The most common way to use the THS4551 with an ac-coupled differential source is to simply couple the input into the R_G resistors through the blocking capacitors. Figure 9-10 shows a typical blocking capacitor approach to a differential input. An optional input differential termination resistor (R_M) is included in this design. The R_M element allows the input R_G resistors to be scaled up and still delivers lower differential input impedance to the source. In this example, the R_G elements sum to show a 1-kΩ differential impedance and the R_M element combines in parallel to provide a net 500-Ω ac differential impedance to the source. Again, the design ideally proceeds by selecting the R_F element values, then the R_G to set the differential gain, and then an R_M element (if needed) to achieve a target input impedance. Alternatively, the R_M element can be eliminated, with the 2 × R_G elements set to the desired input impedance and R_F set to obtain the differential gain (equal to R_F / R_G).

Figure 9-10 Example AC-Coupled Differential Input Design

The dc biasing for an ac-coupled differential input design is very simple. The output V_OCM is set by the input control voltage and, because there is no dc current path for the output common-mode voltage (as long as R_M is only differential and not split and connected to ground for instance), the dc bias also sets the common-mode operating points for the input pins. For a purely differential input, the voltages on the input pins remain fixed at the output V_OCM setting and do not move with the input signal (unlike the single-ended input configurations where the input pin common-mode voltages do move with the input signal). The SLOC341 TINA-TI™ simulation file is available for Figure 9-10.