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Op Amp design

Greetings, I need to know how I can use an Op Amp to take my +/-2.5vdc input, and produce a 0vdc - +5vdc output.

I know that this is a negative feedback type of circuit where the output is wired to the amplifier's inverting (negative) terminal.

I will be using two 9vdc batteries for a dual-supply voltage. I know that my supply center-tap should be my ground, the positive side of my supply powering the positive side of the supply voltage, likewise on the negative. Now, how do I offset my output in proportion to my input?

Thanks

John Doss

Reply to
john.doss
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Inverting or non-inverting?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

I read in sci.electronics.design that " snipped-for-privacy@comcast.net" wrote (in ) about 'Op Amp design', on Mon, 26 Sep 2005:

The key to this sort of op-amp question is to remember that if you are using the op-amp within its gain and frequency limitations, and the output is not driven close to either supply rail, there is substantially no voltage between its two inputs.

So, if you connect the - input through a resistor value R to the output and put -2.5 V on the - input through another resistor of value R, then change the input to the + input from - 2.5 V to + 2.5 V, the output will go from 0 V to +5 V.

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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

I'm going to go out on a limb and assume from some of what you said that the answer to Jim's question is "inverting" - that is, when the input is +2.5v, you want the output to be 0v, and when the input is -2.5v, you want the output to be 5v. So, you want Vout = 2.5v - Vin = -(-2.5v + Vin).

Here's one way to get that:

___ .---|___|--. ___ | R | Vref---|___|--o | NOTE: Vref/Rref = (-2.5V)/R Rref | +9V | ___ | |\\| | Vin ---|___|--o---|-\\ | R | >---o--- Vout .---|+/ | |/| | -9V GND

Vref/Rref must equal -2.5V/R. You could use all R's the same and set Vref=-2.5V, or you could use Rref=2R and use Vref=-5.0V, or 0.5R and -1.25V. And so on. Sometimes it's easier to find positive voltage references; if you had a dual opamp, you could use one half as a voltage inverter. I don't know how precise you need to be, nor what kind of battery life you need.

Since in this circuit the inverting input is a "virtual ground" (meaning that as long as you don't exceed voltage or frequency limits of the opamp it will be close to ground), the input impedance is R.

You might want to put a capacitor in parallel with the feedback resistor to keep the circuit from oscillating. Since you said "DC" I'm assuming you don't need much in the way of frequency response; so, let C = 4.7uF/R, e.g., for R=100k, use C=47pF. Just a ballpark value, doesn't have to be exact.

Reply to
Walter Harley

That 0-5V output is for an A/D possibly- then your OUT to IN equation is OUT= IN + 2.5 which is a simple non-inverting summer, but your application is battery, so you want a rail-to-rail output type of amplifier RRO that is powered by +5V to GND so as to protect the A/D input: View in a fixed-width font such as Courier.

. . . . +---------------------+ . | | . +5V>-+-[2R]-+------+ | . | | | . | | |\\| RRO . IN>-----------[R]-+-----|+\\ . | | >---+->OUT . [2R] +-----|-/ | . | | |/| | . | | | | . +--[R]-+-[R]---|----+ . | | . | | . +--------------+ . | . GND . .

Reply to
Fred Bloggs

If for some strange reason you need OUT= -IN + 2.5 , inverting the IN signal, then same circuit works with exchanged inputs like so: View in a fixed-width font such as Courier. . . . . +---------------------+ . | | . +5V>-+-[2R]-+------+ | . | | | . | | |\\| RRO . +------[R]-+-----|+\\ . | | | >---+->OUT . | [2R] +-----|-/ | . | | | |/| | . | | | | | . IN>-----------[R]-+-[R]---|----+ . | | | . | | | . +---+--------------+ . | . GND . .

Reply to
Fred Bloggs

Take a look at this:

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This shows an op-amp circuit that is designed to do something similar to what you want. This design takes the raw DC coupled signal out of the DAC of a sound card and sums (with adjustment) a negative voltage with it to bring it back down to center over the zero volt line. It also provides a little bit of adjustable gain and it is an inverter. The X, Y scanners take negative and positive voltage as input. The PCAOM controller for color only takes 0 to +5V. This is achieved by only writing positive values to the DAC.

James. :o)

Reply to
James Lehman

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