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Re: Substituting OP27 op-amp

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  • Scotty
    ... requires ... negative ... I didn t do a great deal of searching for single supply op amps. Maybe someone knows of a good one. The requirements for the op
    Message 1 of 5 , Feb 11, 2004
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      --- In spectrumanalyzer@yahoogroups.com, Will Wasson <willwasson@c...>
      wrote:
      > Hi Scotty,
      >
      > As far as I can see the Op27 amplifier is the only device that
      requires
      > a negative voltage, (-5v). Do you know if there is a single supply,
      > rail to rail op-amp that could replace it to save on having a
      negative
      > supply.
      >
      > Will

      I didn't do a great deal of searching for single supply op amps. Maybe
      someone knows of a good one. The requirements for the op amp are:
      The output MUST go down to 1.0 volt or lower to swing the VCO to its
      lowest frequency.
      The input offset current and input noise current should be as low as
      possible, in the pico amp range would be best. Offset voltage is of no
      concern.
      Must allow +20v operation on Vcc, and the output should swing to
      within 2 volts of Vcc to allow VCO to swing to its highest frequency.
      My OP 27 is drawing less than 2 ma from the negative supply. -5v is
      not critical.
      As a matter of fact, my original breadboard's negative voltage was
      created by ac coupling the positive bridge rectifier in my power
      supply and rectifying it to -3v.
      The OP 27 will swing to within 1.5 volts of either rail. Some of the
      newer ones will swing closer than that. I used the OP27 because I had
      a few, and they have been the workhorse of loop filters for many
      years. But, it is really out of date.
      Scotty Sprowls
    • willwasson_94550
      Hi Scotty, Thanks for the reply. Most of the single supply, rail2rail devices satisfy the output swing requirements, but the GBWP of the devices are pretty
      Message 2 of 5 , Feb 11, 2004
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        Hi Scotty,

        Thanks for the reply.

        Most of the single supply, rail2rail devices
        satisfy the output swing requirements, but
        the GBWP of the devices are pretty low. I see
        many around 90KHz or so, and I'm not sure
        if they would be fast enough for your feedback
        loop.

        With the lowering of the +12v requirement to +10v,
        I can see this whole unit being powered by a
        single 12v wall wart supply. That is if I can
        get rid of the -5v supply :-)

        An alternative would be to add a dc to dc converter
        to convert the +12v input to -5v. But this
        has the problem of pushing switching noise into
        your design, and I'm not sure if this would
        be good idea. Idealy, I'm planning on using a
        block of aluminum the size of the circuit board,
        and using my milling machine to make cutouts for
        each module. The walls of the cutouts will contact
        guard rings around each module on the PC board.
        This will provide the interstage shielding along
        with the full ground on the other side of the PC
        board. So maybe a dc to dc converter wouldn't be too
        bad.

        In fact, I was wondering if the cavity filter could
        be milled into the same aluminum block.

        In a previous post you said the you were a real
        stickler for layout design. So I'd really value
        your opinion. Am I getting too far out in left
        field with this aluminum block shield idea :-)

        Thanks!

        Will




        --- In spectrumanalyzer@yahoogroups.com, "Scotty" <wsprowls@y...> wrote:
        > --- In spectrumanalyzer@yahoogroups.com, Will Wasson <willwasson@c...>
        > wrote:
        > > Hi Scotty,
        > >
        > > As far as I can see the Op27 amplifier is the only device that
        > requires
        > > a negative voltage, (-5v). Do you know if there is a single supply,
        > > rail to rail op-amp that could replace it to save on having a
        > negative
        > > supply.
        > >
        > > Will
        >
        > I didn't do a great deal of searching for single supply op amps. Maybe
        > someone knows of a good one. The requirements for the op amp are:
        > The output MUST go down to 1.0 volt or lower to swing the VCO to its
        > lowest frequency.
        > The input offset current and input noise current should be as low as
        > possible, in the pico amp range would be best. Offset voltage is of no
        > concern.
        > Must allow +20v operation on Vcc, and the output should swing to
        > within 2 volts of Vcc to allow VCO to swing to its highest frequency.
        > My OP 27 is drawing less than 2 ma from the negative supply. -5v is
        > not critical.
        > As a matter of fact, my original breadboard's negative voltage was
        > created by ac coupling the positive bridge rectifier in my power
        > supply and rectifying it to -3v.
        > The OP 27 will swing to within 1.5 volts of either rail. Some of the
        > newer ones will swing closer than that. I used the OP27 because I had
        > a few, and they have been the workhorse of loop filters for many
        > years. But, it is really out of date.
        > Scotty Sprowls
      • Scotty
        ... The gain bandwidth product of the op amp does not need to be very high. In the presently designed loop, the 0 gain point is at 41 KHz. The PLL 1 phase
        Message 3 of 5 , Feb 12, 2004
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          --- In spectrumanalyzer@yahoogroups.com, "willwasson_94550"
          <willwasson@c...> wrote:
          > Hi Scotty,

          > Most of the single supply, rail2rail devices
          > satisfy the output swing requirements, but
          > the GBWP of the devices are pretty low. I see
          > many around 90KHz or so, and I'm not sure
          > if they would be fast enough for your feedback
          > loop.

          The gain bandwidth product of the op amp does not need to be very
          high. In the presently designed loop, the "0" gain point is at 41
          KHz. The PLL 1 phase frequency is about 350 KHz and the pre filter
          pole in the loop is 340 KHz. An op amp that is "lousy" at these
          frequencies is actually beneficial. It will help attenuate the side
          bands in the VCO that are created by the PLL's phase discriminator.
          The minimum GBWP of the op amp should be at least 2 times the maximum
          of the loop BW, in this case, 82 KHz. However, the experimentor using
          faster loops may want to have a faster op amp, probably a 500 KHz
          would be the maximum he would need.
          Scotty

          > With the lowering of the +12v requirement to +10v,
          > I can see this whole unit being powered by a
          > single 12v wall wart supply. That is if I can
          > get rid of the -5v supply :-)

          We still need about +20 v for the op amp. The VCO needs about +18v
          control voltage to get to 2013 MHz. I have had best results when
          double regulating all of the voltages in the SSA. For example, the
          +12 regulators should be fed by a +15v or +20v regulator. The +5v
          regulators should be also be fed by a pre-regulator. The +20v and -5v
          to the op amp need not be preregulated. Scotty

          > An alternative would be to add a dc to dc converter
          > to convert the +12v input to -5v. But this
          > has the problem of pushing switching noise into
          > your design, and I'm not sure if this would
          > be good idea.

          If it is well filtered and shielded, it might not create a problem.
          It is certainly worth trying. It would manifest itself when seeing
          the switching frequency as sidebands on high level signals you are
          analyzing.
          Scotty

          Idealy, I'm planning on using a
          > block of aluminum the size of the circuit board,
          > and using my milling machine to make cutouts for
          > each module. The walls of the cutouts will contact
          > guard rings around each module on the PC board.
          > This will provide the interstage shielding along
          > with the full ground on the other side of the PC
          > board. So maybe a dc to dc converter wouldn't be too
          > bad.
          >
          > In fact, I was wondering if the cavity filter could
          > be milled into the same aluminum block.
          >
          > In a previous post you said the you were a real
          > stickler for layout design. So I'd really value
          > your opinion. Am I getting too far out in left
          > field with this aluminum block shield idea :-)

          > Thanks!
          >
          > Will


          I have used the milled cavities in many professional designs. Mainly,
          for RF modules that were part of card bus systems (which are horribly
          noisy). The only problem I have had with the method is good RF
          grounding of the "guard rings" to the aluminum. In a couple of
          designs, having a mounting screw every 1 inch was not enough to keep
          1 GHz from leaking into the next cavity. I made the process
          engineers electroplate the aluminum, screwed it to the board, and
          then "sweat soldered" the alu. to the guard rings. That pretty well
          blew the mechanical engineers design for repairability. But, it
          passed all EMI testing. I would say, proceed with your plan. It will
          make for a beautiful single module.
          As far as using the block for the cavity filter. I suppose you
          could mill the four cavities from a single block with the "top plate"
          integral to the block. The internal 1/4 inch stubs must be well
          soldered to the "bottom plate" and it, in turn, must be well soldered
          to the cavity block. If you are cool enough to have a milling
          machine, you are probably aware of the solderability of aluminum. Do
          you have a TIG welder?
          I use coffee can lids for shielding. Good solderability and can be
          cut with a good, stiff pair of sissors. Don't borrow your wife's.
          Divorce is likely!!
          Scotty
        • Will Wasson
          Hi Scotty, Thanks again! You probably saved me days of lost work milling out an aluminum block. Silly me, I thought that a milled block should provide battle
          Message 4 of 5 , Feb 12, 2004
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            Hi Scotty,

            Thanks again! You probably saved me days of lost work milling out an
            aluminum block.
            Silly me, I thought that a milled block should provide "battle ship"
            type shielding for the
            unit. I was planning on providing screws every few inches to tie it
            down to the PC board.
            It would've taken me days to get the block milled and the screws tapped
            and threaded.
            And it might have worked, but as you noted "Murphy" would have come
            along someday
            and zapped a part , and if the block had to be soldered down that would
            have been it! :-(

            The coffee can lids is an idea that has merit. I do have tin snips, so
            my wife won't chase
            me down the street with her ruined scissors inches from my rear :-)

            A TIG is on my wish list, but until I can find a home repair that
            "needs" a TIG to get it done
            it will have to wait. Of course my wife is still wondering why I needed
            a mini-lathe to
            redo the bathroom floor, ("Oh yeah Hon, there were some special
            connectors under the
            floor that Home Depot doesn't carry anymore, and to reseat the toilet I
            needed to cut
            and turn new brackets". )

            Will

            >
            >
            >
            >
            > I have used the milled cavities in many professional designs. Mainly,
            > for RF modules that were part of card bus systems (which are horribly
            > noisy). The only problem I have had with the method is good RF
            > grounding of the "guard rings" to the aluminum. In a couple of
            > designs, having a mounting screw every 1 inch was not enough to keep
            > 1 GHz from leaking into the next cavity. I made the process
            > engineers electroplate the aluminum, screwed it to the board, and
            > then "sweat soldered" the alu. to the guard rings. That pretty well
            > blew the mechanical engineers design for repairability. But, it
            > passed all EMI testing. I would say, proceed with your plan. It will
            > make for a beautiful single module.
            > As far as using the block for the cavity filter. I suppose you
            > could mill the four cavities from a single block with the "top plate"
            > integral to the block. The internal 1/4 inch stubs must be well
            > soldered to the "bottom plate" and it, in turn, must be well soldered
            > to the cavity block. If you are cool enough to have a milling
            > machine, you are probably aware of the solderability of aluminum. Do
            > you have a TIG welder?
            > I use coffee can lids for shielding. Good solderability and can be
            > cut with a good, stiff pair of sissors. Don't borrow your wife's.
            > Divorce is likely!!
            > Scotty
            >
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