We have found that pin19 in analogin sense mode (sampling at 25 Hz) is significantly noisier than pin20 and pin18 in analogin sense mode. We've tested this on on 2 mbed-M3 devices.

mbed code:

1. include "mbed.h"
2. include "USBSerial.h"

1. define BUF_SIZE 64 Size of sample buffer in samples
2. define SAMP_RATE 25 raw sample rate in Hz

Virtual serial port over USB Serial serial(USBTX, USBRX);

interrupt driven timer Ticker flipper; Ticker flipper2;

analog inputs AnalogIn ain_v1(p19); AnalogIn ain_v2(p20);

DigitalOut ref(p18);

DigitalOut led1(LED1); DigitalOut led2(LED2); DigitalOut led3(LED3);

unsigned short p1_cur[BUF_SIZE]; sample buffer unsigned short p2_cur[BUF_SIZE]; sample buffer unsigned char ind_write; queue write index unsigned char ind_read; queue read index

unsigned int t_local;

void get_readings() {

add nothing to the ISR unless it's strictly required. p1_cur[ind_write] = ain_v1.read_u16()>>4; bottom 4 bits are 0 p2_cur[ind_write] = ain_v2.read_u16()>>4; bottom 4 bits are 0

Update the write index if (ind_write==BUF_SIZE-1){ ind_write = 0; } else { ind_write++; } }

int main() {

reset variables

ind_write = 0; set queue index to 0. ind_read = 0;

int samp_cnt=0;

t_local = 0;

ref = 0; set pin to 0 to see if it helps with noise

Setup timer interrupts for samping the ADC channels flipper2.attach_us(&get_readings, 1000000/SAMP_RATE); the function to be attached and the interval in us

spin in a main loop. flipper will interrupt it to call flip while(1) {

if (samp_cnt>(SAMP_RATE>>2)){ blink an led led2 = 0; } else if (samp_cnt<SAMP_RATE){ led2 = 1; } else { samp_cnt = 0; } samp_cnt++;

if (ind_read != ind_write){ new data, send it out over serial.

If you're going to apply some math to the raw samples, it should be done here

serial.printf("%i,%i,%i\r\n", t_local,p1_cur[ind_read],p2_cur[ind_read]); t_local++; increment local time (just a monotonically increasing sample counter that will overflow Update the read index if (ind_read==BUF_SIZE-1){ ind_read = 0; } else { ind_read++; } }

} }

matlab code for noise sampling on pins:

ser_prt = serial('COM11','BaudRate',9600,'Terminator','CR/LF');

fopen(ser_prt);

of = fopen('data.txt','w');

cnt = 0;

samp_rate = 25; % Hz buff_length = 30; % seconds

databuf = zeros(3,samp_rate*buff_length);

while cnt < 1000 tline = fgetl(ser_prt); cnt = cnt+1;

delim_loc = strfind(tline,',');

if isempty(delim_loc) tlocal = tline(1:delim_loc(1)-1); v1 = tline(delim_loc(1)+1:delim_loc(2)-1); v2 = tline(delim_loc(2)+1:end); fprintf(of,'%s,%s,%s\n\r',tlocal,v1,v2);

databuf(:,1:end-1)=databuf(:,2:end); databuf(:,end)=[str2double(tlocal),str2double(v1),str2double(v2)]; end

if mod(cnt,samp_rate) plot((databuf(1,:)-databuf(1,end))/samp_rate,databuf(2,:),(databuf(1,:)-databuf(1,end))/samp_rate,databuf(3,:)) fprintf('cnt %i\n',cnt) end

end

fclose(of);

fclose(ser_prt); delete(ser_prt); clear ser_prt;

X = fft(databuf(2,:)); Y = fft(databuf(3,:));

f = linspace(0,25,length(X));

figure(1) semilogy(f(f<25/2),abs(X(f<25/2)),f(f<25/2),abs(Y(f<25/2)))

figure(2) plot((databuf(1,:)-databuf(1,end))/samp_rate,databuf(2,:),(databuf(1,:)-databuf(1,end))/samp_rate,databuf(3,:))

After looking at your code ..

#include "mbed.h"

DigitalOut myled(LED1);

//analog inputs 
AnalogIn ain_v1(p19); 
AnalogIn ain_v2(p20);

DigitalOut ref(p18);

DigitalOut led1(LED1); 
DigitalOut led2(LED2); 
DigitalOut led3(LED3);

unsigned short p1_cur[BUF_SIZE]; // sample buffer 
unsigned short p2_cur[BUF_SIZE]; // sample buffer 
unsigned char ind_write; // queue write index 
unsigned char ind_read; //  queue read index

unsigned int t_local;

void get_readings() 
{

    //add nothing to the ISR unless it's strictly required. 
    p1_cur[ind_write] = ain_v1.read_u16()>>4; //bottom 4 bits are 0 
    p2_cur[ind_write] = ain_v2.read_u16()>>4; // bottom 4 bits are 0
    
    // Update the write index 
    if (ind_write==BUF_SIZE-1)
    { 
        ind_write = 0; 
    } 
    else 
    { 
        ind_write++; 
    } 
}



int main()
{

    reset variables

    ind_write = 0;
    set queue index to 0. ind_read = 0;

    int samp_cnt=0;

    t_local = 0;

    ref = 0;
    set pin to 0 to see if it helps with noise

    Setup timer interrupts for samping the ADC channels flipper2.attach_us(&get_readings, 1000000/SAMP_RATE);
    the function to be attached and the interval in us

    spin in a main loop. flipper will interrupt it to call flip 
    while(1) 
    {

        if (samp_cnt>(SAMP_RATE>>2)) 
        {
            blink an led led2 = 0;
        } 
        else if (samp_cnt<SAMP_RATE) 
        {
            led2 = 1;
        } 
        else 
        {
            samp_cnt = 0;
        }
        
        samp_cnt++;

        if (ind_read != ind_write) 
        {
            new data, send it out over serial.

            // If you're going to apply some math to the raw samples, it should be done here

            serial.printf("%i,%i,%i\r\n", t_local,p1_cur[ind_read],p2_cur[ind_read]); 
            t_local++; 
            // increment local time (just a monotonically increasing sample counter that will overflow Update the read index 
            
            if (ind_read==BUF_SIZE-1)
            { 
                ind_read = 0; 
            } 
            else 
            { 
                ind_read++; 
            } 
        }

    } 
}


        }

Correct me if I am wrong,

you are only reading 2 ADC Channels,

I belive that the reason is because Pin 20, has a significant amount of time to settle (charge internal capacitors), in comparison to Pin 19.

there are a few possable compromises/solutions.

Best: External ADC Chip, one for each channel, or one with 2/4/8 seperate ADC blocks (none spring to mind)

compromise 1: Interleaving, sample each ADC alternatly,

who++;
if (who && 1)
{
  // Sample ADC 0
}
else
{
  // Sample ADC 1
}

or you could allow a short 'Charging' delay for each channel ..

{
  read.An1();
  wait_us (10);
  result = read.An1();
}

or alturnatly, what I used to do for PIC's ADC (not quite the same ..

{
  // Result_Previous = ReadADC();
  // select MUX .. for next conversion
}

I am taking an educated guess, that most of your nois is related to the actual switching of the multiplexer,

and not fully charding the sample cap.

Hope this was informative.

Ceri