ESP8266: system_adc_read_fast() always give 1024 as output
I created a test circuit using an AC transformer to produce a 50 cycles / second sine wave signal and tried a number of variations on the system_adc_read_fast sample program in the Espressif SDK manual "2C-ESP8266_Non_OS_SDK_API_Reference__EN". 

The ESP8266 ADC returned realistic values for adc_clk_div = 8 and 16, but, as you found, the values returned were always 1024 when I tried 32. 

I found that the ADC unit was capable of performing ADC conversions in about 6-10 microseconds... 

In a test program to collect data for a 40 millisecond period (ie two complete sine waves - I used a diode to chop off the negative halves) I called the system_adc_read_fast function 40 times collecting 116 samples in each call (using "number of samples" = 116). 

I collected all the readings in a two-dimensional array before printing any values because the Serial.print function causes long delays between calls to the ADC converter. The "gaps" in the chart represent the changes in the sine wave signal between successive calls to the ADC read function.