how much should be the value of ENOB for n-bit ADC

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Hi daer all;
I have a question that for an 8-bit ADC for low-speed industrial applications what is the accepted minimum ENOB for interested bandwidth?

I designed and implemented my Sigma-delta ADC on FPGA; but the ENOB for lower frequencies (20hz) starts from 6.4 and for highest frequency in the bandwidth (8khz) is 4.9 are these values acceptable or must be improved?
thank you;