Calibration#
Data recorded with shepherd is stored in the database as raw binary values, read from the ADC. In order to make sense of that data, we need to map the raw values to physical voltage and current values. Similarly, for emulation, we need to map physical voltage and current values to binary values that can be fed to the DAC.
This mapping is assumed to be linear.
For example, to convert a raw value :math:v_{raw} to its physical equivalent :math:v_{physical}:
There are two “channels” and two “variables”: harvesting and emulation, each with voltage and current. When storing recorded data, these values are stored together with the raw data.
The gain and offset values can be derived from nominal hardware configuration. However, due to component tolerances, offset voltages and other effects, the real mapping can significantly differ from nominal values.
Therefore, we recommend to calibrate shepherd by comparing a number of sample points recorded with shepherd to known reference values measured with accurate lab equipment. From the tuples of reference values and measurements, you can estimate the gain and offset by ordinary least squares linear regression. These values can be stored in the EEPROM on the shepherd cape and, at the beginning of a recording extracted and stored in the database file.