Why the focus on the frequency, if that was all that needed most cards require or can withstand a 5V transition, the fact he trying to produce a voltage appox 1/2 of 5V would suggest to most people that have built ADC
running from a 5V supply that the amplitude of the signal was of concern. I know PDAs run on all ranges of voltage, mine wants 3.6V. If the amplitude is wanted then the ADC input has to deal with 650V P-P not including spikes from switches motors etc.