Digital potentiometer ICs (digital pots, or digipots) are convenient for controlling voltage-controlled devices such as VCOs and as an economical alternative for a DAC. In a typical application (Figure 1
), a MAX5160
digipot controls the frequency of a VCO, via a 3-wire interface.
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
reduces the loading effect of RIN
. Making REXT
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
provides finer resolution around the mid-range frequency, where control is usually the most critical.