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I suppose you are talking about "decibels relative to carrier". You can find it used to measure the noise power density at a given offset from the carrier. E.g. if the noise power 10kHz away in a 1Hz bandwidth is 80dB below carrier power, then you can say SSB noise there is -80dBc. There's no need to convert to dB, the "c" simply tells you which is the reference.
lets give an example: unsigned 10 bit adc => so output is from 0 to 2^(10)-1. The maximum output value 2^(10)-1 is equall to 0 dBc. SNR is in log scale equall to Signal - Noise. Noise in dBc scale is always with '-' as it cant physically be greater then adc resolution ( max output), so SNR is always positive number.
No matter what the scale SNR with '-' means that Noise is much greater then usefull signal.