How much dropout voltage (Vsys - Vout) must I allow for if I the load current greater than the limits in the datasheet electrical table? Looking at a low-voltage op amp like the AD8538 at Vsys = 2.7V, the datasheet lists a Voh with a 10k load, but what if my load is less than 10k?
Op amp dropout Voltage (or max output voltage as a function of load) are traditionally specified as Voh for loads of 100 kOhms and 10 kOhms. The AD8538 is specifed at 10k load and 2.7V supply as 2.67V at room temperature; in otherwords, there must be at least 30 mV of difference between the supply and output voltage. If you want to increase the load source current, refer to the typical plot in the datasheet "Output Saturation Voltage vs. Load Current" (Figure 40). With Vsource = +/- 1.35V, at room temperature, the output will drop 100mV at approximately 0.7mA load.
In many low-voltage applications, every mV counts. In that case, you may want to consider an op amp with rail-to-rail output. A newer generation to the AD8538 is the ADA4051.