# External Resistor Minimizes Digipot Loading

### Abstract

This article discusses how to improve linearity (due to loading of the wiper) and mid-range frequency response of a digital potentiometer in VCO control applications by adding a single external resistor between H and W.

### Introduction

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.

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 (R_{IN}) tends to make the relationship nonlinear and nonsymmetrical. See the "Without R_{EXT}" curve in Figure 2, for which R_{IN} is 0.3 times the digipot's nominal output resistance.

Connecting a resistor (R_{EXT}) from V_{CONTROL} to V_{DD} reduces the loading effect of R_{IN}. Making R_{EXT} = R_{IN} brings the control curve closer to the ideal, and also makes it symmetrical with regard to the middle position. (See "With R_{EXT}" curve.) As an added bonus for VCO control, adding R_{EXT} = R_{IN} provides finer resolution around the mid-range frequency, where control is usually the most critical.