Analog Signature Analyzer...where to place Device Under Test? - Page 1
Forum section: electronics / beginners

author: Skimask
author_rank: Super Contributor
timestamp: February 13, 2013, 01:04:04 am
content: Still trudging along on this analog signature analyzer project of mine...Referring to attached circuit diagram, note the positions of R7 and C14 near the left side of the schematic.R7 is the series resistance, providing a voltage drop for one channel.C7 is actually the D.U.T. (could be anything...a complete circuit, a diode, inductor, whatever...).Question is...do I place the components as is? With R7 (the series resistance) connected to ground?Or swap the positions of R7 and C14, putting the D.U.T. with one leg to ground?Pros/Cons of each? Anything?

author: Skimask
author_rank: Super Contributor
timestamp: February 13, 2013, 04:01:31 am
content: Can't believe I forgot to attach the picture...'cause that's never happened before right?

author: amspire
author_rank: Super Contributor
timestamp: February 13, 2013, 08:03:40 am
content: I think using C7 as the DUT without changing anything may be what you want, but it is an odd circuit. You are providing a DC offset for the two output opamps, and then you put 1N4148 diodes in series with the A/D inputs. This ruins any precision.I am not sure what this "Analog Signature" you are looking for is. If this is a circuit to measure the complex impedance of Caps, Inductors and Resistors, it doesn't look great. In measuring the "analog signature" of a diode, when the diode conducts, it will overload the output of the driving opamp. Something you normally do not want in a test instrument.

author: Skimask
author_rank: Super Contributor
timestamp: February 13, 2013, 08:48:14 am
content: Diodes - That point is actually an ADC input to a PIC. Just in case, I don't want any negative voltage going out there, as well the zeners are there just in case I get too much gain in the opamps. More protection... Another point to be made is that LTSpice doesn't let me model things like switches, digital pot's, etc. There's two caps in there that are basically shorted out by a wire. Sounds useless, but they'll actually be switched in/out by a relay...and the resistors that are labeled DP-Rx will actually be digital pot's for various things.The point of the circuit is to generate a "picture" of a circuit. Don't care what values, don't care about volts, ohms, farads, etc. Just the picture. If two circuits have identical "pictures", then the two circuits are identical.I've got the thing half built, working on the programming, tweaking, etc. In the simulations using LTSpice, it's doing what I want it to do. Doesn't much look like it from the circuit, but...meh...looks can be deceiving...and the circuit as shown is only about 1/10 of the whole picture.I think the main reason why I was asking the original question was some of the "just in case" scenarios. The most "harmful" question I can think (aside from probing a live circuit with a lot of power going thru it) might be what if I put a cap with a decent charge on it across the D.U.T. terminals? What, if anything, happens to any of the other pieces/parts?When I get it done and working, I'm going to post a write-up on it, build-up pictures, testing, videos, etc., and then it should all make perfect sense.Who knows...maybe Dave will give it a beating (or should I say "give me a healthy berating") during a tear-down or mailbag day?