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SNR (Signal-to-Noise Ratio)
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ENOB (Effective Number of Bits)
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Quantization Noise
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Jitter Noise Contribution
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SINAD
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SNR (Signal-to-Noise Ratio) is a measure of how much signal power is present compared to noise power in an ADC. It's typically expressed in decibels (dB) and indicates the quality of the analog-to-digital conversion. Higher SNR values indicate better performance with less noise relative to the signal.
The number of bits in an ADC determines its resolution - how many discrete digital values can represent the analog input range. More bits provide finer resolution and potentially higher SNR. Common ADC resolutions range from 8 to 32 bits, with each additional bit theoretically improving SNR by about 6.02 dB.
Clock jitter introduces timing uncertainty in the sampling process, which appears as noise in the digitized signal. The jitter noise contribution increases with input frequency and jitter amplitude. High-frequency signals are more sensitive to jitter, making low-jitter clocks critical for high-performance ADC applications.
ENOB (Effective Number of Bits) represents the actual resolution performance of an ADC accounting for all noise sources. It's calculated from SNR using the formula: ENOB = (SNR - 1.76) / 6.02. ENOB is typically less than the ADC's nominal bit count due to various noise sources and non-idealities.
SINAD (Signal-to-Noise and Distortion ratio) includes both noise and harmonic distortion in its measurement, while SNR typically considers only noise. SINAD provides a more comprehensive measure of ADC performance as it accounts for all unwanted signal components, not just random noise.
Sampling frequency must be at least twice the input signal frequency (Nyquist criterion) to avoid aliasing. Higher sampling rates can improve SNR by spreading noise over a wider bandwidth, but may also introduce more clock jitter effects. The optimal sampling rate balances these trade-offs.
ADC SNR is limited by several factors including quantization noise (fundamental limit), thermal noise from input circuits, clock jitter, power supply noise, and non-linearities. The theoretical maximum SNR is approximately 6.02N + 1.76 dB, where N is the number of bits.