int sum = 0;
for(int i = 0, index = 0; i < DATA_TO_SEND; ++i) {
    sum += sprintf(&(PromptUSB[index]), "%.4f,%.4f;", ((float)i/(float)SAMPLES)*(float)SAMPLING_FREQUENCY, appData.amplitude[i])
    index += sum;
}
Returned_Value = USB_DEVICE_CDC_Write(USB_DEVICE_CDC_INDEX_0, &appData.writeTransferHandle, PromptUSB, sum, USB_DEVICE_CDC_TRANSFER_FLAGS_DATA_COMPLETE);