The quality of the output signal from an A/D (analog-to-digital) converter is typically measured in terms of the signal to quantization noise ratio. This measurement gives an indication of how well the converter is able to accurately represent the original analog signal in digital form.

Quantization Error:
- Quantization error refers to the difference between the actual analog signal and its digital representation after quantization.
- It occurs due to the finite resolution of the A/D converter, which can only represent the analog signal using a finite number of digital levels.
- Quantization error is a measure of the distortion introduced during the conversion process and can be reduced by increasing the number of digital levels or using a higher-resolution converter.
- However, quantization error alone does not provide a complete picture of the quality of the output signal.

Signal to Quantization Noise Ratio (SQNR):
- The signal to quantization noise ratio is a more comprehensive measure of the quality of the output signal from an A/D converter.
- It is defined as the ratio of the signal power to the quantization noise power.
- The signal power represents the power of the original analog signal, while the quantization noise power represents the power of the error introduced by quantization.
- A higher SQNR indicates a better quality output signal, as it means the signal power is higher relative to the noise power.
- The SQNR can be improved by increasing the resolution of the A/D converter (i.e., using more bits) or by using techniques such as oversampling and noise shaping.

Conversion Constant:
- The conversion constant is not directly related to the quality of the output signal.
- It represents a scaling factor that relates the digital output of the A/D converter to the original analog input.
- It is used to convert the digital representation back to its analog equivalent.
- The conversion constant depends on the reference voltage and the resolution of the A/D converter, but it does not provide information about the signal quality.

In summary, while quantization error is a measure of the distortion introduced during the conversion process, the signal to quantization noise ratio provides a more comprehensive measure of the quality of the output signal from an A/D converter. The conversion constant, on the other hand, is not directly related to the signal quality but is used for converting the digital output back to an analog signal.