ESP32 ADC measurment with variable input voltage?

with "turn wifi off" do you mean
WiFi.disconnect()
?

or do you mean

void setModemSleep() {
    WiFi.setSleep(true);
    if` `(!setCpuFrequencyMhz(40)){
        Serial2.println("Not valid frequency!");
    }
    // Use this if 40Mhz is not supported
    // setCpuFrequencyMhz(80);
}
void wakeModemSleep() {
setCpuFrequencyMhz(240);
}

or

void disableWiFi(){

adc_power_off();

WiFi.disconnect(true);  // Disconnect from the network

WiFi.mode(WIFI_OFF);    // Switch WiFi off

}