resistorstring

Resistor string refers to a series chain of resistors connected between a reference voltage and ground. The junctions between resistors provide a set of stable, proportionally spaced voltages, which can be tapped to yield multiple voltage levels without active components.

Construction and operation: If N equal resistors R are connected in series between Vref and ground, the voltage at the i-th tap (counting from the top) is i/N of Vref, assuming negligible loading. Nonuniform resistor values can produce non-linear tap voltages. The taps are typically connected to switches (such as MOSFETs or transmission gates) whose outputs are combined by a buffer amplifier or fed to another circuit. Selecting a tap produces a voltage proportional to a digital input, an arrangement commonly used in resistor-string digital-to-analog converters (DACs).

Applications: The resistor string provides a simple, low-cost method to generate multiple reference levels in DACs, programmable voltage references, test equipment, and calibration networks. It is often preferred where monotonicity and robustness are important, and where high-precision components are available.

Design considerations: Accuracy depends on resistor matching and temperature coefficients; loading from subsequent stages can distort the voltages at the taps. The output impedance of the string is relatively high, typically requiring buffering. Tolerances introduce differential and integral nonlinearity in DAC use. Resistor-string implementations are well suited for relatively low-speed applications with modest resolution, where simplicity and cost are advantageous.