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Op amp help please

Discussion in 'Electronic Basics' started by seware, Jun 6, 2005.

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  1. seware

    seware Guest

    I'm fairly new to electronics and I am having a bit of a problem
    understanding which path to take. I am needing to construct a receiver stage
    that accepts a differential input signal. So I have been reading about and
    planning to use an op-amp wired as a difference amplifier. One of my
    questions is in powering the op-amp... and also understanding enough about
    different op-amps to make a good selection. I have +12VDC power and am
    uncertain whether I should use a voltage inverter IC, construct a stage to
    produce -6 and +6 or what. Then I started reading about single-supply
    opamps, but I haven't found anything that fully explains them (at least
    enough to make a choice). I know what "rails" are but what is the difference
    in "rail-to-rail", "not rail-to-rail" and "outside the rails" which are some
    terms I found on various chip makers sites. The last part of my confusion
    come from that fact that I intentionally want to clip any negative portions
    of the output of the opamp. One place I read said that I could use ground as
    the negative supply to the opamp to accomplish this, but another book I have
    said that if the input signals go outside the limits of the opamp power
    supply, the opamp can lock-up. Which is it and where can I go for a clear
    understanding? Thanks all and sorry for rambling.
     
  2. Ban

    Ban Guest

    Even if you write all that much, there are many points open which permit to
    give you a reasonable advise.
    First the input: where does the input come from? Impedance of the driver
    stages. What kind of signal? frequency and voltage range, how much common
    mode signal to be suppressed? DC accuracy needed, amplification needed?
    Then the output: What is the next stage(impedance), what range of voltage,
    do you need swing to gnd or have a reference voltage available?
    Then the desired function: how precise has to be the rectification and the
    clamping of the negative portion?
    These are not theoretical questions, but each refer to specific circuit
    constallations.
    I even doubt a differential amplifier will do the job, rather look for an
    "Instrumentation Amplifier".
     
  3. I'll put off giving you advice about the application till we clear up
    some of the opamp generalities.
    All these terms relate the voltage range of input and output signals
    to the supply voltages (rails). All opamps have specific limits on
    what the input voltages (there are two input terminals) can be and
    still have the input pair function as a differential amplifier. This
    is called the common mode voltage (the voltage the two inputs have in
    common while the differential amplifier amplifies their difference)
    range. There is also a different input voltage range (related to the
    supply rails), called absolute maximum voltage limits, that prevents
    damage to the device. The opamp does not actually care if its rails
    are on one side of signal common or straddle it, as long as the
    applied input voltages stay within the absolute limits. It will
    function as intended as long as the input voltages stay within the
    common mode voltage range, with respect to its supply rail voltages.

    Ordinary opamps have a working common mode voltage that reaches to
    within a few volts of either supply rail voltage (regardless of what
    those voltages are with respect to signal common). Single supply
    types typically work from at, or slightly below, the negative rail
    voltage to within a few volts of the positive supply rail.
    Rail-to-rail types have a common mode input range that equals or
    slightly exceeds the voltage range between the rails.

    The output of an opamp is also limited in its voltage swing by the
    voltages applied to the supply pins. Ordinary (not rail to rail
    types) typically can swing their outputs within a few volts of either
    supply rail. Those designated as single supply types can swing their
    output voltage to within millivolts of the negative rail under some
    load conditions, but still swing to within only a few volts of the
    positive rail. Rail-to-rail types can swing their output voltage to
    very nearly either supply rail (under some load conditions).

    So you need to explore your range of input signal voltages and what
    you can do to bias them in different ways, and your requirement for
    output voltage swing, before you are ready to try to fit those
    requirements to a combination of opamp types and available (or
    practical) supply voltages.
    This is a separate problem until you decide how this clipping is to be
    done. The two general ways involve saturating the output (which may
    have recovery problems and input over voltage problems, when the
    negative feedback opens up during saturation) and using some nonlinear
    feedback that abruptly changes the gain as the voltage passes through
    some particular voltage, but the opamp continues to function as a
    closed loop process the whole time.
    A definite possibility for some opamp designs.
    Start with the signals and work out to what you need to do with them,
    to set some boundaries on the problem. Try to keep as much
    flexibility in this part of the process as you can imagine, to not
    rule out some good solutions before they are explored. Then you are
    in a position to begin considering which of the three general kinds of
    opamps and their required supplys fit each way the signals could be
    processed.
     
  4. Chris

    Chris Guest

    Hi, Seware. I think I understand what you want, although your
    description is a little hazy. You have a single 12VDC supply and two
    input signals, both of which go below GND. You want to use an op amp
    to amplify the difference between the two signals, but you only want
    the positive excursions of the output signal -- in other words, you
    want the output to act like a rectifier of the difference between the
    two signals, with unspecified gain.

    I don't think there are any rail-to-rail op amps which can amplify the
    difference of two signals, both of which are below the negative rail of
    the single supply. One thing you might do is use an op amp which has
    an output which can go down to the negative rail with a pulldown, and
    do something like this (view in fixed font or M$ Notepad):

    ` VCC
    ` +
    ` | ___ ___
    ` '-|___|-o-|___|-.
    ` R1 | R2 |
    ` .--------' ===
    ` | GND___
    ` | ___ .--|___|--.
    ` o--|___|---. | Rf |
    ` | | | VCC |
    ` V1 | ___ | | + |
    ` o---|--|___|---o | |\| |
    ` | '---o--|-\ | Vout
    ` | | >---o----o
    ` V2 | ___ .---o--|+/ |
    ` o---|--|___|---o | |/| .-.
    ` | | | | | |Rp
    ` | ___ | | === | |
    ` '--|___|---' .-. GND '-'
    ` | | |
    ` | |Rf |
    ` '-' ===
    ` | GND
    ` |
    ` ===
    ` GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    This looks a little like the difference amp you know from school.
    There are a couple of changes. The voltage divider R1 and R2 sets up a
    DC value which should bring your input signal up out of the weeds so
    it's in play. You sum that DC level with your input voltage to provide
    a composite input your difference amplifier can work on. Note that
    you're going to have to get out your DC electronics book to figure out
    appropriate resistor values to get a composite Ri at a DC voltage level
    that's useful to you. Remember also that, to keep it simple, Ri and Rf
    for V1 should equal Ri and Rf for V2. Then you can use your standard
    difference amplifier equation (if you don't remember, see National
    Semiconductor AN31 for op amp basics).

    The only other quirk is Rp, which is a pulldown necessary for most op
    amps that go down to the negative rail (or within a couple of
    millivolts).

    This difference amplifier will obviously saurate at negative output
    (0V) as V1 exceeds V2, which I guess is what you're looking for. And
    you won't have to worry about amplifying inputs below the negative
    rail. You can start with 1/4 of an LM324, and work your way up in
    specification and expense as your problem demands.

    If this doesn't fill the bill, or if you want more help, you'll have to
    throw a little more light on your problem:

    * What are the maximum and minimum frequencies of interest? You've
    got me half thinking this is RF the way you phrased this.

    * What's the maximum signal amplitude? If your signal is DC or low
    frequency AC, what's the maximum and minimum excursion of the voltage?

    * What's the output impedance of your signal source? Can your signal
    source source and sink current?

    * Is your 12VDC supply floating, or is it referenced to the input
    signals?

    * What's at the output of the gain block you want? What's the load?

    * What's an acceptable level of error or distortion?

    Hope we've been of service, Guv'nor.

    Good luck
    Chris
     
  5. Chris

    Chris Guest

    VCC
    +
    | ___ ___
    '-|___|-o-|___|-.
    R1 | R2 |
    .--------' ===
    | GND___
    | ___ .--|___|--.
    '--|___|---. | Rf |
    | | VCC |
    V1 ___ | | + |
    o------|___|---o | |\| |
    '---o--|-\ | Vout
    | >---o----o
    V2 ___ .---o--|+/ |
    o------|___|---o | |/| .-.
    | | | | |Rp
    ___ | | === | |
    .--|___|---' .-. GND '-'
    | | | |
    | | |Rf |
    | '-' ===
    '--------. | GND
    | |
    | ===
    | GND
    VCC |
    + |
    | ___ | ___
    '-|___|-o-|___|-.
    R1 R2 |
    ===
    GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    And, as inevitable when posting before the second cup of coffee, a
    minor revision. Use two identical voltage dividers R1 and R2 for
    _each_ input as above, instead of sharing. Other than that, OK, I
    guess. Sorry for the confusion.

    Good luck
    Chris
     
  6. Chris

    Chris Guest

    ` VCC
    ` +
    ` | ___ ___
    ` '-|___|-o-|___|-.
    ` R1 | R2 |
    ` .--------' ===
    ` | GND___
    ` | ___ .--|___|--.
    ` '--|___|---. | Rf |
    ` | | VCC |
    ` V1 ___ | | + |
    ` o------|___|---o | |\| |
    ` '---o--|-\ | Vout
    ` | >---o----o
    ` V2 ___ .---o--|+/ |
    ` o------|___|---o | |/| .-.
    ` | | | | |Rp
    ` ___ | | === | |
    ` .--|___|---' .-. GND '-'
    ` | | | |
    ` | | |Rf |
    ` | '-' ===
    ` '--------. | GND
    ` | |
    ` | ===
    ` | GND
    ` VCC |
    ` + |
    ` | ___ | ___
    ` '-|___|-o-|___|-.
    ` R1 R2 |
    ` ===
    ` GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    And, as inevitable when posting before the second cup of coffee, a
    minor revision. Use two identical voltage dividers R1 and R2 for
    _each_ input as above, instead of sharing. Other than that, OK, I
    guess. Sorry for the confusion.

    Good luck
    Chris
     
  7. Chris

    Chris Guest

    ` VCC
    ` +
    ` | ___ ___
    ` '-|___|-o-|___|-.
    ` R1 | R2 |
    ` .--------' ===
    ` | GND___
    ` | ___ .--|___|--.
    ` '--|___|---. | Rf |
    ` | | VCC |
    ` V1 ___ | | + |
    ` o------|___|---o | |\| |
    ` '---o--|-\ | Vout
    ` | >---o----o
    ` V2 ___ .---o--|+/ |
    ` o------|___|---o | |/| .-.
    ` | | | | |Rp
    ` ___ | | === | |
    ` .--|___|---' .-. GND '-'
    ` | | | |
    ` | | |Rf |
    ` | '-' ===
    ` '--------. | GND
    ` | |
    ` | ===
    ` | GND
    ` VCC |
    ` + |
    ` | ___ | ___
    ` '-|___|-o-|___|-.
    ` R1 R2 |
    ` ===
    ` GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    And, as inevitable when posting before the second cup of coffee, a
    minor revision. Use two identical voltage dividers R1 and R2 for
    _each_ input as above, instead of sharing. Other than that, OK, I
    guess. Sorry for the confusion.

    Good luck
    Chris
     
  8. Steve

    Steve Guest

    Sorry for the lack of info. I'll try better.
    Basically I have a two-wire inductive pickup that is used for RPM count of a
    motor shaft. The AC voltage on the two wires are of nearly equal magnitudes
    and opposite polarity. Differential signal right? The voltage range I see on
    my oscope for each wire is +- 8 V. Freq ranges from 0 - 300 Hz (18000RPM).
    I'm not sure what the impedance of the pickup is, but I think I have a
    datasheet on it at home. I currently have available a 12VDC battery and a
    buck-boost circuit that outputs 5VDC. So my desired output of this stage is
    a TTL voltage edge on the positive pulses of the composite signal. My use of
    op amps is limited outside of simple signal amplifying... so reading all
    that I could find, I surmised that using a differential op amp setup would
    allow me to create a composite signal that I could then rectify to
    eliminate the negative swings. I just don't know enough about op amps to
    make an educated guess as to how best to power the thing, what type of opamp
    to use, etc. Do I require pre-filtering for transients or this what CMR is
    for? I'm sure there are circuits available out in www-land that I could
    study and learn from but I have not found exactly what I am looking for yet.
    If you need more info please ask . Thank you for your time.
     
  9. Chris

    Chris Guest

    Sorry - it looks like the diagram got munged -- Google burped. Let's
    try again:

    ` VCC
    ` +
    ` | ___ ___
    ` '-|___|-o-|___|-.
    ` R1 | R2 |
    ` .--------' ===
    ` | GND___
    ` | ___ .--|___|--.
    ` '--|___|---. | Rf |
    ` | | VCC |
    ` V1 ___ | | + |
    ` o------|___|---o | |\| |
    ` '---o--|-\ | Vout
    ` | >---o----o
    ` V2 ___ .---o--|+/ |
    ` o------|___|---o | |/| .-.
    ` | | | | |Rp
    ` ___ | | === | |
    ` .--|___|---' .-. GND '-'
    ` | | | |
    ` | | |Rf |
    ` | '-' ===
    ` '--------. | GND
    ` | |
    ` | ===
    ` | GND
    ` VCC |
    ` + |
    ` | ___ | ___
    ` '-|___|-o-|___|-.
    ` R1 R2 |
    ` ===
    ` GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    Or actually, if you're using a dual or quad op amp, it might be better
    to feed the voltage divider into one of the op amps set up as a voltage
    follower and save the two resistors.

    Good luck
    Chris
     
  10. seware

    seware Guest

    Sorry for the lack of info. I'll try better.
    Basically I have a two-wire inductive pickup that is used for RPM count of a
    motor shaft. The AC voltage on the two wires are of nearly equal magnitudes
    and opposite polarity. Differential signal right? The voltage range I see on
    my oscope for each wire is +- 8 V. Freq ranges from 0 - 300 Hz (18000RPM).
    Pickup resistance is155 ohms +- 20%. I currently have available a 12VDC
    battery and a buck-boost circuit that outputs 5VDC. So my desired output of
    this stage is a TTL voltage edge on the positive pulses of the composite
    signal. My use of op amps is limited outside of simple signal amplifying
    (again an elec. newbie)... so reading all that I could find, I surmised that
    using a differential op amp setup would allow me to create a composite
    signal that I could then rectify to eliminate the negative swings. I just
    don't know enough about op amps to make an educated guess as to how best to
    power the thing, what type of opamp to use, etc. Do I require pre-filtering
    for transients or this what CMR is for? I'm sure there are circuits
    available out in www-land that I could study and learn from but I have not
    found exactly what I am looking for yet. If you need more info please ask
    and thank you for your time.

    Steve
     
  11. This information is very helpful. First of all, if you check the
    leads from the pickup to the case with an ohm meter, I think you will
    find that the coil is isolated from ground, so you can connect the
    pair of wires with freedom, as far as ground is concerned. This kind
    of signal source tends to produce an increasing voltage with
    increasing frequency, but also to produce a fairly constant current
    into a short circuit, over the operating frequency range.

    I suggest that you connect a pair of signal diodes (i.e. 1N4148)
    across the pickup leads, to limit the peak voltage to +- 1 volt,
    regardless of frequency. Bias one input of a single supply opamp
    (powered by 5 volts)
    (i.e. one half of an LM358 http://www.national.com/ds/LM/LM158.pdf or
    1/4 of an LM324 http://www.national.com/ds/LM/LM124.pdf )
    to about 2 volts above the negative rail with a voltage divider or a
    resistor in series with two diodes. Connect the diode clamped pickup
    pair to the two inputs. The bias for the second input passes through
    the pickup. You may want to add a capacitor to ground from the
    divider biased input, to soak up noise spikes that may couple into the
    pickup from other noise sources.

    This means that one input stays around +2 volts and the other swings
    from +1 to +3 volts, well within the common mode voltage range for a
    single supply opamp powered by 5 volts.

    The output will be a TTL compatible signal that is a square wave
    (limited by the slew rate of voltage change of the opamp output of
    about .3 volts per microsecond) representation of the pickup
    frequency. Note that6 on page 11 of the second data sheet, they
    recommend a 240 ohm resistor to ground on the output for best TTL
    compatibility, but you may not need this.

    If this circuit produces too much noise at zero frequency, since it
    has a gain of something like 1 million, there, you may want to add
    some positive feedback to lock the output up once the input falls
    below some minimum frequency. To do this, you add a series resistor
    to the + input, and a feedback resistor as shown at the top of page 13
    of the second data sheet. The ratio of the two resistors will
    determine the minimum operating frequency.
     
  12. seware

    seware Guest

    Thank you very much John.
     
  13. Rich Grise

    Rich Grise Guest

    You're not looking for an opamp at all. You already have 8Vp-p. If your
    sensor has only two wires, and neither of these are connected to the case
    ground, then you can simply ground one of them, clamp the other to the
    +5V rail and to ground with a couple of diodes, and drive the input of a
    schmitt trigger inverter, such as the 74HC14:

    +5V
    |
    V DIODE (e.g., 1N4148)
    --- cathode
    ------- | |\
    | |--------+----+-----------| >o------- Out
    | sensor| | |/
    | |---, V 1/6 74HC14
    ------- | ---
    | |
    GND GND

    This is assuming, of course, that your sensor is simply an inductor,
    and there's a magnet on the shaft.

    The output will be "better than" TTL levels - HC parts give a
    practically rail-to-rail output.

    Have Fun!
    Rich
     
  14. JeffM

    JeffM Guest

    seware wrote [witout context]:
    I notice that you're relatively new to Usenet.
    http://groups-beta.google.com/group...AA3cWgtNo9Dy92HEU9hukdoOPANdqfI6prRsqjc7uCt1A
    I would have sworn that you post from Google Groups--but nope.

    Folks on Usenet like to see a bit of the previous post
    included with (actually, above) the text of your post.
    This is called **context**.
    If you observe how most folks post, you'll see what I mean.

    Most newsreaders blockquote the entire previous post for you
    so that you can easily snip out those portions which don't apply,
    leaving just enough to give context to your addition.

    Rich Grise (pointedly) remarks on context here:
    http://groups-beta.google.com/group...-groups+are-universally-hated+include-context
    The sub-thread was about Google posters;
    you can easily see why I figured that you posted from there.

    The guidelines for Usenet are here:
    http://66.102.7.104/search?q=cache:...gh+at-*-top-*-*-message+do-not-*-*-*-original
    (Worth scanning--especially the parts I've highlighted.)
     
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