Butterworth Filter 2 CH

tom321 · November 19, 2025, 3:08am

Hi. In this program, when Uxf or Uyf are displayed separately on the serial plotter, they are displayed correctly, but when they are displayed simultaneously, the filtered Uxf and Uyf values are the same, where not filtered Ux Uy are different. What is wrong with the code?

 Ux =  -1145.39  Uxf =  -782.38 Uy =  -409.36  Uyf =  -782.36
 Ux =  -1145.32  Uxf =  -782.36 Uy =  -409.38  Uyf =  -782.36
 Ux =  -1145.46  Uxf =  -782.37 Uy =  -409.43  Uyf =  -782.37
 Ux =  -1145.47  Uxf =  -782.38 Uy =  -409.44  Uyf =  -782.38
 Ux =  -1145.34  Uxf =  -782.39 Uy =  -409.43  Uyf =  -782.40

/*
  //PINS generator 3,12, 13, 15,21,
  4 Tx
  0 comp
  32 0 deg
  33 90 deg

  21 Tx dedtime
  //PINS analog 32,33,35, 36, 39,
  32 detector x
  33 amp x
  35 detector y
  34 amp y
  36 mA
  39 bat
  ADS1220
  23 mosi
  19 miso
  27 csx
  33 csy
*/
#include <Filters.h>
#include <AH/Timing/MillisMicrosTimer.hpp>
#include <Filters/Butterworth.hpp>

#include <ADS1220_WE.h>
#include <Adafruit_GFX.h>
#include <Adafruit_ST7789.h>
#include <SPI.h>
#include <driver/ledc.h>
#include <Preferences.h>
Preferences preferences;

#define TFT_MOSI 23  // Data out
#define TFT_SCLK 18  // Clock out
#define TFT_CS   22// 
#define TFT_DC    21 //
#define TFT_RST   -1  // pin# 2

Adafruit_ST7789 tft = Adafruit_ST7789(TFT_CS, TFT_DC, TFT_RST);
//================================ ADS  =============================
// Pin definitions
#define SPI_MISO_PIN 19
#define ADS1220_1_CS_PIN    25   // Chip Select
#define ADS1220_1_DRDY_PIN  34   // Data Ready

#define ADS1220_2_CS_PIN    26   // Chip Select
#define ADS1220_2_DRDY_PIN  35   // Data Ready

#define SPI_INITIALIZED true
#define SPI_NOT_INITIALIZED false
////////

/* Create ADS1220 objects */
ADS1220_WE ads1 = ADS1220_WE(ADS1220_1_CS_PIN, ADS1220_1_DRDY_PIN);
ADS1220_WE ads2 = ADS1220_WE(ADS1220_2_CS_PIN, ADS1220_2_DRDY_PIN);


//===============================  end ADS  ======================
void setup() {
  Serial.begin(115200);
  //===========================  ADS  =============================
  while (!Serial);

  /* Choose SPI clock speed here. */
  // ads1.setSPIClockSpeed(4000000); // set SPI clock speed, default is 4 MHz
  // ads2.setSPIClockSpeed(4000000);

  digitalWrite(ADS1220_1_CS_PIN, HIGH);
  pinMode(ADS1220_1_CS_PIN, OUTPUT);
  digitalWrite(ADS1220_2_CS_PIN, HIGH);
  pinMode(ADS1220_2_CS_PIN, OUTPUT);

  if (ads1.init()) {
  }
  if (ads2.init()) {
  }

  ads1.setGain(ADS1220_GAIN_1);
  ads1.setDataRate(ADS1220_DR_LVL_2);
  ads1.setConversionMode(ADS1220_CONTINUOUS);

  ads2.setGain(ADS1220_GAIN_1);
  ads2.setDataRate(ADS1220_DR_LVL_2);
  ads2.setConversionMode(ADS1220_CONTINUOUS);

  float vRef1 = ads1.getVRef_V();
  float vRef2 = ads2.getVRef_V();
  //===========================  end ADS  =============================

  pinMode(22, OUTPUT);

  pinMode(5, INPUT_PULLUP);  //amp ++
  pinMode(17, INPUT_PULLUP);  // amp --
  pinMode(12, INPUT_PULLUP);  //phase ++
  pinMode(14, INPUT_PULLUP);  // phase --
  pinMode(2, INPUT_PULLUP); // sw

  /////////////////
  ledc_timer_config_t timerConfig = {
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .duty_resolution = LEDC_TIMER_10_BIT,
    .timer_num = LEDC_TIMER_0,
    .freq_hz = 15300,
    .clk_cfg = LEDC_AUTO_CLK
  };
  ledc_timer_config(&timerConfig);

  //0°,F1
  ledc_channel_config_t channelConfig1 = {
    .gpio_num = 32,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_0,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 0
  };
  ledc_channel_config(&channelConfig1);

  //90°, F2
  ledc_channel_config_t channelConfig2 = {
    .gpio_num = 33,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_1,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 256
  };
  ledc_channel_config(&channelConfig2);
  //==========================================

  //F3 , Tx GB
  ledc_channel_config_t channelConfig3 = {
    .gpio_num = 4,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_0,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 0
  };
  ledc_channel_config(&channelConfig3);
  //++++++++++++++++++++++++++++++++++++++++

  // F4 comp
  ledc_channel_config_t channelConfig4 = {
    .gpio_num = 0,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_0,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 0
  };
  ledc_channel_config(&channelConfig4);
  //===========================================

  //GB, F4
  ledc_channel_config_t channelConfig5 = {
    //.gpio_num = 32,
    //.gpio_num = 16,
    // .gpio_num = 4,
    .gpio_num = 15,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_0,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 0
  };
  ledc_channel_config(&channelConfig5);

}
///////////////////////////
const double f_s = 100; // Hz
// Cut-off frequency (-3 dB)
const double f_c = 12; // Hz
// Normalized cut-off frequency
const double f_n = 2 * f_c / f_s;

// Sample timer
Timer<micros> timer = std::round(1e6 / f_s);

// Sixth-order Butterworth filter
auto filter = butter<6>(f_n);


////////////////////////////
void loop()
{
  //if (timer)

  ads1.setCompareChannels(ADS1220_MUX_0_1);
  float Ux = ads1.getVoltage_mV();
  //  Ux = ads1.getVoltage_muV();

  ads2.setCompareChannels(ADS1220_MUX_0_1);
  float Uy = ads2.getVoltage_mV();
  // Uy = ads2.getVoltage_muV();


  Serial.print(" Ux =  ");
  Serial.print(Ux + 5 );
  Serial.print("  Uxf =  ");
  Serial.print(filter(Ux));

  Serial.print(" Uy =  ");
  Serial.print(Uy + 5 );
  Serial.print("  Uyf =  ");
  Serial.print(filter(Uy));

  Serial.println();

}

build_1971 · November 19, 2025, 5:30am

This does not change the value of Ux..
Maybe you mean:

Serial.print(Ux += 5);

Or maybe better:

Ux=Ux+5;
Serial.print(Ux);

tom321 · November 19, 2025, 8:59am

5 is to move Ux up to see easier Ux and Uxf.

tom321 · November 19, 2025, 9:00am

b707 · November 19, 2025, 10:01am

It seems not only is Uxf = Uyf , but both of them are completely inconsistent with values Ux and Uy.
Looks like you filter outputs ~~random noise.~~ (see below)

Addition
Everything became clear as soon as I looked at the code. For two series of values, you need TWO SEPARATE filters.

Currently, your values Ux and Uy are treated as a single data serie, and the filter produces the average between them.

tom321 · November 19, 2025, 3:01pm

I was using this sample

https://www.google.com/search?q=butterworth+filter+2+channels+arduino&sca_esv=5bde5639e0944a75&sxsrf=AE3TifOTnGbEto

The differences between the two channels only occur in the void() loop

tom321 · November 19, 2025, 3:13pm

I created separate channels before void loop() and program is working properly.
Thanks for suggestion.

const double fx_s = 100; // Hz
// Cut-off frequency (-3 dB)
const double fx_c = 25; // Hz
// Normalized cut-off frequency
const double fx_n = 2 * fx_c / fx_s;

// Sample timer
//Timer<micros> timer = std::round(1e6 / f_s);

// Sixth-order Butterworth filter
auto filterx = butter<6>(fx_n);

// Sampling frequency
const double fy_s = 100; // Hz
// Cut-off frequency (-3 dB)
const double fy_c = 25; // Hz
// Normalized cut-off frequency
const double fy_n = 2 * fy_c / fy_s;

// Sample timer
//Timer<micros> timer = std::round(1e6 / fy_s);

// Siyth-order Butterworth filter
auto filtery = butter<6>(fy_n);

tom321 · November 19, 2025, 9:58pm

After adding DC removing filters it is occurred that Ux (green ) is not filtered by LP filter.


#include <Filters.h>
#include <AH/Timing/MillisMicrosTimer.hpp>
#include <Filters/Butterworth.hpp>

#include <ADS1220_WE.h>
#include <Adafruit_GFX.h>
#include <Adafruit_ST7789.h>
#include <SPI.h>
#include <driver/ledc.h>
#include <Preferences.h>
Preferences preferences;

#define TFT_MOSI 23  // Data out
#define TFT_SCLK 18  // Clock out
#define TFT_CS   22// 
#define TFT_DC    21 //
#define TFT_RST   -1  // pin# 2

Adafruit_ST7789 tft = Adafruit_ST7789(TFT_CS, TFT_DC, TFT_RST);
//================================ ADS  =============================
// Pin definitions
#define SPI_MISO_PIN 19
#define ADS1220_1_CS_PIN    25   // Chip Select
#define ADS1220_1_DRDY_PIN  34   // Data Ready

#define ADS1220_2_CS_PIN    26   // Chip Select
#define ADS1220_2_DRDY_PIN  35   // Data Ready

#define SPI_INITIALIZED true
#define SPI_NOT_INITIALIZED false
////////

/* Create ADS1220 objects */
ADS1220_WE ads1 = ADS1220_WE(ADS1220_1_CS_PIN, ADS1220_1_DRDY_PIN);
ADS1220_WE ads2 = ADS1220_WE(ADS1220_2_CS_PIN, ADS1220_2_DRDY_PIN);


//===============================  end ADS  ======================

//===============================  hp filter  ======================
float inputSignalx;
float filteredSignalx;
float previousInputSignalx;
float previousFilteredSignalx = 0;
float alphax = 0.98; // Adjust for

float inputSignaly;
float filteredSignaly;
float previousInputSignaly;
float previousFilteredSignaly = 0;
float alphay = 0.98; // Adjust for cutoff frequency (closer to 1 for higher cutoff)
//=============================== end hp filter  ======================

void setup() {
  Serial.begin(115200);
  //===========================  ADS  =============================
  while (!Serial);

  /* Choose SPI clock speed here. */
  // ads1.setSPIClockSpeed(4000000); // set SPI clock speed, default is 4 MHz
  // ads2.setSPIClockSpeed(4000000);

  digitalWrite(ADS1220_1_CS_PIN, HIGH);
  pinMode(ADS1220_1_CS_PIN, OUTPUT);
  digitalWrite(ADS1220_2_CS_PIN, HIGH);
  pinMode(ADS1220_2_CS_PIN, OUTPUT);

  if (ads1.init()) {
  }
  if (ads2.init()) {
  }

  ads1.setGain(ADS1220_GAIN_1);
  ads1.setDataRate(ADS1220_DR_LVL_2);
  ads1.setConversionMode(ADS1220_CONTINUOUS);

  ads2.setGain(ADS1220_GAIN_1);
  ads2.setDataRate(ADS1220_DR_LVL_2);
  ads2.setConversionMode(ADS1220_CONTINUOUS);

  float vRef1 = ads1.getVRef_V();
  float vRef2 = ads2.getVRef_V();
  //===========================  end ADS  =============================

  pinMode(22, OUTPUT);

  pinMode(5, INPUT_PULLUP);  //amp ++
  pinMode(17, INPUT_PULLUP);  // amp --
  pinMode(12, INPUT_PULLUP);  //phase ++
  pinMode(14, INPUT_PULLUP);  // phase --
  pinMode(2, INPUT_PULLUP); // sw

  /////////////////
  ledc_timer_config_t timerConfig = {
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .duty_resolution = LEDC_TIMER_10_BIT,
    .timer_num = LEDC_TIMER_0,
    .freq_hz = 15300,
    .clk_cfg = LEDC_AUTO_CLK
  };
  ledc_timer_config(&timerConfig);

  //0°,F1
  ledc_channel_config_t channelConfig1 = {
    .gpio_num = 32,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_0,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 0
  };
  ledc_channel_config(&channelConfig1);

  //90°, F2
  ledc_channel_config_t channelConfig2 = {
    .gpio_num = 33,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_1,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 256
  };
  ledc_channel_config(&channelConfig2);
  //==========================================

  //F3 , Tx GB
  ledc_channel_config_t channelConfig3 = {
    .gpio_num = 4,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_0,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 0
  };
  ledc_channel_config(&channelConfig3);
  //++++++++++++++++++++++++++++++++++++++++

  // F4 comp
  ledc_channel_config_t channelConfig4 = {
    .gpio_num = 0,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_0,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 0
  };
  ledc_channel_config(&channelConfig4);
  //===========================================

  //GB, F4
  ledc_channel_config_t channelConfig5 = {
    //.gpio_num = 32,
    //.gpio_num = 16,
    // .gpio_num = 4,
    .gpio_num = 15,
    .speed_mode = LEDC_LOW_SPEED_MODE,
    .channel = LEDC_CHANNEL_0,
    .timer_sel = LEDC_TIMER_0,
    .duty = 512,
    .hpoint = 0
  };
  ledc_channel_config(&channelConfig5);

}
/*
  ///////////////////////////
  const double f_s = 100; // Hz
  // Cut-off frequency (-3 dB)
  const double f_c = 12; // Hz
  // Normalized cut-off frequency
  const double f_n = 2 * f_c / f_s;

  // Sample timer
  Timer<micros> timer = std::round(1e6 / f_s);

  // Sixth-order Butterworth filter
  auto filter = butter<6>(f_n);
*/
//============================   LP filter  ===================================
//lp x
const double fx_s = 100; // Hz
// Cut-off frequency (-3 dB)
const double fx_c = 12; // Hz
// Normalized cut-off frequency
const double fx_n = 2 * fx_c / fx_s;

// Sample timer
//Timer<micros> timer = std::round(1e6 / f_s);

// Sixth-order Butterworth filter
auto filterx = butter<6>(fx_n);

// lp y
const double fy_s = 100; // Hz
// Cut-off frequency (-3 dB)
const double fy_c = 12; // Hz
// Normalized cut-off frequency
const double fy_n = 2 * fy_c / fy_s;

// Sample timer
//Timer<micros> timer = std::round(1e6 / fy_s);

// Siyth-order Butterworth filter
auto filtery = butter<6>(fy_n);

//============================  eng LP filter  ===================================
void loop()
{
  //if (timer)

  ads1.setCompareChannels(ADS1220_MUX_0_1);
  // float Ux = ads1.getVoltage_mV();
  float  Ux = ads1.getVoltage_muV();

  ads2.setCompareChannels(ADS1220_MUX_0_1);
  // float Uy = ads2.getVoltage_mV();
  float Uy = ads2.getVoltage_muV();

  //===============================  hp filter  ======================
  // Simple IIR high-pass filter (difference equation)
  // This removes the DC component and lets through changes
  inputSignalx = filterx(Ux);
  filteredSignalx = alphax * (previousFilteredSignalx + inputSignalx - previousInputSignalx);
  previousInputSignalx = inputSignalx;
  previousFilteredSignalx = filteredSignalx;

  inputSignaly = filtery(Uy);
  filteredSignaly = alphay * (previousFilteredSignaly + inputSignaly - previousInputSignaly);
  previousInputSignaly = inputSignaly;
  previousFilteredSignaly = filteredSignaly;
  //===============================  end hp filter  ======================
  /*
    Serial.print(" Ux =  ");
    Serial.print(Ux + 5 );
    Serial.print("  Uxf =  ");
    Serial.print(filterx(Ux));

    Serial.print(" Uy =  ");
    Serial.print(Uy + 5 );
    Serial.print("  Uyf =  ");
    Serial.print(filtery(Uy));
  */

  Serial.print("  filteredSignalx =  "); //good, gray
  Serial.print(filteredSignalx);
  Serial.print("  filteredSignaly =  "); // not so good, green
  Serial.print(filteredSignaly);

  Serial.println();

}

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Butterworth Filter 2 CH

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