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Is this circuit stable?

Discussion in 'Electronic Components' started by Lostgallifreyan, Apr 15, 2007.

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  1. http://preview.tinyurl.com/36vncy

    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?
     
  2. Hmm, meant to crosspost to s.e.d and s.e.c, and borked it, but nm....
     
  3. 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.

    http://preview.tinyurl.com/36vncy ('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 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.

    The circuit works ok with supplies other than 24V, and the two 100K
    resistors in the divider are meant to keep the zero point fixed even if the
    zener voltage drifts with temperature. The zener value itself is chosen for
    low thermal drift.

    Is there something about this circuit that is inherently unstable or
    inaccurate? If so, what should best be changed?
     
  4. It looks like it should function.
    That is a confusing dual use of the word "supply".
    A complete schematic would help this description make more
    sense.

    Is the gain stage intended to produce only positive output
    voltages?

    By the way, there might be a better way to do the gain
    adjustment. Your method puts DC current through the wiper
    contact, and that might, eventually, make its contact noisy.

    Since the opamp bias current is very low, you can avoid
    almost all contact point current by connecting the wiper to
    the input and adding a pair of resistors at each end to
    complete the feedback divider. This method also gives you a
    larger gain range for a given total divider resistance and
    pot resistance.

    A high resistance path to signal zero volts is handy if you
    might ever disconnect the input signal. That will keep the
    opamp from saturating under that condition.
    The reference voltage section must also supply the full
    "ground" return current the right opamp produces. If the
    output current gets anywhere near the opamp current limit, I
    would expect to see the reference voltage amplifier start to
    malfunction.
    An integrated voltage reference chip would probably do a
    better job (less temperature sensitive, sharper voltage
    regulation and lower noise) than any zener. Take a look at
    this data sheet:
    http://cache.national.com/ds/LM/LM431.pdf
    With two resistors, you can program it to whatever voltage
    you need.
    Not so much inaccurate as just having a limited current
    capability that you need to be aware of.

    That is not to say this is the best circuit for the job,
    because you have not very well specified exactly what the
    job is.

    What range of input voltage?
    What minimum input impedance?
    What gain range?
    What load impedance?
    What frequency response?
    What temperature range?
     
  5. John Popelish wrote:

    Hold on. I just realized that the zener reference section
    shows no connections at all to either the opamp supply pins
    or the input/output common (the bottom right pin, I assume).

    Either you missed some important connections in the
    schematic, or this circuit cannot function.
     
  6. Thankyou, lots of good stuff there, it helps me to think. I added some
    context to that file.

    http://preview.tinyurl.com/36vncy ('DC Gain Stage' diagram, PNG format).
    It does, till I move it into its permanent home. :) The meter instability
    is in 5mV range, at worst, 50mV, but did not exist at all until I moved the
    circuit from pinboard to PCB is a metal case with chassis mounted BNC
    sockets. Seems that context matters, so I agree, I should have posted more
    info than I did.
    Ok. First, the supply is +5V to +15V or so, sent to an LM2904T 5V LDO
    regulator. This allows battery use as well as most cheap DC output PSU's.
    The supply ground is commoned to the chassis at the regulator's tab, OR at
    the sensor input ground if the LED panel meter's floating ground is shorted
    to the supply ground, which I chose as best (and recommended) option,
    isolating the regulator tab from the case to prevent a ground loop, which
    lessened the reading instability problem but did not remove it.

    The regulated 5V feeds the LED panel meter as directly as possible to keep
    the meter's ground current from affecting the gain stage signal ground. The
    gain stage gets 24V from a 1W power converter that takes from the 5V
    regulator and isolates the 24V supply to the gain stage so that its new
    ground can become the real system (signal) ground.
    Nice, I'll give that some thought and testing time. If I make new boards
    for this thing, I'll probably do that. It's not a high priority right now
    though, I've seen similar devices do it the way I have without problems. I
    can see it's not the best way, but I don't think that changing it will cure
    the problem caused by the LED panel meter's changing current draw.
    I'll load it lightly by meters with huge input resistances. This is what
    bothers me; as there is not supposed to be a significant current, I don't
    know how the LED panel meter's current draw can change the gain stage's
    signal levels so much when it's directly supplied by the main 5V regulator.
    I first thought of poor load regulation in the LM2904, but it's not that.
    I've thought of using something like this, but would rather use a fixed
    voltage 2-pin device that could occupy similar volume to the zener, if
    possible. I abandoned the tightly packed box I was hoping to use, so I
    don't mind if the thing stands up off the board a bit. :) It just has to
    fit the space on its plane. I need to consider other sources of inaccuracy
    first. If the op-amp offset voltage drifts much, that will be a weaker
    point.

    The divider made by two 100K resistors will halve the voltage at the zener,
    whatever it is. I'm more concerned with asymmetry, if it can exist, between
    the new ground set by that midpoint, and the offset adjust to zero made by
    the offset tweak pot. Thermal drift in the op-amp might be one cause of
    asymmetry, but that wouldn't cause the main problem of output changing when
    the LED meter reading changes.
    In my first use of this design, a laser power meter, an activated charcoal
    coated TEC in a metal barrel to shield it from ambient heat and provide a
    thermal mass. (I reconditioned a cheap Scientech head I got off eBay...)
    +10 µV to +2V DC.
    Unknown. I designed it for the highest input impedance a modest priced op-
    amp can give me, so it reads the voltage of a sensor while drawing a
    current so small I need not consider it. If I had to adapt it to something
    requiring a load resistance, I'd add a resistor between input and ground.
    About 11, for the TEC's voltage boosted to give a 1mV per mW output for
    power to be read on a voltmeter. I'll adjust the values of the gain stage
    network for wider adjustment than I currently have for scale, but this
    isn't where the problem is...
    High as possible. I noted your points about current, but I'd intended this
    to read and write voltages. The Fluke meter I used, and the LED panel meter
    that caused the bother, both have extremely high input resistances. If I
    needed current drive, I'd add a voltage follower, or arrange an entirely
    different circuit with that in mind.
    Nothing strict. I chose an op-amp that would be stable without special HF
    filter requirements for stability. I only need this to work fast enough to
    show changes a human eye or ear can keep up with, so perhaps 200 Hz. At
    fastest, I might supply an audio input with DC blocking disabled so I can
    turn a wave recorder program into a 2KHz sample rate data logger, so
    definitely never higher than 1 KHz.
    Not thought of that much because the op-amp will drift a bit whatever else
    I do... Any temperature a human might be comfortable in. Maybe extended to
    0°C to 85°C would be good, more secure. I'll just calibrate the thing at a
    settled room temperature, most times.
    Mentioned in the original text, but I agree, it should be in the diagram.
    Op-amp is supplied by the same 24V that feeds the zener and divider
    network. I've added to that diagram:
    http://preview.tinyurl.com/36vncy
     
  7. To clarify: I'm not talking about repeated oscillation here. I know we can
    hear to maybe 20 KHz, but a one-shot change shorter than about 5
    milliseconds isn't easily resolvable. In short, I'm monitoring aperiodic
    signal changes that might be perceptible to human senses.

    I don't think any of this will bear on the meter reading instability
    problem, but I might as well be complete in detail if I want nice full help
    in answer. :)
     
  8. Jamie

    Jamie Guest

    < LOTS OF STUFF DELETED >

    Ok, from what I have read here, it appears that you maybe getting
    a common connection or, near it with the supply to your common ground.
    The common in the meter is going to cause the virtual ground you
    made to tie together.
    Even if you have a meter with a isolated common on the input, some
    where along the line it's going to join to the common of the chassis...

    The supply your using should be totally isolated, no common of the
    supply should be connected to the chassis. This means a supply with a
    transformer in it or a DC-DC converter.

    AT this point, you can use that virtual ground like you did but only a
    higher current type so that you can supply the meter with its regulated
    5 Volts from one side of the main source and the TAB (common) goes to
    your virtual ground.
    The virtual ground can also be connected to the chassis but not the
    main supply..

    So for example, lets say we use a 15 volt battery. The +&- of this
    source should never come in contact with any commons/chassis grounds,
    only does the virtual ground you made comes in contact with the common ..
    You can then use the + side of the battery for example to operate
    the regulator. The reg common goes to the virtual ground.. etc....

    I think you get it.
    P.S.
    I think you need to use a higher amp handling OP-AMP of you want
    to go this way.
    It would be better to simply use a DC-DC with Dual output converter
    for your voltage gain circuit and tie the CT (center Tap) of the
    converter to the main common along with everything else.

    If you want, you could always create a - volt generator that is
    regulated via a -post regulator..
    For example, assume you have a 15 volt supply, using a 555 timer in
    astable mode into a CAP that is rectified for - volts, then pass that
    into a -REG..
    You can then use the + Reg witch will have plenty of current on
    reserve to supply the meter and the -reg to supply the -side of your
    voltage gain Op-amp.

    The 555 is good for 200 ma's if memory serves?

    This is a commonly done trick in cases where single supplies were used
    and - voltage was needed.
     
  9. (snip)

    This helps a lot.

    The LDO I am guessing is actually an LM2940T.
    http://cache.national.com/ds/LM/LM2940.pdf
    These things are notorious for oscillating without the
    correct output capacitor effective series resistance (ESR).
    The graph on page 10 shows this allowable range for the
    resistance in series with the output capacitor. I usually
    use a very low ESR capacitor with an external .47 ohm
    resistor, to make them behave. If you have a scope you
    might check yours. The circuit board may add less
    resistance than the plug board did.
    Again, the trouble sounds like it is related to the regulator.
    If I am following your schematic, the regulator ground is
    connected to the DPM, and then to the output of LF412A, yet
    that output is trying to produce 2.5 volts with respect to
    that ground, powered by the 5 volt supply. I don't think
    you can ground the output of LF412A.


    (snip)
    I think the circuit has a major flaw relating to tying the
    meter input ground to the power supply ground. I think the
    meter input ground has to be only the 2.5 volt reference, so
    cannot tie back to ground through the case.

    (snip)
    (snip)

    The circuit components should work pretty well (with the
    exception of reference drift from the zener).
     
  10. Guest

    The LED DPM must be muxing the display. This will generate noise on
    the pseudo ground. Sounds like a bad idea to me.

    TI used to make a pseudo ground generator. I don't have the number
    handy, but I got a sample and used it in a project. It worked fine,
    but this isn't rocket science. I also favor using a regulator over an
    op amp for such applications.
     
  11. Hold on again. I keep forgetting that the 24 volt supply is
    isolated and powers the two opamps (I assume). So the
    reference opamp does not so much produce a +2.5 volt level
    as it forces the - rail of the 24 volt supply to -2.5 volts.

    That voltage will be perturbed by the high frequency
    capacitive currents that couple the switching in the 24 volt
    supply to the output, relative to ground.

    You cannot just bypass the -2.5 volt rail to ground with a
    big capacitor, to absorb this noise current, because the
    opamp doesn't like a big capacitor effectively on its output
    (even if its effective output, in this case is its negative
    supply pin). But a capacitor in series with a 10 to 100 ohm
    resistor might keep it happy.

    I still think the problem revolves around the LDO breaking
    into oscillation and quenching out from the noise provided
    by the isolation converter.

    You also don't show any bypass capacitor across the power
    supply rails of the opamps, which would help stabilize them
    against breaking into oscillation, but perhaps the isolation
    converter includes a good output capacitor that is very
    close to them. Still, I would probably try adding a .1 uF
    across each opamp. Another from the -2.5 volt rail to the
    divider input to the reference opamp might help, too.
     
  12. You don't have a lot of good choices for the capacitor on
    the output of the LDO.

    Here is an example of a low ESR capacitor that can use an
    external resistor to make sure the total resistance is
    inside the single decade that makes for stable operation:
    http://www.panasonic.com/industrial/components/pdf/ee_sp_cap_cd_dne.pdf
    I would probably use the 8 volt 22 uF cap because it has the
    lowest ESR.
     
  13. 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...
     
  14. The power converter isolates between the 5V input and it's own 24V (+/-
    12V) supply out, so you can. That op-amp's first half output is the gain
    stage section's only contact with chassis or earth or any other kind of
    ground that might exist.
     
  15. wrote in
    I had half an op-amp spare, and a tight space...
    This thing works. It's only when it goes into the metal case and ground is
    wired to it that things go agley. I considered basic proximity too, on the
    pinboard, and lifted the gain stage (wired to pinboard, mounted on box
    baseplate), inverted it, held it as close to the other stuff as I could,
    and could not provoke the error that way. Only by deliberately complicating
    the ground circuit and making the DPM draw current from points remote from
    the regulator's own connection, could I reproduce the problem on the
    pinboard.

    The small power converter throws much more noise on the line than the DPM,
    I scoped this, it was prominent, though low level. I couldn't see any noise
    that I couldn't attribute to this, or an even lower level of general white
    noise mush. Neither affected the working of the circuit. I tried using
    capacitors to reduce the noise from the power converter, successfully, but
    that didn't solve the problem.
     
  16. Isolated... There is another reason why I have to consider that tie though,
    the common mode to the DPM allows 1V excursions. Maybe I could risk a lower
    zener voltage, below 2V, to allow floating meter input with excursion
    within bounds, but as I tried the other recommendation on the meter's badly
    uninformative data sheet, of placing a 10K resistor in place of that tie,
    and so no change in results apart from a small (about 2mV) offset in the
    readings I was getting with gain stage input shorted, I don't think the tie
    was a problem. I got better results that way on the pinboard too. The metal
    box made things worse to the point where the improvement was masked, but I
    still think it's probably the right way. That's why I isolated the 5V
    regulator tab from the box to break a ground loop. That helped, it lowered
    the erratic reading variance. Before doing that, (and also making the main
    supply to the box ground at the regulator gnd and not at the box at first
    contact opportunity). I got the reading errors to range over about 0.8mV
    instead of over 50mV as they had when I'd just grounded with no thought for
    the loops that might be formed.
     
  17. 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?
    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.
     
  18. The LDO has it's best interest cared for. Tantalum chosen for the exact
    best ESR as specified by data sheet, solder directly to its pins, can't get
    closer or better contact than that.

    I tried caps in various places. Not one cap placement affected the meter
    reading causing feedback to the gain stage. Only messing with ground
    circuitry did that.

    I scoped the output, input, gnds, everything, at various frequencies and
    gains. The only periodic waveform was a very low level spike at around 150
    KHz (if I remember right) from the power converter. It got everywhere in
    the system, but had no effect. I removed most of it with a cap, saw that
    this did not make any change to the feedback problem, so omitted the cap.

    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.

    What I might do is just forget the DPM and provide the gain stage output to
    a BNC socket as before, and also to a pair of 4mm sockets for a multimeter.
    The DPM was just a nice thought, but the gain stage works perfectly if I
    just use a multimeter. That won't have any shared power ground to think
    about.
     
  19. 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.
     
  20. Thankyou, but look again. There is one. There is no fault of commoning
    different grounds. If there was, this would never work, anywhere. It DOES
    work, on the pinboard, it only gets problematic at 5mV levels or so when
    boxed in a metal case. None of which has anything to do with a gross error
    between a floating ground tied to a -2.5V line by mistake. The NMA0512DC
    power converter provides the needed isolation, it's why I chose it.
     
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