Arduino Giga using AdvancedADC in a class to read from 3 analog pins; problem

[gjg_p320]

Hello to all,
I tested the following slightly modified sketch from the examples of Arduino_AdvancedAnalog and it works fine.
But up to now I didn't succeed in packing the used function into a class in order to automate the whole analog-reading procedure in a project.

AnalogIn.ino:
#include <Arduino_AdvancedAnalog.h>
char tmp[64] = "";
AdvancedADC adc(A0, A1, A2);
uint64_t last_millis = 0;
void setup() {
	Serial.begin(9600);
	// Resolution, sample rate, number of samples per channel, queue depth.
		if (!adc.begin(AN_RESOLUTION_12, 16000, 32, 96)) { // ok
			Serial.println("Fehler Start adc");
		while (1);
	}
}
void loop() {
	if (adc.available()) {
		SampleBuffer buf = adc.read();
		// Process the buffer.
		if (millis() - last_millis > 333) {
			sprintf(tmp, "Wert A0: %d", buf[0]);
			Serial.println(tmp);  // Wert von Kanal 1
			sprintf(tmp, "Wert A1: %d", buf[1]);
			Serial.println(tmp);  // Wert von Kanal 1
			sprintf(tmp, "Wert A2: %d", buf[2]);
			Serial.println(tmp);  // Wert von Kanal 1
			last_millis = millis();
		}
		// Direct access to the underlying buffer.
		// buf.data()
		// Release the buffer to return it to the pool.
		buf.release();
	}
}

I tried to do the following schematic:
analogIn.h:

class z;
extern z Z;
class z {
  advancedADC *adc;
  bool StartADC();
  int getVal(int n);
...}

analogIn.cpp:

z Z;
AdvancedADC adc(A0, A1, A2);
  ...
}
z::StartADC() {
  return adc->begin(AN_RESOLUTION_12, 16000, 32, 96);
}
z::getVal(int n) {
if (adc->available()) {
    SampleBuffer buf = adc->read();
    return buf[n];
  } else {
    return -1;
  }
  buf.release();
}

So analogIn.h is included in the main program, usb started, StartADC() called and then getVal(i). When calling getVal(i), the program crashes. What I'm doing wrong? Your help is appreciated!
Regards
Gjg_P320

P.S.: I tried also this and it doesn't work:

analogIn.h:

class z;
extern z Z;
class z {
  bool StartADC();
  int getVal(int n);
...}

analogIn.cpp:

z Z;

AdvancedADC adc(A0, A1, A2);
	
z::StartADC() {
  return adc.begin(AN_RESOLUTION_12, 16000, 32, 96);
}
z::getVal(int n) {
if (adc.available()) {
    SampleBuffer buf = adc.read();
    return buf[n];
  } else {
    return -1;
  }
  buf.release();
}