AN629 Datasheet | Maxim Integrated

DIGITAL POTENTIOMETERS Jul 01, 2001 Digitally Adjustable Voltage References A digitally adjustable voltage reference is useful in many applications. For instance, it can be used to finely tune the reference voltage when absolute accuracy is required or to adjust the reference voltage to match the full-scale voltage range of an analog-input signal to maximize the dynamic range of an analog-to-digital conversion. You can easily create one by bining an adjustable voltage reference with a digital potentiometer (pot). Using the digitally variable aspect of the digital pot to control the adjustment voltage on the reference results in a flexible and useful circuit. Three basic circuit topologies are presented here. The first achieves a coarse adjustment over a wide voltage range, the second a fine adjustment over a narrow voltage range, and the third a fine adjustment over a wide voltage range. These simple circuits can be used directly or can form the basis for more plex control systems. The output voltage of an adjustable voltage reference, like the MAX6160, is set by the ratio of two resistors, R1 and R2, in Figure 1. The relationship between the voltage and the resistors is given by the equation next to the circuit in the figure. The equations for calculating the values for R1 and R2 are also shown. Figure 1. The MAX6160 adjustable output circuit with fixed precision resistors Coarse Adjustment over a Wide Voltage Range Figure 2 bines the MAX6160 with the MAX5462 (a 100-kilohm, 32-tap digital pot). This circuit is capable of digitally adjusting the output over the range from 1.23V to 5.3V. The ADJ pin voltage limits the minimum output voltage, and the supply voltage for the MAX5462 and the MAX6160 dropout voltage limit the maximum output voltage. This circuit is useful when the voltage reference needs to be programmed in the field to different levels and absolute accuracy is not critical but low drift is required. Figure 2. The MAX6160 digitally...