SBOS710D October 2014 – February 2018 LMH5401
When using the LMH5401 with an AC-coupled differential source, the input can be coupled in through two blocking capacitors. An optional input differential termination resistor (RM) can be included to allow the input RG resistors to be scaled up while still delivering lower differential input impedance to the source. In Figure 62, the RG elements sum to show a 200-Ω differential impedance and the RM element combines in parallel to give a net 100-Ω, AC, differential impedance to the source. Again, the design proceeds by selecting the RF element values, then the RG to set the differential gain, then an RM element (if required) to achieve a target input impedance. Alternatively, the RM element can be eliminated, the RG elements set to the desired input impedance, and RF set to the get the differential gain (= RF / RG). The DC biasing in Figure 62 is simple. The output VOCM is set by the input control voltage and, because there is no DC current path for the output common-mode voltage, that DC bias sets the input pins common-mode operating points.