# avr adc differential mode frequencies

I know that in single ended free running mode the following equation holds:

f_ADC = f_clk/N = 13f_s

where f_ADC is the ADC frequency, f_clk is the avr's clock frequency and f_s is the sampling rate.

The above equation means that if I want to convert an analog signal with a max frequency of 4KHz then I should choose f_s=8Khz (Nyquist theorem), and then choose N such that f_ADC >= 13fs=13*8KHz.

Now, I've read in the ATMega128 datasheet that in case of differential channels gain (eg to convert the difference of two analog signals), an internal f_ADC2 frequency is produced which is f_ADC2 = f_ADC/2 (or is it f_ADC2 = 2*f_ADC ????) but can't really figure out what happens with timings.

So the actual question is: what equation holds (like the one above) if I use differential gain channels at free running mode?