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RE: [Electronics_101] Op Amp selection

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  • Larry Beaty
    Steve, Bob Pease advised me that RR output requires a crossover in the input stages of the op amp. I just use more voltage for the op amp than I need in the
    Message 1 of 4 , May 10, 2013
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      Steve,



      Bob Pease advised me that RR output requires a crossover in the input stages
      of the op amp. I just use more voltage for the op amp than I need in the
      output.



      Larry



      From: Electronics_101@yahoogroups.com
      [mailto:Electronics_101@yahoogroups.com] On Behalf Of Steve Greenfield
      Sent: Friday, May 10, 2013 3:10 PM
      To: Electronics_101 Mailing List
      Subject: [Electronics_101] Op Amp selection





      Phew!

      I'm looking for an inexpensive Op Amp that I can use for a lot of projects.
      I'm not looking for an all-purpose, works on everything Op Amp. Just close.

      So far the OPA365 isn't looking too bad. RRIO within 10mV of the rails on
      the output and inputs that can go 100mV outside of the supply rails, 2.2 to
      5.5V (+-1.1 to +-2.75V), 50MHz GBW, slew rate 25V/uS, offset voltage
      100uV...

      But the price is higher than I'd like at about $1.50 each from Newark, a few
      cents more from Mouser but without that nasty $20 freight charge. And most
      sensors don't require such high frequencies.

      Links:
      http://www.ti.com/product/opa365

      <http://www.mouser.com/ProductDetail/Texas-Instruments/OPA365AIDBVR/?qs=sGAE
      piMZZMutXGli8Ay4kAIC6Ja3Zf0ILqDG3m9fyLs%3d>

      My head is spinning. It is like speed dating for the person you'll marry.

      I thought this was a clever implementation of a precision rectifier, it does
      not rely on switching through a diode and so the frequency limits and slew
      rates should be the same as the Op Amp ratings:
      http://www.planetanalog.com/document.asp?doc_id=528005
      <http://www.planetanalog.com/document.asp?doc_id=528005&site=planetanalog>
      &site=planetanalog


      Steve Greenfield AE7HD
      http://www.linkedin.com/in/stevenjgreenfield

      [Non-text portions of this message have been removed]





      [Non-text portions of this message have been removed]
    • AlienRelics
      For a current project (silicon diode to Arduino) I m trying to find something that can go below the negative (ground/common for single supply) on the input,
      Message 2 of 4 , May 10, 2013
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        For a current project (silicon diode to Arduino) I'm trying to find something that can go below the negative (ground/common for single supply) on the input, and down to the negative for the output. I'm not really in need of the positive rail for this, as I plan on using the ADC input modes on AVR chips at 1.1V or 2.5V to avoid the positive rail.

        But I'd prefer to find something that also does the positive rail in the same manner so I can just order 100 or so and have them on hand.

        Don't you mean that it requires crossover input for RR -input-?

        <http://www.edn.com/electronics-blogs/bakers-best/4400897/What-does--rail-to-rail--input-operation-really-mean->

        Although for feedback purposes, you likely do need RR input if you are going to take advantage of RR output. It is a question of design. After all, done correctly, I can do high side current monitoring while powering the Op Amp from a supply voltage much smaller than that being monitored, with inputs that won't allow operation even up to the Op Amp's V+. It is just a matter of design so the Op Amp never operates outside its acceptable range.

        For the output, you are correct, I really need a -V supply to actually get to ground. In the case of the diode to Arduino signal conditioner, I'm using a diode in the output with a pull-down resistor load to create an offset, so that the output really only has to go down to 0.6V above common (single supply).

        Steve Greenfield AE7HD

        --- In Electronics_101@yahoogroups.com, "Larry Beaty" <lbeaty@...> wrote:
        >
        > Steve,
        >
        >
        >
        > Bob Pease advised me that RR output requires a crossover in the input stages
        > of the op amp. I just use more voltage for the op amp than I need in the
        > output.
        >
        >
        >
        > Larry
        >
        >
        >
        > From: Electronics_101@yahoogroups.com
        > [mailto:Electronics_101@yahoogroups.com] On Behalf Of Steve Greenfield
        > Sent: Friday, May 10, 2013 3:10 PM
        > To: Electronics_101 Mailing List
        > Subject: [Electronics_101] Op Amp selection
        >
        >
        >
        >
        >
        > Phew!
        >
        > I'm looking for an inexpensive Op Amp that I can use for a lot of projects.
        > I'm not looking for an all-purpose, works on everything Op Amp. Just close.
        >
        > So far the OPA365 isn't looking too bad. RRIO within 10mV of the rails on
        > the output and inputs that can go 100mV outside of the supply rails, 2.2 to
        > 5.5V (+-1.1 to +-2.75V), 50MHz GBW, slew rate 25V/uS, offset voltage
        > 100uV...
        >
        > But the price is higher than I'd like at about $1.50 each from Newark, a few
        > cents more from Mouser but without that nasty $20 freight charge. And most
        > sensors don't require such high frequencies.
        >
        > Links:
        > http://www.ti.com/product/opa365
        >
        > <http://www.mouser.com/ProductDetail/Texas-Instruments/OPA365AIDBVR/?qs=sGAE
        > piMZZMutXGli8Ay4kAIC6Ja3Zf0ILqDG3m9fyLs%3d>
        >
        > My head is spinning. It is like speed dating for the person you'll marry.
        >
        > I thought this was a clever implementation of a precision rectifier, it does
        > not rely on switching through a diode and so the frequency limits and slew
        > rates should be the same as the Op Amp ratings:
        > http://www.planetanalog.com/document.asp?doc_id=528005
        > <http://www.planetanalog.com/document.asp?doc_id=528005&site=planetanalog>
        > &site=planetanalog
        >
        >
        > Steve Greenfield AE7HD
        > http://www.linkedin.com/in/stevenjgreenfield
        >
        > [Non-text portions of this message have been removed]
        >
        >
        >
        >
        >
        > [Non-text portions of this message have been removed]
        >
      • AlienRelics
        Speaking of which, a negative bias voltage generator makes -0.23V so you can use a 5.5V Op Amp down to the negative rail: http://www.ti.com/product/lm7705 For
        Message 3 of 4 , May 10, 2013
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          Speaking of which, a negative bias voltage generator makes -0.23V so you can use a 5.5V Op Amp down to the negative rail:

          http://www.ti.com/product/lm7705

          For a quick DIY, you can always do something like generate -V by using a voltage doubler from an oscillator built from a spare inverting gate, 555 timer, or set an output from a uController to putting out a square wave.

          Keep in mind, from 5V power, that is a 2.5Vp square wave. Doubled, with two diode drops subtracted is about -3.6V, -not- -10V as you might expect.

          Steve Greenfield AE7HD

          --- In Electronics_101@yahoogroups.com, "Larry Beaty" <lbeaty@...> wrote:
          >
          > Steve,
          >
          >
          >
          > Bob Pease advised me that RR output requires a crossover in the input stages
          > of the op amp. I just use more voltage for the op amp than I need in the
          > output.
          >
          >
          >
          > Larry
          >
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