Transfer Functions: Converting digital sensor data into physical units

Sensor data acquired with BITalino and biosignalsplux devices are represented in the digitized sensor values ranging between 0 and 2 ^{(sampling resolution – 1)} due to the analog to digital conversion of the acquired sensor signal.

Using OpenSignals, you can export sensor data in their digital value format or convert the signals into their original physical units.

Signals recorded using the PLUX APIs record only the digital sensor values and need to be converted back into their original physical units by applying the sensor’s transfer function, which can be found in your sensor’s user manual or datasheet:

• BITalino Sensor Documentation
• biosignalsplux Sensor Documentation

You can find below an example of how to apply the transfer function.

Example: Converting biosignalsplux ECG Signals

From the ECG’s user manual, the sensor’s transfer function is as follows:

We can define the parameters of the transfer function as follows:

# ADC: Digital values of the ECG signal
ecg_digital = [...]

# n: Sampling resolution of the signal, by default 16-bit
sampling_resolution = 16

# VCC: the sensor's operating voltage of 3V
vcc = 3

# Gecg: the sensor's gain
gain = 1019

Using these parameters, the resulting transfer function to convert the ECG signal back into mV would be as follows:

# Convert ECG signal into mV
ecg = (ADC/2**(sampling_resolution-1) - 0.5) * vcc * gain