External Resistor Minimizes Digipot Loading
Figure 1. Adding a resistor (REXT) to this VCO-control circuit minimizes nonlinearity while enhancing the mid-range frequency resolution.
The ideal relationship between wiper position and control voltage is a straight line (Figure 2). If the digipot's output resistance is high, however, the loading effect of the VCO input resistance (RIN) tends to make the relationship nonlinear and nonsymmetrical. See the "Without REXT" curve in Figure 2, for which RIN is 0.3 times the digipot's nominal output resistance.
Figure 2. These curves for wiper position vs. normalized control voltage in Figure 1 show the effect of adding a simple resistor (RIN) to the circuit.
Connecting a resistor (REXT) from VCONTROL to VDD reduces the loading effect of RIN. Making REXT = RIN brings the control curve closer to the ideal, and also makes it symmetrical with regard to the middle position. (See "With REXT" curve.) As an added bonus for VCO control, adding REXT = RIN provides finer resolution around the mid-range frequency, where control is usually the most critical.