7.3.1 Analog Output

The DAC9881 offers a force and sense output configuration for the high open-loop gain output amplifier. This feature allows the loop around the output amplifier to be closed at the load (as shown in Figure 66), thus ensuring an accurate output voltage. The output buffer V_OUT and R_FB pins are provided so that the output op amp buffer feedback can be connected at the load. Without a driven load, the DAC9881 output typically swings to within 15 mV of the AGND and AV_DD supply rails. Because of the high accuracy of these DACs, system design problems such as grounding and wiring resistance become very important. A 18-bit converter with a 5-V full-scale range has an LSB value of 19 µV. The DAC9881 has a typical feedback resistor current of 0.5 mA; thus, a series wiring resistance of only 100 mΩ (R_W1) causes a voltage drop of 50 µV. In terms of a system layout, the resistivity of a typical 1-ounce copper-clad printed circuit board (PCB) is 0.5-mΩ per square. For a 0.5-mA current, a 0.25-mm wide printed-circuit conductor 25-mm long results in a voltage drop of 25 µV.

Figure 66. Analog Output Closed-Loop Configuration
(R_W1 and R_W2 Represent Wiring Resistance)