Is there a simple way to guarantee that my AD8476 will be in its linear
operating range for a given input?
There are three separate limits to the linear range of the AD8476. The inputs
of the internal amplifier and the AD8476 outputs must stay within the supply
rails. Together, these set the limit on the input common-mode and
differential-mode voltages. The final limit to consider is the overvoltage
range, which is 18V beyond the supply rails. The AD8476 datasheet shows in
figure 53 on page 18 of the datasheet (Rev. B) the equation to determine the
voltage at the inputs of the internal amplifier.
It also says that the inputs are rail-to-rail, but for minimal distortion, the
internal amplifier inputs should be kept between –Vs and +Vs – 1V. Using the
fact that RF = RG for AD8476, and also that VIN,cm is defined as ½ (VP + VN),
the above equation can be simplified to the following:
Vinputs= ½ VOCM + ½ Vin,cm
For best linearity, this must be between –Vs and +VS – 1. For example, with a
single +5V supply, VOCM is naturally at 2.5V. Using the equation and limits
above, the common-mode input can be from -2.5V to +5.5V. The differential mode
input voltage, VIN,dm, is limited to ±4.95 V because of the output swing. And
finally, the inputs have an absolute limit of 18V beyond each rail, which is
-18V to +23V in the example.