Data Acquisition Basics—Using Frequency to Determine the Correct Sample Rate

The characteristics of a particular body signal are dictated by its amplitude and frequency. The frequency of a signal can help determine the ideal rate at which to sample a particular signal, thus ensuring the resulting waveform is of the highest quality. Basically, the higher the frequency of a signal, the faster the sample rate that should be used. As an analogy – if we liken the various signals to a freeway and assume all cars are traveling in the same direction, but at different speeds, we can then determine the signal’s frequency band by looking at the slowest cars and the fastest cars. For comparison, a “slow car” would be an electrogastrogram (EGG) signal (0-1 Hz) and a “fast car” would be an electromyogram (EMG) signal (10-2000 Hz). In order to acquire the information correctly, we need to ensure the sample rate is set high enough to sweep up in our dragnet the fastest cars along with the slower cars. In the case of the EMG signal, if the sample rate is set to only 100 Hz, many of the faster “cars” would not be captured, and the definition of the upper frequency ranges would be lost in the resulting waveform. Conversely, 100 Hz would be more than sufficient to capture all of the “cars” on the EGG signal freeway. (Other frequency range examples for particular signals are: ECG = .05-100 Hz, EEG = 1-100 Hz, EOG 01.-100 Hz.) The good news is that BIOPAC sets the acquisition sample rate to a default of 2000 Hz in our  MP160 or MP36 systems for AcqKnowledge or Biopac Student Lab software, more than sufficient for most applications. That said, it’s good practice to determine the maximum frequency response of a particular signal and multiply that maximum by four times to arrive at the sample rate that will capture all the cars moving down the freeway.

BIOPAC offers a wide array of wired and wireless equipment that can be used in your research. To find more information on solutions for recording and analyzing signals such as ECG, heart rate, respiration and more using any platforms mentioned in this blog post, you can visit the individual application pages on the BIOPAC website.