Why LTC6900CS5 is needed in the circuit EVAL-ADM3053?
The EVAL-ADM3053EBZ evaluation board demonstrates the pcb layout techniques and external components used to meet EN55022 Class B emissions. The LTC6900 low power oscillator is used to provide stimulus to the ADM3053 isolated CAN transciever's TXD input pin for emissions testing.
The EVAL-ADM3053EBZ works fine for testing functionality as well. You will likely want to provide your own input signal to TXD in that case, so remove the LTC6900 entirely, or lift the LTC6900 oscillator's pin 5 so its output is not in contention with with whatever device is driving the other TXD input signal.
The User Guide UG-234 can be found here: