The output considerations remain the same as for the ac-coupled design. Again, the input can be dc-coupled when the output is ac coupled. A dc-coupled input with an ac-coupled output can have some advantages to move the input V_ICM down by adjusting the V_OCM down if the source is ground referenced. When the source is dc-coupled into the THS4551 (see Figure 9-4), both sides of the input circuit must be dc-coupled to retain differential balance. Normally, the non-signal input side has an R_G element biased to whatever the source midrange is expected to be, provided that this midscale reference gives a balanced differential swing around V_OCM at the outputs. Often, R_G2 is simply grounded for dc-coupled, bipolar-input applications. This configuration provides a balanced differential output if the source swings around ground. If the source swings from ground to some positive voltage, grounding R_G2 gives a unipolar output differential swing from both outputs at VOCM (when the input is at ground) to one polarity of the swing. Biasing R_G2 to an expected midpoint for the input signal creates a differential output swing around VOCM.

One significant consideration for a dc-coupled input is that VOCM sets up a common-mode bias current from the output back through R_F and R_G 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 R_G element is set, check that the voltage divider from V_OCM to V_IN through R_F and R_G (and possibly R_S) establishes an input V_ICM at the device input pins that is in range. If the average source is at ground, the negative rail input stage for the THS4551 is in range for applications using a single positive supply and a positive output V_OCM setting because this dc common-mode 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). TINA-TI™ simulations of the intended circuit offer a good check for input and output pin voltage swings (see Figure 9-7).