0.0000 Volt at ADC pin after booting the ESP32

Hi
the problem is that after booting my esp32 the voltage is 0.0000 Volt at adc pin 34. so measurement is not working. if i disconnect and reconnect the adc pin the measurement works and i get realistic values.
my sensor is running with 24volts. i use a step up to get the voltage.
whats the problem of my setup?

substitutions:
  device_name: test_spannug
  device_description: test
  friendly_name: Zisterne Füllstand
  
esphome:
  name: ${device_name}
  comment: ${device_description}
  platform: ESP32
  board: nodemcu-32s
  #on_boot:
   # then:
    #  - delay: 1s
     # - switch.turn_on: relay
      #- delay: 9s
      #- switch.turn_off: relay
  
wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password
  fast_connect: true
  
  manual_ip:
    static_ip: 192.168.1.99
    gateway: 192.168.1.1
    subnet: 255.255.255.0

#mqtt:
 # broker: 192.168.1.43
  #port: 1883
  #username: openhabian
  #password: openhabian
  #birth_message:
  #will_message:
  #discovery: True
  #on_message:
    #- topic: zisterne/ota_mode
    #  payload: 'ON'
    #  then:
    #    - deep_sleep.prevent: deep_sleep_1
    #- topic: zisterne/sleep_mode
    #  payload: 'ON'
    #  then:
    #    - deep_sleep.enter: deep_sleep_1


captive_portal:

# Enable logging
logger:
  level: debug

ota:
api:
web_server:
  port: 80

switch:

  - platform: restart
    name: "Neustart"

sensor:
  - platform: adc
    pin: GPIO34
    name: "Spannung"
    update_interval: 1s
    attenuation: 11db


What is this sensor? What is your wriring?