Larger capacitor for low frequency ADC with Teensy Audio Board?

[Audioinfra]

Dear all,

I successfully created the low frequency sawtooth wave, which is 0.5 Hz, the results is really good, however, I tried to record it by the recorder example, I found out the AC coupling in the Audio Board filtered out the low freq part. Which result as below:


I have to choose two option, first is remove the coupling cap and shorten it, second is changing the larger one. I am a Mech guys so have no idea what should be the best option for ADC low freq signal. If the latter is recommended, what would be the size of the cap. Please help me out

Cheers.

[Audioinfra]

this is the code of creating saw tooth wave by sum of sine wave

#include <Bounce.h>
#include <Audio.h>
#include <Wire.h>
#include <SPI.h>
#include <SerialFlash.h>
#include "Arduino.h"
#include "arm_math.h"

// GUItool: begin automatically generated code
AudioSynthWaveform       sine1;          //xy=111,149
AudioPlayQueue           queue2;         //xy=282,571
AudioRecordQueue         queue1;         //xy=312,488
AudioOutputI2S           i2s1;           //xy=412,572
AudioAnalyzeFFT1024      fft1024_1;      //xy=434,618
AudioConnection          patchCord21(sine1, 0, i2s1, 0);
AudioConnection          patchCord24(sine1, 0, i2s1, 1);
AudioConnection          patchCord22(queue2, fft1024_1);
AudioConnection          patchCord23(sine1, queue1);

AudioControlSGTL5000     sgtl5000_1;     //xy=111,635
// GUItool: end automatically generated cod

#define MAX_BLOCKSIZE   256
#define NO_COMPONENT 20

#define TFT_DC      20
#define TFT_CS      21
#define TFT_RST    255  // 255 = unused, connect to 3.3V
#define TFT_MOSI     7
#define TFT_SCLK    14
#define TFT_MISO    12

elapsedMillis press_time;
int modePlay = 0;
int modeRecord = 0;

//double coeff_play[] = { 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
double coeff_play[] = { 0,0,0,0,0,0.5,0,0,0,0,0,0,0,0,0,0,0,0,0,0};

int offsets = 0;
int i = 0;
int ii = 0;
byte bufferblock[512];
elapsedMillis timeElapsed;
int on = 1;
float32_t  sinFloatComponent[NO_COMPONENT];
float32_t  sinCoef[NO_COMPONENT];
float32_t  sinFloat[MAX_BLOCKSIZE + 1];
int16_t    sinInt[MAX_BLOCKSIZE + 1];

// Void setup
void setup() {
  Serial.begin(9600);
  Serial1.begin(38400);

  AudioMemory(100);
  sgtl5000_1.enable();
  sgtl5000_1.volume(0.5);
  //sgtl5000_1.lineOutLevel(23);
  SPI.setMOSI(7);
  SPI.setSCK(14);
  sine1.begin(0.5, 0.8, WAVEFORM_ARBITRARY);
  int16_t *saw = GetSawtoothWave();
  sine1.arbitraryWaveform(saw, 0.8);
  delay(20);
  modePlay=1;
  startFFT();
}

void loop() {

//  switch (on) {
//
//    case 0: // Main menu
//
//      break;
//
//    case 1:
//
//      if (modePlay == 0 ) {
//        startFFT();
//      }
//      if (modePlay == 1 ) {
//        stopFFT();
//      }
//      break;
//
//
//  }

  if (modePlay == 1) continueFFT();
}
//
//  float vol = analogRead(15);
//  vol = vol / 1024;
//  sgtl5000_1.volume(vol);
//
/*AudioWindowHanning1024 (default)
  //AudioWindowBartlett1024
  //AudioWindowBlackman1024
  //AudioWindowFlattop1024
  //AudioWindowBlackmanHarris1024
  //AudioWindowNuttall1024
  //AudioWindowBlackmanNuttall1024
  //AudioWindowWelch1024***
  //AudioWindowHamming1024
  //AudioWindowCosine1024
  //AudioWindowTukey1024*/
void startFFT() {
  modePlay = 1;
  queue1.begin();
  fft1024_1.windowFunction(AudioWindowHamming1024);
}

void continueFFT() {

  if (queue1.available() >= 2) {
    byte buff[512];
    memcpy(buff, queue1.readBuffer(), 256);
    queue1.freeBuffer();
    memcpy(buff + 256, queue1.readBuffer(), 256);
    queue1.freeBuffer();
    while (i < 256  &&  215 * i + offsets < 256) {
      memcpy(bufferblock + ii * 2, buff + 2 * (215 * i + offsets), 2);
      //Serial.println(ii);
      i ++;
      ii++;
    }
    offsets = 215 * i + offsets - 256;
    i = 0;

    if (ii >= 256 && queue2.available()) {
      memcpy(queue2.getBuffer(), bufferblock, 256);
      queue2.playBuffer();
      memcpy(queue2.getBuffer(), bufferblock + 256, 256);
      queue2.playBuffer();
      ii = 0;
      printtft();
      //Serial.println(queue2.available());
    }
  }
  //  if(1) {
  //  if(millis() - last_time >= 5000) {
  //    Serial.print("Proc = ");
  //    Serial.print(AudioProcessorUsage());
  //    Serial.print(" (");
  //    Serial.print(AudioProcessorUsageMax());
  //    Serial.print("),  Mem = ");
  //    Serial.print(AudioMemoryUsage());
  //    Serial.print(" (");
  //    Serial.print(AudioMemoryUsageMax());
  //    Serial.println(")");
  //    last_time = millis();
  //  }
  //}
}


void stopFFT() {
  queue1.end();
  //Serial.println("stopPlaying");
  //tft.fillRect(5, 30, 320, 240, BLACK);
  modePlay = 0;
}



/*
  huge arguement list
  &d name of your display object
  x = x data point
  y = y datapont
  gx = x graph location (lower left)
  gy = y graph location (lower left)
  w = width of graph
  h = height of graph
  xlo = lower bound of x axis
  xhi = upper bound of x asis
  xinc = division of x axis (distance not count)
  ylo = lower bound of y axis
  yhi = upper bound of y asis
  yinc = division of y axis (distance not count)
  title = title of graph
  xlabel = x asis label
  ylabel = y asis label
  gcolor = graph line colors
  acolor = axi ine colors
  pcolor = color of your plotted data
  tcolor = text color
  bcolor = background color
  &redraw = flag to redraw graph on fist call only
*/


void printNumber(float n) {

  if (n >= 0.0005) {
    long a = (n*100000);
    Serial1.print("<");
    Serial1.print(a);
    Serial1.print(">");
  } else {
    Serial1.print("<");
    Serial1.print(0);
    Serial1.print(">");
  }
}



void printtft() {
  Serial1.print("A");
  printNumber(fft1024_1.read(0));
  printNumber(fft1024_1.read(1));
  printNumber(fft1024_1.read(2));
  printNumber(fft1024_1.read(3));
  printNumber(fft1024_1.read(4));
  printNumber(fft1024_1.read(5));
  printNumber(fft1024_1.read(6));
  printNumber(fft1024_1.read(7));
  printNumber(fft1024_1.read(8));
  printNumber(fft1024_1.read(9));
  printNumber(fft1024_1.read(10));
  printNumber(fft1024_1.read(11));
  printNumber(fft1024_1.read(12));
  printNumber(fft1024_1.read(13));
  printNumber(fft1024_1.read(14));
  printNumber(fft1024_1.read(15));
  printNumber(fft1024_1.read(16));
  printNumber(fft1024_1.read(17));
  printNumber(fft1024_1.read(18));
  printNumber(fft1024_1.read(19));
  printNumber(fft1024_1.read(20));
  printNumber(fft1024_1.read(21));
  printNumber(fft1024_1.read(22));
  printNumber(fft1024_1.read(23));
  printNumber(fft1024_1.read(24));
  printNumber(fft1024_1.read(25));
  printNumber(fft1024_1.read(26));
  printNumber(fft1024_1.read(27));
  printNumber(fft1024_1.read(28));
  printNumber(fft1024_1.read(29));
  printNumber(fft1024_1.read(30));
  printNumber(fft1024_1.read(31));
  printNumber(fft1024_1.read(32));
  printNumber(fft1024_1.read(33));
  printNumber(fft1024_1.read(34));
  printNumber(fft1024_1.read(35));
  printNumber(fft1024_1.read(36));
  printNumber(fft1024_1.read(37));
  printNumber(fft1024_1.read(38));
  printNumber(fft1024_1.read(39));
  printNumber(fft1024_1.read(40));
  Serial1.print("B");
  //
  //  Serial.println();
  //  Serial.print("all=");
  //  Serial.print(AudioProcessorUsage());
  //  Serial.print(",");
  //  Serial.print(AudioProcessorUsageMax());
  //  Serial.print("    ");
  //  Serial.print("Memory: ");
  //  Serial.print(AudioMemoryUsage());
  //  Serial.print(",");
  //  Serial.print(AudioMemoryUsageMax());
  //  Serial.print("    ");
  //  Serial.println();
}


float dspSin(float x) {
  float result = arm_sin_f32(x);
  return result;
}

float radian(int i)
{
  float radianStep = 2 * PI * i / MAX_BLOCKSIZE;
  return radianStep;
}

int16_t * GetSawtoothWave() {
  for (int i = 0; i < NO_COMPONENT; i++)
  {
    sinCoef[i] = -0.5705 * (coeff_play[i]);
    //sinCoef[i] = 1;
    //Serial.println(sinCoef[i], 5);
  }
  for (int i = 0; i <= MAX_BLOCKSIZE; i++)
  {
    sinFloat[i] = 0;
    for (int j = 0; j < NO_COMPONENT; j++)
    {
      sinFloatComponent[j] = dspSin(radian(i) * (j + 1));
      sinFloat[i] += sinCoef[j] * sinFloatComponent[j];
      //Serial.println(sinFloatComponent[j], 5);
      //Serial.println(sinFloat[i], 5);
    }
  }
  arm_float_to_q15 ( &sinFloat[0], &sinInt[0], MAX_BLOCKSIZE);
  //  for (int i = 0; i <= MAX_BLOCKSIZE; i++)
  //  {
  //    //Serial.println(sinFloat[i], 5);
  //    //Serial.println(sinInt[i]);
  //  }
  return sinInt;
}

[PaulStoffregen]

I'm confused. You're asking about ADC (analog to digital conversion), but your code appears to be synthesizing a waveform and sending it to the output which is the opposite process, converting digital data to analog signals.

The code has what looks like a lot of unrelated leftover stuff, especially regarding the FFT, which only confuses me more. Maybe a concise but complete program with only the stuff relevant to your question would help?

On top of all that, when it comes to the frequency response of series capacitors, the impedance of whatever other stuff you're connecting matters. I can't even tell if you're really talking about inputs or outputs, not to mention *what* sort of thing you're connecting and what its impedance might be. The graph you posted shows a waveform, but the X axis is labeled with arbitrary units which I can barely read. Or maybe that's seconds, where the entire thing represents about 13 minutes? Even knowing impedance, the specific time scale (spectral content) matters for estimating the capacitance required.

If you could state clearly which pins and what you're connecting and what you're doing, maybe it'd be possible to help more.