Electronics Forum / CAD / September 2007

Forgot to mention:

It will draw current if one of the opamps is burnt out (with the
soldering iron).

Vitaliy

Time to get your trusty VOM out. Verify that there is continuety between the
ground pins on the chips and the ground of the power supply. Now start
measuring voltages. You might need a 'scope to see where you lose the
signal.

Tam

Obviously you have a continuity problem somewhere...
A multimeter will be able to tell you what's wrong.

Why do you need a 4 layer board if your 2 layer board was already
working?
Why are you dedicating the entire inner layers to +5V and -5V?

Dave.

Why?  If it was working perfectly fine, why change it?

That's an unusual configuration.  Most analog designs benefit
more from a ground plane than from power planes without a
ground plane.

Also, +5V and -5V are slightly unusual voltages for powering
opamps. +/-15V, +/-12V, or even +/-10V are more commonly used.
Are you using +5V so you can power some logic chips?

Is this a through-hole or surface mount design?  If it is
through-hole, which layer a trace is on has nothing to do
with whether it goes to a resistor.

Here is how to troubleshoot a new analog board layout that
supposedly has the same circuit (is built to the same schematic)
as an existing board:

First, test them both and verify that they both work the same.
If one of them is missing a signal starting at a particular
stage, that's the place to look at first.

You have found that they don't work the same -- they draw 0mA
and 20mA.  Feed appropriate DC level into all the inputs, go
through every circuit with a voltmeter, and make a list of
everything else that is different.

There are only three possible reasons why the two would work
differently when fed DC as described above:

[1] The circuits are not the same.  Somewhere a trace is
added, missing, or goes to the wrong place.
 
[2] A component is bad or has the wrong value.

[3] (Rare) a circuit that didn't oscillate on the old board
oscillates on the new board, or a circuit that did oscillate
on the old board does not oscillate on the new board.  

To troubleshoot [1] (The circuits are not the same): Get an
assistant, have one of you use a DMM in continuity mode to
see what each pin connects to, while the other highlights
the connections on the schematic.  It is important to do
this single-blind.  I do it like this:

Me: (looking at schematic) "Does U1 pin 3 go to U3 pin 6?"

Him: (looking at board with meter) "Yes."

I then use a yellow highlighter to color the connection on
the schematic.

Me: "Good. Does it go anywhere else?"

Him: (looking at where the traces go on the board and checking
the other pins of U1, U2, and any components close to them)
"No."  

Note that some meters won't beep if you aren't on the trace
long enough.  You want one that beeps when you slide the
probe from pin-to-pin on a connector or IC and get a
connection for a very short amount of time.

Me: "Good." ...and so on.  To reduce the chances of errors,
every few minutes I ask whether a pin that *doesn't* connect
to to another connects, and say "good" when he says "No."

If he gives an unexpected answer, we look at the board, at the
artwork, and at the schematic, and occasionally refer to the
old board and its artwork.

To troubleshoot [2] (component is bad or has the wrong value),
use an LCR meter and DMM in diode mode to measure every discrete
component, a logic probe and pulser to measure every logic gate,
and a signal generator and oscilloscope to test every opamp and
transistor.  If you do this a lot, get a Huntron Tracker (see
http://www.huntron.com/) and use that first.

To troubleshoot [3] (oscillations), start by making sure that
troubleshooting method [2] didn't find anything, then start
looking at signals with an oscilloscope.  Check amplifier and
oscillator circuits first.

Or, if you are doing this at work, hire a good electronics
engineering technician from a temp agency.  *Most* electronics
engineers can do the same job, but there is an occasional one
that graduated with no hands-on experience making things work.

Unless there is a mistake in the net then I cant see what is wrong.
Have you run a "continuity check" and an "integrity check" on your
layout ?
I am assuming your software has these?  if not dump it !

www.ckp-railways.talktalk.net/pcbcad21.htm