> > You are very close to the answer. =A0Try removing R9 - this will change
> > the gain of the input amp from +2V to +1V. =A0With an offset adjust val=
ue
> > around +1V, you should get what you are looking for.
> >
> > Why the change? =A0You have an input voltage range of -2V to +2V =3D=3D=
> =A04V
> > span. =A0You want an output span of +4V to 0V =3D=3D> =A04V span also. =
=A0Thus, you
> > need a system gain of -1 which your second stage already has.
>
> Of course, I meant
> "Try removing R9 - this will change the gain of the input amp from +2 to
> +1."

If the P version of the opamp  is essentially the same as this

                       http://www.st.com/stonline/books/pdf/docs/2298.pdf

then it's output is not guaranteed to swing to within more than about
2 volts of either supply at +/- 5V supply.
How closely it does swing when you drive it into 'forbidden' areas may
relate to how much overdrive you provide, leading to unpredicatble and
non-linear effects.

Start off running it on say 2 x 9V batteries to get the circuit
working correctly and then investigate the implications of lower power
supply voltages.

You have a rail to rail input amplifier but not R2R output.
In this case, if you only have +/- 5 Volt available you want R2R on
input and output.


   Russell McMahon.

-- =

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