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 combining 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 complex 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 combines 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
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