8.2.1.2 Detailed Design Procedure

First, we determine the biasing for the 2.5-V reference. With the 5-V supply voltage, we would use a voltage divider consisting of one resistor from the supply to IN- and an second resistor from IN–. The 25 nA of input current bias should be < 1% of the bias current for Vref. With a 100-kΩ resistor from IN– to V+ and an additional 100-KΩ resistor from IN– to ground, there would be 25 µA of current through the two resistors. The 3-kΩ pullup shown will need 5 V/3 kΩ → 1.67 mA, which exceeds our current budget.

With the 400-µA supply current and 25 µA of VREF bias current, there is 575 µA remaining for output pullup resistor; with 5-V supply, we need a pullup larger than 8.7 kΩ. A 10-kΩ pullup is a value that is commonly available and can be used here.