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Adding a Fixed Constant

Many equations require the inclusion of a fixed constant, which has no connection with any of the variables. For example, the general equation for a straight line is Y = mX + b, where m is the slope of the line and b is an offset, which defines the point at which this particular line will cross the Y axis on a graph. The next question to be answered, then, is how we can include such a fixed constant in our analog computer circuits.

A potentiometer is used to set a constant.

To insert a constant into the equation, we need to introduce a fixed voltage. This is most easily done with a simple potentiometer, as shown to the left. The potentiometer is set to an appropriate voltage, and then connected to its own input resistor to the op amp. Thus, this input represents an "unvarying variable," which is a constant.

In an analog computer, a number of steps are taken to help ensure both accuracy and precision:

If the required constant is in the range of ±10, it can be applied directly, with the gain of that input set to 1. If the constant is larger, it is scaled down and then amplified. In some cases, the constant might be very small. In that case, it can be scaled up and its input set to a fractional gain. This allows accurate setting of the potentiometer.

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