Those familiar with Digital Signal Processing (DSP) know that any time you sample a signal you will alias frequencies higher than the sample frequency back into the target bandwidth. To help mitigate this as well as to reduce high frequency noise on the board (e.g. FPGA switching at 100 MHz) we implemented a 3 pole anti-aliasing filter using the LT1567.
The LT1567 is essentially two high precision op amps that are specially configured for filter designs. Their Gain Bandwidth Product (GBWP) of 5 MHz made them an acceptable choice since, when configured for unity gain, they can provide a 5 MHz filter. After using the design tool to set the corners for the three filters at 4 MHz, we looked at the resulting Bode plot and determined that the filter attenuation would not drastically affect the target bandwidth. The result of the filter design tool is shown below.
It is worth noting that the LT1567 takes in a single ended input and then converts it to a differentially signaled output. This was one of main reasons for placing the filter out front. However, because the VGA also takes in a single ended input, we terminated one of the outputs by matching the input impedance of the VGA using passive components. The VGA had the same problem so swapping the two components greatly complicated things. The final deciding factor was that the filter has significantly better overvoltage protection than the VGA, making it much more suitable to handle the incoming signal.