Please let me know if this schematic has any glaring errors or problems.It's a DC gain stage, with adjustable offset and scale. Precise component values might change, but the basic form will not, unless it is bad in some way I can't see.The first half of the op-amp makes a new ground, so that there is enough negative voltage to allow through-zero offset tweak to set to zero. The supply will be a small 1 watt 5V to 24V DC-DC power converter, so that the op-amp can have >20V supply allowing a wide range output while the supply is only +5V. The power converter will also isolate the supply input so that the new ground can be connected to the signal input and output ground which will be commoned to the chassis and earth. The op-amp and the 2K2/Zener network are both fed directly from this isolated 24VDC supply.The second half of the op-amp amplifies a DC signal on the non-inverting input. It's set up so the input impedance is as high as possible, but a resistor might be placed across the input in any finished circuit.When I built this, it worked accurately and quietly on a pinboard, and on a small PCB I made for it, when reading the output on a Fluke voltmeter, but when wired to an LED panel meter in a small metal box the reading was erratic. I traced this to the meter changing its current draw when different segments lit, and the ground circuit current seemed to be affecting the gain stage output, causing a feedback effect that I couldn't eradicate, no matter how I arranged the ground wiring.Is there something about this circuit that is inherently unstable or inaccurate? If so, what should best be changed?

This is the first appearance in this thread on S.E.D., I didn't get the cross-post set correctly late last night. On S.E.C., please ignore the earlier post, this one is updated with an extra paragraph near the end. ('DC Gain Stage' diagram, PNG format).Please let me know if this schematic has any glaring errors or problems. It's a DC gain stage, with adjustable offset and scale. Precise component values might change, but the basic form will not unless it is bad in some way I can't see.The first half of the op-amp makes a new ground, so that there is enough negative voltage to allow through-zero offset tweak to set to zero. The supply will be a small 1 watt 5V to 24V DC-DC power converter, so that the op-amp can have >20V supply allowing a wide range output while the supply is only +5V. The power converter will also isolate the supply input so that the new ground can be connected to the signal input and output ground which will be commoned to the chassis and earth. The op-amp and the...

John Popelish wrote innews: Just crediting where credit is due... Last post quoted me instead in error.

John Popelish wrote in news:0Zydnek0FeM-Yes, I'll try that. I considered it, but there was no space, in the next attempt there will be.

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

Is this circuit stable?

L

Lostgallifreyan 19 years ago

Lostgallifreyan wrote in news:Xns99146FEED5672zoodlewurdle@140.99.99.130:

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.

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Lostgallifreyan 19 years ago

John Popelish wrote in news:gPGdnTYY1-nRc7_bnZ2dnUVZ snipped-for-privacy@comcast.com:

Just crediting where credit is due... Last post quoted me instead in error.

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J

John Popelish 19 years ago

f=20

C,=20

o be=20

In that case, I am probably wrong about the involvement of=20 the regulator.

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J

John Popelish 19 years ago

The offset adjustment won't necessarily be set to the mid position.

Understood.

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J

John Popelish 19 years ago

Lostgallifreyan wrote: (snip)

(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.

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John Popelish 19 years ago

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.

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Lostgallifreyan 19 years ago

John Popelish wrote in news:0Zydnek0FeM-

877bnZ2dnUVZ snipped-for-privacy@comcast.com:

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

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L

Lostgallifreyan 19 years ago

John Popelish wrote in news:0Zydneg0FeNh877bnZ2dnUVZ snipped-for-privacy@comcast.com:

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.

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Lostgallifreyan 19 years ago

John Popelish wrote in news:M_ydnWTbPsJp8b7bnZ2dnUVZ snipped-for-privacy@comcast.com:

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

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L

Lostgallifreyan 19 years ago

John Popelish wrote in news:M_ydnWfbPsIQ8L7bnZ2dnUVZ snipped-for-privacy@comcast.com:

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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Is this circuit stable?

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