it a 2nd time.

That is called Cross-posting.

Everyone who reads the question will see ALL the responses and will know that the proper answer has already been given in another group (or a bogus solution has been offered and needs correcting).

It is easy enough to use a resistor divider to measure the higher voltage but measuring a 10uV drop under real world conditions seems unrealistic.

-- Dan Hollands

1120 S Creek Dr Webster NY 14580

I'm no expert on Usenetiquette but I was always under the impression that it's cross-posting that's frowned upon, and that it's better to post to each group individually.

As to the OP's questions, detecting a 10uV difference would be difficult but not impossible. One of the high-precision, low-drift opamps running from a regulated power supply, with perhaps a temperature-controlled environment should be able to do it.

And yes, as already stated by another poster, measuring a voltage higher than the power supply is very simple with a resistor divider. E.g., to measure 50V, use a 10:1 divider to step it down to 5V.

--
Most of us who post to the electronics newsgroups prefer to
crosspost because it makes it much easier to keep up with what\'s
going on and a single reply will be read by all.

I wonder if maybe a "trick" could be used for this application. The problem is that you need to measure this tiny percentage change in a large DC value. If you try to set up a comparator with precision resistors, you are going to have a heck of a time with stability issues.

Here's the idea: Run the raw 50V through a super-long TC filter. A simple RC will be OK. This will be the reference voltage. Think of this as "zero". Now you just need to detect -10 uV, not

49.9999 V. In order for this idea to work without a precision divider, you will need to "float" the comparator. That will be a bit more work, but probably worth the trouble.

The other possibility is to capacitively couple to the line and just detect the transient when the load changes. You can amplify the heck out of that for detection, as long as you can be assured the line is otherwise clean.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

--
LOL, that\'s the _perfect_ reason to cross-post!

Why do otherwise?  If I post to several groups separately with a
query which is on-topic to all of them, say:  "Please, I need a
schematic and the source code for a BASIC code-practice oscillator
which I can run through my home entertainment system using a PIC and
X-10, but I don\'t know how to design something like that."  followed
by the the inevitable "TIA", then I\'ll have to harvest my replies by
schlepping from group to group, and everyone who replies to my query
will have to do the same to make sure that they\'re not repeating
what someone else has posted and to keep track of the thread. 

That\'s ridiculous.

It would be vastly simpler for me and everyone else if I just
cross-posted.  It\'s not like there\'s a problem with bandwidth, and
even it there was, that\'s the kind of thing that drives the
technology forward.  Usenet isn\'t supposed to dictate terms; it\'s
our servant, and as long as we shelter it, feed it, and clothe it,
it needs to do do what we tell it to.

There is disagreement on this topic. :-)

The original intent of crossposting was to handle situations like this, where one post (and all its replies) are on-topic for several groups. Crossposting has gotten a bad reputation because some people use it to incite flame wars: you can crosspost to two very different groups, and by default peoples' responses will be crossposted also, and especially if people don't *realize* that the thread is in multiple groups this can produce a lot of flamage really quickly.

But, IMHO, crossposting *is* the right thing to do if your post is on topic for two groups whose cultures aren't terribly different, like say, three different groups in sci.electronics.*. Or comp.arch.embedded or comp.arch.fpga, to name two other groups whose subject matter overlaps with sci.electronics a bit. It's polite to mention in your message that it is crossposted, though, just in case the respondent doesn't read the headers and is writing a response that's only on topic for one of the groups. You can also use the Followup-To: header in your original post, if it seems appropriate.

--
   Wim Lewis , Seattle, WA, USA. PGP keyID 27F772C1

But once again, if people routinely cross-post we might as well return to 1995, when there was only sci.electronics (and sci.electronics.repair). sci.electronics was split up because traffic was getting heavy, and split into what seemed to be a logical set of subsets.

There may be times when cross-posting is appropriate. But much of the time, it's simply someone too lazy to figure out the most appropriate place. Or too impatient to see if the first newsgroup gets much of a response (which also may mean laziness, because they may not have taken the time to read the newsgroups they post to for a while before posting, so they have no real understanding of whether the post is appropriate).

Just because a post is on topic to various newsgroups does not mean that cross-posting is the solution.

Michael

No. When you crosspost *one* message to *a few* appropriate groups, a modern newsreader will notice this and mark the posting "read" across all crossposted groups. This means that if you have chosen not to display viewed messages, you only have to view the message (and the replies) once.

When you multipost, on the other hand, the same message (and all the replies) is still there in *every* group you multiposted into, even after you have already read the article in another group. This is obviously a very unefficient and annoying way to read usenet, especially if you read groups with similar interests..

Pretty much only problem with crossposting is when it crosses the boundaries of the groups, i.e., the incessant political prattling that goes on in strictly non-political groups, and also spamming, but these are not so much problems with crossposting so much as they are problems of basic respect (or lack thereof) of others in usenet.

Jon