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Is this circuit stable?

L

Lostgallifreyan

Jan 1, 1970
0
The circuit board may add less

This is useful. It's why I think it's current-induced, but I'll think more
on this. While it's not causing bother with the regulator (tested with
standard 78S05 too, same results), there might be some other aspect where
that tiny current difference between pinboard contacts and soldered
connections makes a difference. The error is in the mV range as it is. I
might try a 1 ohm resistor between the gain stage ground and the meter
input ground, but it will have to wait. It was not a happy occasion, 16
hours straight and I snapped at the end, bust up the box and all. I'll get
some more pots and make up a new gain stage soon (got three boards left of
four). I won't be trying that same small box again though, whatever else I
do, so the problem might not re-appear, given that that box was where it
happened. The easiest guaranteed answer seems to be to use a PLASTIC box.
 
L

Lostgallifreyan

Jan 1, 1970
0
Just crediting where credit is due... Last post quoted me instead in error.
 
J

John Popelish

Jan 1, 1970
0
Lostgallifreyan said:
I thought of that, and scoped everything I could scope...
The capacirot is exactly as specified, 22µF with 0.5 ohm ESR. It's a
tantalum, SMT, soldered directly across GND and OUT right at the base of
the TO220 package. (And no, I didn't damage anything with heat. :)

If there is any better way to satisfy the exact requirements for this IC,
I'd like to know what it is. I also tried with a standard 78S05, just to be
sure. Same problem with the meter reading causing feedback to the gain
stage...

In that case, I am probably wrong about the involvement of
the regulator.
 
J

John Popelish

Jan 1, 1970
0
Lostgallifreyan said:
And if that drift is halved along with the actual reference itself? (Two
100K's...) Shouldn't it cancel, being symmetrical about the zero point?

The offset adjustment won't necessarily be set to the mid
position.
That is the plan, anyway, it's why I didn't go for some more exotic and
precise reference. I did consider drift, even so, it's why I chose that
voltage, I read that Z5V1 (or sometimes Z5V6) has the smallest thermal
drift.

Understood.
 
J

John Popelish

Jan 1, 1970
0
Lostgallifreyan wrote:
(snip)
The changes on the output were seen to be in sync with the LED changes.
It's not an HF noise thing, it;s a gross-current-change thing. BUT, it
happens even when the gain stage's gnd is deliberately not in a loop, or
even shares a length of wire, with the DPS's supply ground. This
arrangement solves the problem, on the pinboard, but not in the metal case.

The only thing I can think of is that the thing might still be HF or even
RF related, but for VERY short durations, during the change in current when
the reading updates. If the meter reads the instantaneous voltage it finds
at update time, it might be reading a tiny one-shot pulse caused by itself.
That sounds wrong, because you'd think that the measurement would be taken
and finished before the meter can change the display, but what if it's not
so? What if there is a small fraction of a second during which the reading
is changing at same time as input is being sensed? Any overlap could cause
a spike to be caused and read back, and the duration would be too short to
catch on my scope, which can't see one-shot events.
(snip)

I don't think the DPM needs a very well regulated supply.
Have you tried adding an RC low pass filter between the LDO
regulator and the DPM supply pin? The current may be high
enough that an LC filter would be better. This would help
break the feedback loop from the DPM supply current, back
through the 24 volt supply to the signal amplifier and back
to the DPM input.
 
J

John Popelish

Jan 1, 1970
0
Lostgallifreyan said:
So true. I actually got lucky, I just happened to have the SMT tantalums
around, they had the exact spec wanted. Perfect fit on the legs of the IC
too, it really doesn't get better.

A perfect fit if the spec is not a maximum ESR, instead of
the actual ESR. I never found capacitors that has specs I
could rely on, except for maximum ESR. That is why I always
use an external resistor.
 
L

Lostgallifreyan

Jan 1, 1970
0
I don't think the DPM needs a very well regulated supply.
Have you tried adding an RC low pass filter between the LDO
regulator and the DPM supply pin? The current may be high
enough that an LC filter would be better. This would help
break the feedback loop from the DPM supply current, back
through the 24 volt supply to the signal amplifier and back
to the DPM input.

Yes, I'll try that. I considered it, but there was no space, in the next
attempt there will be.
 
L

Lostgallifreyan

Jan 1, 1970
0
A perfect fit if the spec is not a maximum ESR, instead of
the actual ESR. I never found capacitors that has specs I
could rely on, except for maximum ESR. That is why I always
use an external resistor.

I thought if anything, the ESR might not be as low as claimed, so a series
resistor would raise it further. I did take it on trust, but if a maker
says 0.5 ohm ESR and it were easily found not to be so when tested by
people far more qualified than be, wouldn't there be enough trouble over it
for the maker to either meet or change the specs?

If you're saying that this thing might have lower than 0.25 ohm and be on
the inside limit for stable use, I can try a 0.5 ohm series resistor, but I
still think it's been ok. I didn't see ANY problems that I did not also see
with a standard 78S05.
 
L

Lostgallifreyan

Jan 1, 1970
0
In that case, I am probably wrong about the involvement of
the regulator.

OK, you got it. :) I just pre-empted that in the last post before I saw
this..
 
L

Lostgallifreyan

Jan 1, 1970
0
The offset adjustment won't necessarily be set to the mid
position.

Agreed. This is one point I need to suss out, while the offset is small, it
will be bigger than the op-amp's own offset, as it uses the 10M resistor
like a long lever with a fulcrum close to the op-amp input.

As far as I know, the only thing that could seriously affect this is a
significant change in the 24V supply. I never saw such, I specifically
looked for that. It would have had to change in obvious step with DMP
display updates, it would have been a clue as obvious as a slap in the
face, and it wasn't there.

I'll still look into this as a possible source of error, if it doesn't
counteract the op-amp's own thermal drift. None of which is ultimately
bothersome though, I'll probably arrange to set for zero before any
critical measurements are made. Small drifts are fine, if I'm monitoring
anything long-term with this, I'll be controlling the ambient conditions
for best stability anyway.
 
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