# SQNR vs SNDR (with regards to Delta Sigma Modulation)

With regard to delta-sigma modulation, I understand that SQNR is the ratio of the signal power within our frequency bandwidth to the noise power within our bandwidth (in-band quantization noise). What I don't understand is how this is functionally different from SNDR (signal-to-noise distortion ratio). Is there even a difference or is this just a terminology quirk?

$$SNQR = 6.02 \text{ dB/bit} + 1.76 \text{ dB}$$
Please see this other post where I explain how this formula is derived. This formula is then used again with $$SNDR$$ replacing $$SNQR$$ to come up with the Effective Number of Bits or ENOB:
$$ENOB = \frac{SNDR-1.76 \text{ dB}}{6.02 \text{ dB}}$$
So when we use $$SNQR$$ we are referring to the actual number of bits in the converter, and determining the theoretical noise due to quantization alone, while with $$SNDR$$ and $$ENOB$$ we are including additional noise sources and equating that to the perfect ADC that would provide similar performance.