Since power supply limitation, sometimes we need convert CMOS signal level to meet receiver requirement. Such as 3.3V to 2.5V, 1.8V to 1.2V
Please refer to below interfacing from AN408 for this conversion.
This resistor attenuator network should consider device source impedance Rs (usually this information can be found in datasheet). The R1 and R2 calculation should consider below two items:
1. R2 parallel with R1+ Rs to get 50 ohms resistor for impedance matching.
2. R2 /(R1+Rs) equal to ratio of receiver swing/ source swing
Attachment is one excel tool to calculate R1 and R2 according to source/receiver level and source impedance