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Rail-to-rail op-amp suggestion?

Discussion in 'Electronic Design' started by [email protected], Feb 22, 2006.

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  1. Guest

    Not being an analog guy, I'm having trouble choosing an op-amp from the
    vast array available. I just want to use it as a voltage follower to
    handle a slight impedance mismatch between a sensor and an ADC.

    5V supply with output swing from at least 0.25 to 4.5V.
    Low-bandwidth application (DC to 300Hz)
    Would like a dual-gate 8-pin SOIC or (T)SSOP if possible.

    I realize this question sounds stupid, but is an LM358 suitable for
    this application?
     
  2. Guest

    No. The input common mode voltage range of the LM358 only extends up to
    1.5V below the rail - 3.5V with a 5V rail. A 4.5V output voltage
    doesn't seem too realistic either.

    You might want to look at the Motorola (now Freescale?) MC33201. It's
    particular virtue is a realtively low output impedance, which is handy
    when you are driving nasty capacitative ADC inputs.
     
  3. PeteS

    PeteS Guest

    An amp I used successfully for driving nasty capacitive loads with rail
    to rail in and out (within 100mV of the rail for both), rated at 2.5V
    to 30V supply (typical 5V) is the LM2861. I was using this as a unity
    gain buffer, which is your application.

    http://www.national.com/pf/LM/LM8261.html

    Cheers

    PeteS
     
  4. PeteS

    PeteS Guest

    For the OP - unity gain is usually the worst case for capacitive
    loading, as it typical (as Bill alluded) on an ADC input. There's an
    excellent app note about it from National (marketing their designs, of
    course, but hey, that's business)

    http://www.national.com/appbriefs/files/AppBrief108.pdf

    Cheers

    PeteS
     
  5. Phil Hobbs

    Phil Hobbs Guest

    Interesting. There was one thing in the datasheet that made my brain
    hurt a bit though--maybe I've been doing device work too long, but I'm
    puzzled by this passage from Page 15:

    The output stage Figure 1 is comprised of complementary
    NPN and PNP common-emitter stages to permit voltage
    swing to within a VCE(SAT) of either supply rail. Q9 supplies
    the sourcing and Q10 supplies the sinking current load.
    Output current limiting is achieved by limiting the VCE of Q9
    and Q10; using this approach to current limiting, alleviates
    the draw back to the conventional scheme which requires
    one VBE reduction in output swing.

    Q9 and Q10 are the output transistors. But how does limiting VCE
    perform current limiting? Maybe they meant VBE? I can think of ways to
    do that, but VCE?

    How do you IC guys do current limiting in a bipolar RRO amplifier?

    Cheers,

    Phil Hobbs
     
  6. No, for reasons Bill described. You might be able to get the output
    swing with a pullup, but the input stage can't handle signals close to
    the +ve (ObJohnLarkin: 'positive') rail. So, forget it.

    Last time I had this requirement for a moderate quantity design, I
    picked a Microchip MCP602x series part for its low max Vos** (and a
    number of other factors), but there are many, many parts which will
    meet your stated requirements.

    ** Note that with RRIO opamps the Vos can behave a bit strangely as
    you transition between the two input stages (Vos shifts suddenly by
    scores of uV with CM input voltage (and the temperature sensitivity
    changes in the case of the MCP602x series)). If your ADC is ~10 bits
    you can virtually always ignore this, because it's quite a bit less
    than the quantization error (~5mV in this case if Vref = 5.0V/10bits).


    Best regards,
    Spehro Pefhany
     
  7. James Beck

    James Beck Guest

    Microchip has some parts made just for this type of application. I use
    one of their MCP6271 for a little signal conditioning on one of our
    products. Just go to microchip.com and take a look. Should be several
    that they list as rail to rail input and output.

    Jim
     
  8. Guest

    Hi Bill (et al - this is a global reply):

    The main reason I was looking at the 358 is that I bought twelve reels
    of them a while ago, and still have six and a half left. It's the only
    dual op-amp in an 8-pin package that I have in my home lab, in fact. I
    have a regulated +12V rail in my application, and for that matter a
    rather wiggly +/-10V available at very low current. I had some crazy
    plan to power the 358 off +12, and bias the input up off the 0V rail,
    but I kind of stopped there.

    I've downloaded the datasheets for the parts suggested by everybody,
    thanks for the input. I happen to have a friendly Microchip rep keen to
    get her parts into my work, so it will PROBABLY wind up being the uChip
    part....
     
  9. Guest

    I think this factor can be ignored in my application. The sensor value
    is never going to be static - it wanders constantly. The software
    filters the input; it maintans a running average of sixteen samples in
    a ring buffer. If "this" sample differs from the average by more than a
    threshold, a second ring buffer is populated with a copy of the current
    ring buffer, and the new value is put in the second buffer. If, after
    sixteen further samples, the average of buffer 2 is further away from
    the original average than 1/16th the difference between the first
    maverick and the original average (i.e. if a trend is detected), the
    second buffer is copied into the first buffer and it goes back to
    "quiet" mode. I'm not quite sure what to call this sort of filter, but
    it works very well in the application.

    The software that needs the sensor reading only sees the filtered
    average.
     
  10. I think I didn't explain this fully... it's a DC effect.

    "Suddenly" refers to the slope of Vos vs. Vcm (offset voltage vs.
    common mode voltage) not Vos vs. time (or imagine you were increasing
    common mode voltage at certain rate). But it's well under a mV (100 or
    200uV, IIRC) in size, so unless you have a lot of bits or are
    dithering with noise or something like that to get more effective bits
    it will be well hidden.


    Best regards,
    Spehro Pefhany
     
  11. Well, you didn't say anything about having 12V available (and we
    assumed you didn't). In that case, you probably can use the LM358, but
    you'd have to clamp the output so it doesn't cause problems with the
    ADC. The LM358 also has a rather large maximum Vos (that means a DC
    offset error! perhaps several bits), but aside from that it works
    almost down to the - rail. You could get a -0.6V with a diode and
    resistor and a +12V from your regulated supply if it was necessary to
    get down to the rail. If your ADC can tolerate a couple of K you could
    use the second amplifier as a clamp using a diode and the 5V supply as
    a reference. If you only need to go to 50mV or so then you wouldn't
    need the - supply, ground would do.
    The CMOS parts from Microchip are nice, and not insanely expensive,
    but they cost a lot more than an LM358 bought in bulk (probably under
    a dime). You do get 500uV Vos, almost no bias current, and few
    external parts for the money. OTOH, there are some applications where
    the bias current is actually an advantage (detect an open connection).


    Best regards,
    Spehro Pefhany
     
  12. Fred Bartoli

    Fred Bartoli Guest

    Not all the RRI opamps exhibit this effect.
    See the National's range of CMOS opamps.

    For example:
    http://cache.national.com/ds/LM/LMC6484.pdf
     
  13. Yes, DS011714-60 and DS011714-61. Much better.


    Best regards,
    Spehro Pefhany
     
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