10.2.3 Application Curves

The LMH5401-SP has on-chip series output resistors to isolate the output of the amplifier. These resistors provide the LMH5401-SP extra phase margin in most applications. When the amplifier is used to drive a terminated transmission line or a controlled impedance filter, additional external resistance is required to match the transmission line of the filter. In these matched applications, there is a 6-dB loss of gain. When the LMH5401-SP is used to drive loads that are not back-terminated or matched, there is a loss in gain resulting from the on-chip resistors. Figure 76 shows that loss for different load conditions. In most cases the loads are between 50 Ω and 200 Ω, where the on-chip resistor losses are 1.6 dB and 0.42 dB, respectively. As an example, if the LMH5401-SP were to drive an ADC with a differential input impedance of 100 Ω without any matching components the signal loss would be 0.83 dB compared to 6 dB in a matched configuration. Of course, this is only feasible if the LMH5401-SP and the ADC are placed in close proximity (< 1/4 wavelength of the frequency of interest) so as to avoid standing waves from reflections due to the mismatch in impedances). Figure 77 shows the net gain realized by the amplifier for a large range of load resistances when the LMH5401-SP is configured for 16-dB gain.

Figure 76. Gain Loss Resulting from On-Chip Output Resistors

Figure 77. Net Gain versus Load Resistance