The LMH5485-SEP provides a simple interface to a wide variety of precision SAR, ΔΣ, or higher-speed pipeline ADCs. To deliver the exceptional distortion at the output pins, considerably wider bandwidth than typically required in the signal path to the ADC inputs is provided by the LMH5485-SEP. For instance, the gain of 2 single-ended to differential design example provides approximately a 500 MHz, small-signal bandwidth. Even if the source signal is Nyquist bandlimited, this broad bandwidth can possibly integrate enough LMH5485-SEP noise to degrade the SNR through the ADC if the broadband noise is not bandlimited between the amplifier and ADC. Figure 10-4 shows an example DC-coupled, gain of 2 interface with a controlled, interstage-bandwidth filter.

Designed for a DC-coupled 50 Ω input match, this design starts with a 499 Ω feedback resistor, and provides a gain of 2.35 V/V to the LMH5485-SEP output pins. The third-order interstage, low-pass filter provides a 20 MHz Bessel response with a 0.85 V/V insertion loss to the ADC, providing a net gain of 2 V/V from board edge to the ADC inputs. Although the LMH5485-SEP can absorb overdrives, an external protection element is added using the BAV99 low-capacitance device, shown in Figure 10-4. For DC-coupled testing, pins 1 and 2 of JP1 are jumpered together. When the source is an AC-coupled, 50 Ω source, pins 2 and 3 of JP1 are jumpered to maintain differential balance.