Just found the problem of this ADC. When switching between channels, if the voltage is quite different, it will cause the wrong result.
for example, connect Ch4 to 1.6V, connect Ch7 to 3.3V, and reference is 3.3V, if continuously reading either Ch4 or Ch7, the result will be good, Ch4 is about 0x800, and Ch7 is around 0xfff, but if you read ch4, ch7, ch4, ch7....
the result for ch4 will become 0xaa5, while ch7 is still 0xfff.