In many ways, this method is a much simpler way to operate the FDA from a design equations perspective. Again, assuming that the two sides of the circuit are balanced with equal R_F and R_G elements, the differential input impedance is now just the sum of the two R_G elements to a differential inverting summing junction. In these designs, the input common-mode voltage at the summing junctions does not move with the signal but must be dc biased in the design range for the input pins and must take into account the voltage headroom required to each supply. Slightly different considerations apply to ac- or dc-coupled differential input to differential output designs, as described in the following sections.