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OP AMP HELP

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  • Kayclifton@aol.com
    The project that I am working on needs to be run on battery only, and I do not know if it is possible to get a -5 input using dc. I want to have a +5 power
    Message 1 of 8 , Oct 5, 2000
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      The project that I am working on needs to be run on battery only, and
      I do not know if it is possible to get a -5 input using dc.
      I want to have a +5 power supply and reference the other input to
      ground.
      I have heard that there are single supply op amps.I may have the
      terminology wrong.
      Can anyone recommend one?
      Thanks
      Kay Clifton
    • Alejandro Fubini
      You can produce -5v with dc. It s all relative to what the op-amp sees, if you hook up the op-amp with +5 at the Vcc+ and 0v at Vcc- and you make your ground
      Message 2 of 8 , Oct 5, 2000
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        You can produce -5v with dc. It's all relative to what the op-amp sees, if
        you hook up the op-amp with +5 at the Vcc+ and 0v at Vcc- and you make your
        ground to be +2.5v then the op-amp will see the Vcc- as a negative voltage
        (with reference to ground which is actually positive 2.5v)


        |-----------| |------------|
        a[ 1.5v (AA) | b[ 1.5v (AA) |c
        |-----------| |------------|

        Here's a poor attempt at drawing 2 AA batteries, if you can see them...
        If your reference is point 'b', then at 'a' you'll have 1.5v relative to 'b'
        and at point 'c' the voltage will be -1.5v relative to 'b'. Still the
        voltage between point 'a' and 'c' is 3v as you'd expect.

        Remember that everything is relative.

        I hope this makes sense and hasn't confused you more, I'm sure someone here
        can give you a clearer explanation.

        Alex
        -----Original Message-----
        From: Kayclifton@... [mailto:Kayclifton@...]
        Sent: Thursday, October 05, 2000 6:26 PM
        To: basicstamps@egroups.com
        Subject: [basicstamps] OP AMP HELP


        The project that I am working on needs to be run on battery only, and
        I do not know if it is possible to get a -5 input using dc.
        I want to have a +5 power supply and reference the other input to
        ground.
        I have heard that there are single supply op amps.I may have the
        terminology wrong.
        Can anyone recommend one?
        Thanks
        Kay Clifton
      • Richard Warner
        Check out Burr-Brown s web site. They have single supply op amps (and every other flavor). Rich ...
        Message 3 of 8 , Oct 5, 2000
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          Check out Burr-Brown's web site. They have single supply op amps (and every
          other flavor).

          Rich


          >From: Kayclifton@...
          >Reply-To: basicstamps@egroups.com
          >To: basicstamps@egroups.com
          >Subject: [basicstamps] OP AMP HELP
          >Date: Thu, 05 Oct 2000 22:25:54 -0000
          >
          >The project that I am working on needs to be run on battery only, and
          >I do not know if it is possible to get a -5 input using dc.
          >I want to have a +5 power supply and reference the other input to
          >ground.
          >I have heard that there are single supply op amps.I may have the
          >terminology wrong.
          >Can anyone recommend one?
          >Thanks
          >Kay Clifton
          >
          >
          >
          >
          >

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        • Brian Gracia
          ... Try Max406BCPA is a 5V chip rail-to-rail Brian
          Message 4 of 8 , Oct 5, 2000
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            At 05:25 PM 10/05/2000 , you wrote:
            >The project that I am working on needs to be run on battery only, and
            >I do not know if it is possible to get a -5 input using dc.
            >I want to have a +5 power supply and reference the other input to
            >ground.
            >I have heard that there are single supply op amps.I may have the
            >terminology wrong.
            >Can anyone recommend one?
            >Thanks
            >Kay Clifton
            >

            Try Max406BCPA is a 5V chip rail-to-rail

            Brian
          • Tracy Allen
            Kay, here is the circuit I think you were referring to (from www.tekscan.com): Rf ;------/ / --; ... Rs | | +9 | -5 Vt --/ / -o--|- | |
            Message 5 of 8 , Oct 5, 2000
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              Kay,

              here is the circuit I think you were referring to (from www.tekscan.com):
              <TT>
              Rf
              ;------/\/\--;
              | |
              Rs | |\ +9 |
              -5 Vt --/\/\-o--|-\ | |
              sensor | \ |
              | >-----o---Vout
              | /
              ;---|+/|
              | |/ |
              | -9
              grnd
              </TT>
              The input to this circuit is the resistance Rs, which is (roughly)
              inversely proportional to applied force. The circuit converts resistance to
              voltage.

              Vout=5*Rf/Rs
              =~ constant * (applied force)

              The feedback resistor Rf is chosen to scale the volts into the range of
              force you want to measure. The output from the op-amp feeds an A/D
              converter. If the minimum resistance (which occurs at maximum pressure)
              happens to be 1kohm, and if you want 5 volts to go to your A/D at that
              point, then you would choose Rf=1kohm.

              Here is a single supply version of the circuit:
              <TT>

              Rf
              ;------/\/\--;
              | |
              Rs | |\ +5 |
              ;--/\/\-o--|-\ | |
              | | \ |
              | | >-----o---Vout
              | | /
              | Vr----|+/|
              | |/ |
              | |
              '------o------o-------grnd Vss
              grnd
              </TT>
              The reference voltage can come from a voltage divider or from a reference.
              My personal favorate is the LM10, a single supply op-amp that has a
              built-in reference, and that can swing all the way to the rails (0 volts to
              +5 volts) at the output.

              Vout = Vr*(Rf/Rs+1)
              Say you do use an LM10--then Vr=0.2 volts, and choose Rf=24kohm to achieve
              Vout=5 volts at Rs=1000ohms. In your software calculate
              (applied force) = constant * (Vout-Vr)

              Or let's say you use a CA3130 op amp (which also can swing to the rails at
              the output--you need that!), and a simple voltage divider to give Vr=1.0
              volts. Then choose Rf=4000 ohms to give a full scale output of 5 volts
              when the input Rs=1000 ohms.

              ---------------

              Maybe you don't need the op-amp though. FSRs are not precision circuit
              elements.

              To use RCtime instead of the op-amp/ADC, here is a circuit:
              <TT>
              Rsensor 220
              +5 Vdd ---/\/\----o---/\/\----P0 on BS2
              |
              === 0.22uf Cf
              | film
              Vss
              </TT>
              The RCtime value is proportional to Rs. Capacitor Cf sets the scale factor.

              Trc var word
              loop:
              rctime 0,0,Trc
              low 0
              debug dec Trc,tab,dec 65535/Trc,cr
              pause 200
              goto loop

              This should give a reading of about Trc=30 when Rsensor=1000ohms.
              65535/Trc inverts to give a result proportional to applied force.
              (applied force)=~ constant/Trc


              -- Tracy Allen
              Electronically Monitored Ecosystems
              http://www.emesystems.com
            • Al Williams
              Hi Kay, This is a tricky question with a lot of things you have to consider. A normal op amp can t go right to either supply rail. So if you run an op amp at
              Message 6 of 8 , Oct 6, 2000
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                Hi Kay,

                This is a tricky question with a lot of things you have to consider.

                A "normal" op amp can't go right to either supply rail. So if you run an op
                amp at -5 and 5 V, your output might range from -4.8V to 4.8V. The more
                current you take from the output, the lower the voltage range is. So if
                those numbers are at 10mA, at 50mA, it might drop to, say, +/- 4.7V.

                Now, the problem is, you want a single supply. But is that practical? That
                depends on what you are doing. They do make single-supply op-amps. These are
                especially designed to go to at least one rail (usually the - rail). So if
                you supply ground (0V) and 5V, the output could range from 0 to 4.8V (for
                example). What does that mean? Well suppose you are buffering a DAC or PWM
                output. The output only goes from 0 to 5V anyway. So you'll never need that
                negative output swing. If you know your output would always range from 1V to
                4V, you might not even need a special op amp at all.

                However, what happens if you are using the op amp as an audio amplifier with
                a capacitor coupling the input? Well, then you are in trouble because the
                input is going to have + and - components and you are going to throw away
                half of them (horrible distortion). So for that sort of thing, you have to
                have an op amp that can go + or - or a way to bias the input so that it is
                never negative (and you don't invert the signal).

                So if you need that, the question becomes, "How do I get a negative
                voltage?" There are a few common ways of doing this:

                1) Use 2 9V batteries. Ground the center. Use the 9V to power your 5V
                regulator and use +/- 9V on the op amp.

                2) Use a dc to dc converter module to convert some voltage you already have
                to -5V. Power Trends has some switching regulators that take +5 and
                deliver -12 (with the magnetics built in). I haven't used them, but we use
                the 38V to 5V regulator module in a product and other than being
                unnecessarily large, it works OK. Maxim and others make chips for this, but
                they require external inductors (that I know of). The Power Trends part is a
                module like the Stamp -- not a true IC.

                The simplest DC to DC converter is just an oscillator of some sort (555, RC
                and an inverter, etc.), and a capacitive charge pump switched with diodes
                (see
                http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#5volts.gif).
                You can also use a transformer/rectifier scheme. Look up DC to DC converter
                on the Web. Switching regulators can do it by manipulating charge/magnetic
                field. Maxim and others make "voltage inverters" that have all you need
                except the capacitors.

                3) An ugly trick I have used before works if you have low power requirements
                on the op-amp. First make sure the op amp's inputs and outputs are DC
                isolated (capacitor coupling on all in and out). This isn't uncommon anyway.
                Then take a 9V battery and build a voltage divider (50%). Ground the center
                of the voltage divider. Now the battery terminals are at 4.5V and -4.5V
                (just don't ground the battery ground -- it isn't ground, it's -4.5V. The
                downside is that the divider draws power, and the load will "upset" the 50%.
                Still, I have used this to make a small circuit run off a single 9V battery
                with no extra DC to DC converters, etc. The whole thing was just a fraction
                bigger than a 9V battery by itself -- two jacks, two capacitors, the op amp,
                two resistors for power, and a few parts in the feedback loop. This is
                somewhat less ugly if you use more parts (see
                http://www.sound.au.com/project43.htm who describes it better than I can).
                If you need more power (grunt, grunt) look at
                http://www.aaroncake.net/circuits/vinvertr2.htm -- a novel use of an LM386.


                As usual, I've gone on too long. However, the point is:

                a) You can get single supply op amps, but they are not always appropriate
                for what you want to do.

                b) If they are not, generating negative voltage is not all that scary after
                all.

                Hope that is helpful.

                Regards,

                Al Williams
                AWC
                *Floating point math for the Stamp, PIC, SX, or any microcontroller at
                http://www.al-williams.com/awce/pak1.htm




                > -----Original Message-----
                > From: Kayclifton@... [mailto:Kayclifton@...]
                > Sent: Thursday, October 05, 2000 5:26 PM
                > To: basicstamps@egroups.com
                > Subject: [basicstamps] OP AMP HELP
                >
                >
                > The project that I am working on needs to be run on battery only, and
                > I do not know if it is possible to get a -5 input using dc.
                > I want to have a +5 power supply and reference the other input to
                > ground.
                > I have heard that there are single supply op amps.I may have the
                > terminology wrong.
                > Can anyone recommend one?
                > Thanks
                > Kay Clifton
                >
                >
                >
              • Stephen Roberts
                Just a bit of philosophy, and a practical comment. Although certain op amps are optimized for single-supply operation, there s really no distinction between
                Message 7 of 8 , Oct 6, 2000
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                  Just a bit of philosophy, and a practical comment.

                  Although certain op amps are optimized for single-supply operation, there's
                  really no distinction between "single-supply" and "dual-supply" op amps.
                  Notice that the op amp has no "ground" terminal. All it knows about is the
                  (+) and (-) supply voltages, and how the inputs compare to the supply
                  voltages.

                  Now for the practical comment. In addition to Tracy's and Al's suggestions,
                  another approach is to use an ICL7662 CMOS "Voltage Converter". This is an
                  8-pin DIP part which (among other things) gives you the negative of
                  whatever input voltage you supply. Digikey has p/n TC7662ACPA for $2.25.
                  Mouser has p/n 570-ICL7662CPA for $1.92. You provide two 10uF tantalum caps
                  and input your +5V, and you get -5V out. Hope this helps.

                  Steve

                  --------------------------------------------------------------------
                  Steve Roberts: sroberts@...
                  --------------------------------------------------------------------
                • Technology Electronics, Ltd.
                  Hello Kay, There are a number of inexpensive (less than a dollar) CMOS opamps that may work. I also have a great circuit that costs about a dollar or two in
                  Message 8 of 8 , Oct 10, 2000
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                    Hello Kay,

                    There are a number of inexpensive (less than a dollar) CMOS opamps that may
                    work. I also have a great circuit that costs about a dollar or two in basic
                    parts to generate a negative voltage to drive CMOS opamps.

                    If interested in details, e-mail your regular mailing address, and we can
                    send you a schematic that may work for you. Also, please include the total
                    current requirements of the supply and the maximum allowable current drain
                    from the battery.

                    Chuck Page
                    Technology Electronics, Ltd.

                    -----Original Message-----
                    From: Kayclifton@... <Kayclifton@...>
                    To: basicstamps@egroups.com <basicstamps@egroups.com>
                    Date: Thursday, October 05, 2000 6:26 PM
                    Subject: [basicstamps] OP AMP HELP


                    >The project that I am working on needs to be run on battery only, and
                    >I do not know if it is possible to get a -5 input using dc.
                    >I want to have a +5 power supply and reference the other input to
                    >ground.
                    >I have heard that there are single supply op amps.I may have the
                    >terminology wrong.
                    >Can anyone recommend one?
                    >Thanks
                    >Kay Clifton
                    >
                    >
                    >
                    >
                    >
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