SBOS695A August 2014 – December 2014 LMH3401
The LMH3401 can be used with either split or single-ended power supplies. The ideal supply voltage is a 5.0-V total supply, split around the desired common-mode of the output signal swing. For example, if the LMH3401 is used to drive an ADC with a 1.0-V input common mode, then the ideal supply voltages are 3.5 V and –1.5 V. The GND pin can then be connected to the system ground and the PD pin is ground referenced.
Using a 5-V power supply gives the best balance of performance and power dissipation. If power dissipation is a critical design criteria a power supply as low as 3.3 V (±1.65) can be used. When using a lower power supply, the input common-mode and output swing capabilities are drastically reduced. Make sure to study the common-mode voltages required before deciding on a lower-voltage power supply. In most cases the extra performance achieved with 5-V supplies is worth the power.
Single-supply voltages from 3.3 V to 5 V are supported. When using a single supply check both the input and output common-mode voltages that are required by the system.
In general, split supplies allow the most flexibility in system design. To operate as split supply, apply the positive supply voltage to VS+, the negative supply voltage to VS–, and the ground reference to GND. Note that supply voltages do not need to be symmetrical. Provided the total supply voltage is between 3.3 V and 5.25 V, any combination of positive and negative supply voltages is acceptable. This feature is often used when the output common-mode voltage must be set to a particular value. For best performance, the power-supply voltages are symmetrical around the desired output common-mode voltage. The input common-mode voltage range is much more flexible than the output.
Power-supply decoupling is critical to high-frequency performance. Onboard bypass capacitors are used on the LMH3401EVM; however, the most important component of the supply bypassing is provided by the PCB. As illustrated in Figure 71, there are multiple vias connecting the LMH3401 power planes to the power-supply traces. These vias connect the internal power planes to the LMH3401. Both VS+ and VS– must be connected to the internal power planes with several square centimeters of continuous plane in the immediate vicinity of the amplifier. The capacitance between these power planes provides the bulk of the high-frequency bypassing for the LMH3401.