2 Gain error

As shown in Figure 1, a typical DA comprises an op amp and four resistors. The midpoint of the R1/R2 resistor ladder connects to the inverting terminal of the op amp, while the R3/R4 midpoint connects to the noninverting terminal.

Equation 1 depicts the transfer function of a typical DA. Notice that the ratio of R2 and R1 (assuming that R1 = R3 and R2 = R4) determines the gain. You will need matched resistors if you are trying to achieve a low gain error. Resistors with a tolerance of ±1% can induce a gain error as high as 2%. While discrete resistors may exhibit large variations, the monolithic resistors found in integrated DAs are often trimmed to achieve a gain error of just 0.01%.