The anti-aliasing filters deploy AD8622 OpAmp buffers at the input of each signal line. This is before any filtering. So, these OpAmps need to be able to handle the full signal bandwidth. Unfortunately, the slew rate limit of the AD8622 is only 0.28V/µs. This is an order of magnitude lower than the ubiquitous OP27. In other words, the largest signal amplitudes at 1MHz, 100kHz and 10kHz are 45mV, 450mV and 4.5V, respectively. The readbacks of the EOM driver have a signal BW of about 300kHz and a signal amplitude of ~0.5V. This was too much and produced excess in-band noise—more than a magnitude above the digitization noise. We swapped the OpAmps in this path with AD8672 which have a slew rate of 4V/µs.

Plot 1. Green trace is readback with AD8672, amber curve is AD8622, and red trace as inication of digitization noise level.