reading raw data until condition is met, then stopping

[ikejima]

Do you want to clear the file (/home/pi/data_log.csv) every 5 seconds and start write to the same file ?

file = open("/home/pi/data_log.csv", "a") -> the second parameter is mode, try use w
'w' open for writing, truncating the file first

i=0 
 
while adcx.get_last_result() < 840:
    # Create a empty file
    file = open("/home/pi/data_log.csv", "w")
    file.write("Time,Sensor1,Sensor2,Sensor3,Sensor4\n")
    file.write(bytes(datetime.now())+"\n")
    print('Reading ADS1x15 channel 0 for 5 seconds...')
    start = time.time()
    # write for 5 seconds
    while (time.time() - start) <= 5.0:
        now = datetime.now()
        usec = "%02d%06d" % (now.second, now.microsecond)
        value = adcx.get_last_result()
        file.write(str(usec) + "," + "{0}".format(*value) + "\n")

    file.close()