[quote user="Radoslaw Michalak"]a) In doc. there is sth like min Tsclk = 50ns so Fscl = 20Mhz ( is it minimum frequency for SPI communication between ads1298 and any other device ?[/quote]
The SCLK speed is spec'd at a minimum SCLK period of 50ns. That means the maximum frequency is 20MHz.
[quote user="Radoslaw Michalak"]b) Tclk - master clock signal - I am not sure what is it . Is it internal clock in master device like atmega8 ?[/quote]
tclk is the master clock period. The master clock is either the internal master clock or the external clock signal on the CLK pin.