In analog-to-digital circuits, SNR, SINAD, and ENOB parameters are often seen. So what do they mean and how are they measured?
The signal-to-noise power ratio SNR, in dB, is the ratio of the average power of the input signal to the noise (excluding any harmonics and DC components), and is used to measure the internal noise of the device [1]. Calculated as follows:
In an ideal state, only the quantization noise of the ADC is considered, and the ideal SNR value can be derived. The formula is as follows [6] to evaluate the SNR value of the test system. (N: ADC resolution)
SNR = 6.02 * N + 1.76
SINAD（Signal to Noise and Distortion Ratio）
The ratio of signal to noise and distortion, SINAD, in dB, is the power ratio of the input signal and all output signal distortion (including harmonic components, excluding DC), used to evaluate the nonlinearity of all transfer functions of the output signal plus the system The cumulative effect of all noise (quantization, jitter, and aliasing) [2]. Calculated as follows:
ENOB（Effective Number of Bits）
The effective number of bits ENOB, the unit is bit, is used to describe the effective resolution of the ADC/DAC system. For an ideal N-bit ADC system, its ENOB should be infinitely close to N, but the real circuit will inevitably introduce noise, thereby reducing the actual resolution of the ADC [3]. Generally, the formula for calculating the ideal value of SNR is used to replace SNR with SINAD in the formula to calculate ENOB. The formula is as follows [6]:
example
Taking Texas Instruments' ADC14X250 analog-digital conversion chip as an example, the typical SNR, SINAD, ENOB indicators and test methods are given [4].
Table 1 ADC14X250 SNR, SINAD, ENOB indicators
Table 1 The amplitude of the input signal to be measured is -3dBFS (and -1dBFS is commonly used), which represents the ratio of the input signal to the full-scale amplitude. Different input frequencies correspond to different SNRs. The signal and noise power can be calculated based on FFT according to the digital conversion result of the input signal [5]. ENOB is calculated by SINAD.