Arduino Due Fast Sampling positive Negative Voltage

Hi, Im trying to achieve a Negative Positive reading with Arduino Due,
i ve reach the 1 uS with Free Running Mode but cant figure out how to read the negative Voltage.

int x;
long t;
int m[10000]={0};

int i=0;
void setup()
{
Serial.begin(9600); // initialize the serial port:
ADC->ADC_MR |= 0x80; // these lines set free running mode on adc 7 (pin A0)
REG_ADC_MR = 0x10380180; // change from 10380200 to 10380180, 1 is the PREESCALER and 8 means FREERUN
ADC->ADC_CR=2;
ADC → ADC_CHER = 0x80; // enable ADC on pin A0

}

void loop() {
//t = micros(); // init time an elapsed time, in micro seconds
while((ADC->ADC_ISR & 0x80)==0); // wait for conversion
x = ADC->ADC_CDR[7];
if (x>1000){
for(int i = 0; i < 10000; i++) {

// instrucction to measure
while((ADC->ADC_ISR & 0x80)==0); // wait for conversion
x = ADC->ADC_CDR[7]; // read value A0
m*=x;*

  • // 2 us is nothing measured time*
  • } *
  • for(int i=0; i<10000; i++){*
    _ byte j=map(m*,0,4095,0,255);*_

* Serial.write(j);*

* delay(12);*
* }*
* }*

// t = micros()-t; // 2 us is nothing measured time
//Serial.println(t);
}
i have an idea for using DC offset but dont know how to put it in a Code
thanks everyone!

For a bipolar ADC :