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External LNA Control Latency

Question asked by MikeSc on Oct 22, 2013
Latest reply on Oct 23, 2013 by MikeSc


I am using the AD9361 evaluation board (FMCOMMS2) and a ZC706 development board to test the speed at which a GPO output can switch when a large signal is present.  This GPO signal will be used to enable an external attenuator to prevent signals greater than 2.5 dBm from appearing at the input port of the AD9361.  The response of the AD9361 must be quick in order to  prevent an RF burst from exceeding the 2.5 dBm limit.


I have configured the device for fast attack AGC mode with a full table.  I modified the two highest gain settings (address 0x4c and 0x4b) to set GPO0 to a 1 - all other gains set it to a 0.  Therefore, a single index step of 2 or greater will cause GPO0 to de-assert.  The AGC configuration is set as described in the Gain Control and RSSI User Guide v2.9 example #3.  The receive frequency is set to 915 Mhz and I am injecting a tone at 914 Mhz, -20 dBm.


I am observing a latency of around 100 nanoseconds.  I have a few questions -


Is this the quickest response possible with the default sampling rate of 30.72 Mhz?  What is the limiting factor? - the settling time of the LMT detector is only 7 nanoseconds.  The LMT Detector Settling Time (0x103) has no effect on this measurement - any idea why this would have no effect?  Also the LMT End Overload Counter values (0x121) and Immed Gain Change (0x123[D7]) have no effect on the latency. 




If the sampling rate is increased I do notice a proportional decrease in latency.  What is the maximum ADC sampling rate the part can handle?