Resistor ladder

A resistor string can function as a DAC by having the bits of the binary number control electronic switches connected to each tap.

[4] The binary weighted configuration uses power of two multiples of a base resistor value.

The R–2R network causes these digital bits to be weighted in their contribution to the output voltage Vout.

The actual value of Vref (and the voltage of logic 0) will depend on the type of technology used to generate the digital signals.

[7] For a digital value VAL, of a R–2R DAC with N bits and 0 V/Vref logic levels, the output voltage Vout is: For example, if N = 5 (hence 2N = 32) and Vref = 3.3 V (typical CMOS logic 1 voltage), then Vout will vary between 0 volts (VAL = 0 = 000002) and the maximum (VAL = 31 = 111112): with steps (corresponding to VAL = 1 = 000012) The R–2R ladder is inexpensive and relatively easy to manufacture, since only two resistor values are required (or even one, if R is made by placing a pair of 2R in parallel, or if 2R is made by placing a pair of R in series).

These can be filtered with capacitance at the output node (the consequent reduction in bandwidth may be significant in some applications).

Since the output impedance is independent of digital code, the analog output may equally-well be taken as a current into a virtual ground, a configuration called current mode (or sometimes inverted mode).

Using current mode, the gain of the DAC may be adjusted with a series resistor at the reference voltage terminal.

And within integrated circuits, R–2R networks may be printed directly onto a single substrate using thin-film technology for higher accuracy.

Such on-chip resistor ladders for digital-to-analog converters achieving 16-bit accuracy have been demonstrated.