[sdiy] Precision voltage offset

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At 11:29 AM -0800 01/25/02, GothGeek Sysadmin wrote:
>
>I'm trying to offset a CV by a set amount.  Offset it down in fact.  The
>idea here is to offset F0 to 0 volts so I can use a standard Midi/CV with
>my Moog without having to patch it to a keyboard and use its transpose
>feature.  ( yes, I'm on the same quest I mailed about a month or 2 ago ).

So you want to add a negative voltage to another voltage??  Do you know
excactly how much voltage you need to add??

I'd build an inverting summing amplifier followed by an inverting amplifier
using two op-amps.  For VCO voltages I'd use a LT1013 dual op-amp.

For an inverting amplifier, the non-inverting (positive) input of the
op-amp is connected to ground.  Let's say there is a feedback resistor
going from the output to the inverting (negative) input (R), and an input
resistor going to the inverting input (R1), so Vout/R = -(Vin/R1), and has
a voltage gain of -(R/R1).

To build a summing amp, add more inputs to the inverting input.  In this
case, the inverting input is considered a "virtual ground" because the
op-amp will swing its output through the feedback loop to keep the
inverting and non-inverting inputs at the same voltage, and the
non-inverting input is connected to the real ground.  However, unlike
ground, the inverting input draws very little current (ie. it has a very
high impedance).

As you add inputs the voltages are added together.  The inputs can be
negative or positive voltages.  If all the resistors are the same, then
Vout is simply the sum of the voltages.  If the resistors are different,
then each input voltage is divided by that fraction, such that:
Vout = - (V1R/R1 + V2R/R2 + ...)

You can build the inverting follower with resistors that are exactly the
same or without resistors, such that Vout = -Vin.

The precision will depend on the resistors, the choice of op-amp, and the
power supply running it.