Resistive ladder, or R-2R DACs, use a three terminal structure as shown above, with resistors switched between terminals A and B and a third terminal C having a fixed impedance. The resistor ratios shown implement a binary ladder with each leg corresponding to a single bit of the DAC. Note that the impedances at terminals A and B are highly code dependent. The matching of both the resistors and switches will affect the monotonicity and accuracy of the structure. Such DACs are normally trimmed or factory calibrated, and monolithic 16-bit resistive ladder DACs with ±1 LSB INL and DNL have long been commonly available. So-called "current-output" R-2R DACs are really "impedance-output"; they use an R-2R network to vary the impedance between the voltage reference (V_REF) and the output. As long as the output is held at ground, this causes a proportional current to flow and then the on-chip feedback resistor is used to convert this to a voltage with the use of an external amplifier.