For my design, I am using an external reference for the ADC (AVDD). Since the external reference doesn't need to be warmed up (unlike the internal references), can the warm-up time be reduced from 5uS to something less - maybe 1.25uS ? How much time does the ADC take to warm up -vs- internal bandgap references ?
Using an ADC_CLK freq of 16MHZ, if the TIMEBASE field of ADC_CTRL were set to "3", then the warm-up would be 5 x (3+1) or 20 ADC clock cycles (1.25uS).
The goal here is to keep the ADC in NORMAL mode all the time (EM0 and EM2). When the timer interrupt occurs, I want to be able to take the first sample as quickly as possible in the ISR without having to wait the 5uS warm-up time before the first sample is taken. Also, I want to keep power as low as possible in EM2 mode.
Are there any other tricks here ? I know that I could keep the ADC "WARM", but there is no specification for what the non-converting idle current is for the ADC in KEEPWARM mode. If it were only a few micro-amps, then it might be worth leaving the ADC warmed up. It would also be nice if the internal references could be turned off if they are not being used.
If you do not need to wait for the external reference to warm up, the time will be the same as if you are using the already warmed up internal reference. The ADC does this in the KEEPINSTANDBY mode, and according to the reference manual requires a 1us warmup period for the ADC.