There are three main methods for triggering an A/D conversion:
- Software trigger – The application program executes a command to initiate the A/D conversion. This is used for “occasional” sampling, where the A/D conversions are done on demand by the application at low speeds, for example measuring a backup battery voltage once an hour.
- External signal – An external signal is used to trigger the A/D conversion. This allows the A/D to be synchronized to some external device or event. For example, a digital input from a rotary encoder initiates the A/D conversion every time a wheel rotates to the 12 o'clock position.
- Clock trigger – An on-board programmable counter initiates A/D conversions at precise intervals. This is used for precise timing for high speed sampling or waveform analysis.
NOTE: Many A/D boards quote high sample rates but do not inclulde an on-board counter/timer or an external clock input for sample rate control. These boards require software commands to trigger the A/D conversions. Because it is impossible to control the timing of the A/D conversions precisely when using software triggers, you cannot obtain an accurate representation of a waveform with these boards. If you are planning to perform A/D sampling at high rates and/or you need to digitize a waveform, you will require either an on-board counter/timer or an external clock source and a means of connecting it to the board.