SBAS438C May 2008 – November 2019 DAC9881
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 VOUT and RFB 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 AVDD 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 (RW1) 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.
the wiring resistance of RW2 is not critical as long as the feedback resistor (RFB) is connected at the driven load.