Timing for Frequency Meter

Projects General Guidance
1

I am using the MEGA2560 to measure a 4V pk-pk sine wave (2.5V DC offset) and a smaller DC sine wave from a shunt. I use the analogRead function for both and then post process the array to determine the frequency. My problem is time. I timed the analog read by adding a digitalWrite toggle at the beginning and ending of the reads and found the readings took between .07-.1ms. The variation kills my accuracy as my sample rate is significant compared to that number. I am an amateur programmer. Is this a problem I can solve using the timer or do I need to strap on my big boy pants and figure out how to use interrupts?

//Record Data
  for(i = 0; i < arraysize; i++)
  {
    VoltageArray[i] = analogRead(9);
    CurrentArray[i] = analogRead(10);
    TimeArray[i] = time;
    time = time + 0.00024; // This is amount of time for the delay and approximate time of both analog reads
    delay(.1);
  }
2

You can do analog reads asynchronously (see above page). They still take around 104 uS, but you can be doing something else in the meantime.

If you want faster sampling than that overall, you will need to get a dedicated ADC chip. I think I was using one recently that lets you sample at SPI access rates, that is around once every few uS.

3

Could you provide more details on what you trying to achieve? Why not measure freq. with digital input? What is the nature of signal, is it single tone, noisy etc,