SBOS778D April 2016 – April 2021 THS4551
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 VICM down by adjusting the VOCM 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 RG element biased to whatever the source midrange is expected to be, provided that this midscale reference gives a balanced differential swing around VOCM at the outputs. Often, RG2 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 RG2 gives a unipolar output differential swing from both outputs at VOCM (when the input is at ground) to one polarity of the swing. Biasing RG2 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 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 VIN 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 THS4551 is in range for applications using a single positive supply and a positive output VOCM 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).