According to UG-570, page 11 & 33, the RX TIA is a single pole low pass filter with a corner frequency at 2.5x the baseband bandwidth. Furthermore, according to page 33, the Rx TIA LPF corner frequency is programmable with the ad9361_set_rx_rf_bandwidth function call. The function ad9361_set_rx_rf_bandwidth function eventually calls function ad9361_rx_tia_calib, which sets all related registers.
According to ad9361_rx_tia_calib as well as the AD9361 Register Map (UG-671), there are only three different bandwidth settings for REG 0x1DB, i.e., < 3MHz, 3-10MHz, and >10MHz.
In addition, REG 0x1DC, 0x1DD, 0x1DE, and 0x1DF are set according to values in REG 0x1EB, 0x1EC and 0x1E6. However, it is unclear how these registers are set according to the baseband bandwidth.
So, how exactly is the Rx TIA corner frequency set to be 2.5x the baseband bandwidth?
As a comparison, the Rx BB LPF registers 0x1FB and 0x1FC provide a way to directly input the BBBW.