2 AAF design approach

Assuming that you have decided on the correct FDA for your application and whether to use either a low-pass or band-pass filter in order to achieve optimum performance (bandwidth, SNR and SFDR) in front of the ADC, follow these three steps:

1. Understand the amplifier’s characterized load impedance (RL). In order for the amplifier to perform at its best, the amplifier should “see” the correct DC load or RL listed in the data sheet for optimum performance. This is the characterized impedance typically found at the top of the specification tables.
2. Determine a starting point for the correct amount of output series resistance to use closest to the amplifier’s outputs. This helps prevent unwanted peaking in the pass band. You’ll also typically find this information in the FDA’s data sheet - LMH5401 8-GHz, Low-Noise, Low-Power, Fully-Differential Amplifier Data Sheet.
3. Determine whether to use one or more external parallel resistors to back-terminate the input to the ADC, and the starting value of the input series resistance to isolate the ADC from the filter. These series resistors also help reduce unnecessary peaking in the pass band and “kickback” commonly found in unbuffered ADCs.

Figure 1 shows an example of the specification table.

The generalized circuit shown in Figure 2 and filter parameter list in Table 1 apply to most high-speed differential FDA and ADC interfaces; you can use both as a basis for the AAF design.

Although not every filter construction will be exactly the same, Figure 2 can serve as a blueprint on how to kick-start your design. Using this design approach will tend to minimize the insertion loss of the filter by taking advantage of the relatively high input impedance of most high-speed ADCs and the relatively low output impedance of the driving source (the FDA).