The output considerations remain the same as for the ac-coupled design. Again, the input can be dc-coupled while the output is ac-coupled. A dc-coupled input with an ac-coupled output can have some advantages to move the input Vicm down if the source is ground referenced. Figure 7-2 shows how when the source is dc-coupled into the LMH5485-SP, both sides of the input circuit must be dc coupled to retain differential balance. Normally, the nonsignal input side has an Rg element biased to whatever the source midrange is expected to be. Providing this midscale reference gives a balanced differential swing around Vocm at the outputs.

One significant consideration for a dc-coupled input is that Vocm sets up a common-mode bias current from the output back through Rf and Rg to the source on both sides of the feedback. Without input balancing networks, the source must sink or source this dc current. After the input signal range and biasing on the other Rg element is set, check that the voltage divider from Vocm to V_IN through Rf and Rg (and possibly Rs) establishes an input Vicm at the device input pins that is in range. If the average source is at ground, the negative rail input stage for the LMH5485-SP is in range for applications using a single positive supply and a positive output Vocm setting because this dc current lifts the average FDA input summing junctions up off of ground to a positive voltage (the average of the V+ and V– input pin voltages on the FDA).