ADC startup time in AT91SAM7 - what is it?
Looking at the datasheet, this is the time required for the ADC to properly restart from idle mode (there is also an ADC sleep mode which is confusing). Idle mode relates to the processor core, but the startup time appears to also be required in ADC sleep mode (see section 36.5.6 of the datasheet)


It is programmable as the number of clock cycles will vary based on the clock used in the ADC module.

Calculation:

STARTUP: Start Up Time

Startup Time = (STARTUP+1) * 8 / ADCClock 

Therefore you should program the register with

STARTUP = (Startup time * ADCCLK / 8) -1 where Startup time is 20 microseconds.

This particular ADC has a fixed minimum time required for (apparently) the analog cell to come back up.