read negative voltage with mcp3008

how can i read a negative voltage by the mcp3008 ?
, i tried to do that with this code but i I could not read
it , i see that the most readings are 0.00 ,
this is my code :

import RPi.GPIO as GPIO
import pigpio
import time
import spidev

GPIO.setwarnings(False)

spi = spidev.SpiDev()
spi.open(0,0)
pi=pigpio.pi()
if not pi.connected:
exit()

def analogInput(channel):
 spi.max_speed_hz =  1442      #f_cpu/frequency 1442
 adc=spi.xfer2([1,(8+channel)<<4,0])
 data = ((adc[1]&3)<<8) + adc[2]
 return data

def Volts(data):
 volts=(data*3.3)/float(1023)      #1024
 volts=round(volts,2)
 return volts  

dc=718000
pi.hardware_PWM(18, 70000, dc) 
while True:
    out=analogInput(0)
    out1=Volts(out)
    print(out1)
    time.sleep(3)