4 Software Filtering

Filtering in software is also used for noise reduction. Software filtering acts as a low-pass filter and is implemented simply using the formula in Equation 1.

With software filtering, the raw data is smoothed according to a smoothing factor, α. An α of 1 indicates no filtering, and an α of 0.99 allows most noise to be passed through, whereas an α of 0.01 eliminates a lot of noise. Figure 4-1 shows the impact of different levels of software filtering on noise reduction.

Using the following pseudocode, the α value of the filter can be easily adjusted to meet system requirements.

int sensorPin = ; // EDIT indicate pin # for analog input to read VTEMP from TMP6
float Vbias = ; // EDIT indicate Vref voltage for ADC
int ADC_bits = ; // EDIT number of bits of resolution for ADC
int ADC_resolution = pow(2, ADC_bits) - 1; // number of ADC steps

int rawADC; // variable for measured ADC value
float VTEMP; // variable for measured ADC voltage
float alpha = ; // EDIT 0.001 < alpha < 1, adjust as required. Smaller alpha means stronger filter
float TMP6raw; // variable for raw TMP6 temperature reading
float TMP6filtered; // variable for filtered temperature reading

/* EDIT 4th order polynomial coefficients from Thermistor Design Tool */
float THRM_A0 = ;
float THRM_A1 = ;
float THRM_A2 = ;
float THRM_A3 = ;
float THRM_A4 = ;

/* place in SETUP or INITIALIZATION code */
// this code initializes the filtered temperature to the initial raw temperature value
rawADC = analogRead(sensorPin);
VTEMP = Vbias / ADC_resolution * rawADC;
TMP6filtered = (THRM_A4 * powf(VTEMP, 4)) + (THRM_A3 * powf(VTEMP, 3)) + (THRM_A2 * powf(VTEMP, 2)) + (THRM_A1 * powf(VTEMP, 1)) + THRM_A0;

/* MAIN code */
rawADC = analogRead(sensorPin); // read ADC value
VTEMP = Vbias / ADC_resolution * rawADC; // convert to voltage
// convert voltage to temperature
TMP6raw = (THRM_A4 * powf(VTEMP, 4)) + (THRM_A3 * powf(VTEMP, 3)) + (THRM_A2 * powf(VTEMP, 2)) + (THRM_A1 * powf(VTEMP, 1)) + THRM_A0;
// calculate filtered temperature using previous filtered temp and raw temp
TMP6filtered = TMP6filtered - (alpha * (TMP6filtered - TMP6raw));