### Q

There are a lot of equations for input range on the data sheet.Is there a simple way to guarantee that my AD8476 will be in its linear

operating range for a given input?

### A

There are three separate limits to the linear range of the AD8476. The inputsof 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.