How can I sample a signal without ADC?

Hello,

You can also build a DAC with your FPGA outputs using a few opamps and an use additional opamp as a comparator that compares the output of the improvised DAC with the signal you want to monitor. One digital input of your FPGA checks whether the DAC output value is below or above your signal. You start the measurement cycle with your DAC at Vref/2 then see if you are above for below Vref/2 and then proceed with Vref/4 or 3Vref/4 etc. In fact some ADC work like this internally. On the other hand, you might consider using an LTC1863 that does the job for you. (A good ADC does sample & hold like it is ment to be done, my proposal does not.)

Best Regards,

Johi.