The LMH5485-SEP combines a core differential I/O, high-gain block with an output common-mode sense that is compared to a reference voltage and then fed back into the main amplifier block to control the average output to that reference. The differential I/O block is a classic, high open-loop gain stage with a dominant pole at approximately 900 Hz. This voltage feedback structure projects a single-pole, unity-gain Aol at 850 MHz (gain bandwidth product). The high-speed differential outputs include an internal averaging resistor network to sense the output common-mode voltage. This voltage is compared by a separate Vcm error amplifier to the voltage on the V_OCM pin. If floated, this reference is at half the total supply voltage across the device using two 100-kΩ resistors. This Vcm error amplifier transmits a correction signal into the main amplifier to force the output average voltage to meet the target voltage on the V_OCM pin. The bandwidth of this error amplifier is approximately the same bandwidth as the main differential I/O amplifier.

The differential outputs are collector outputs to obtain the rail-to-rail output swing. These outputs are relatively high-impedance, open-loop sources; however, closing the loop provides a very low output impedance for load driving. No output current limit or thermal shutdown features are provided in this lower-power device. The differential inputs are PNP inputs to provide a negative-rail input range.

To operate the LMH5485-SEP connect the OUT– pin to the IN+ pin through an Rf, and the OUT+ pin to the IN– pin through the same value of Rf. Bring in the inputs through additional resistors to the IN+ and IN– pins. The differential I/O op amp operates similarly to an inverting op amp structure where the source must drive the input resistor and the gain is the ratio of the feedback to the input resistor.