Datasheet

ADC08831, ADC08832
www.ti.com SNAS015C –SEPTEMBER 1999–REVISED MARCH 2013
Signal-to-Noise Ratio
Signal-to-Noise Ratio (SNR) is the ratio of RMS magnitude of the fundamental to the RMS sum of all the non-
fundamental signal, excluding the harmonics, up to 1/2 of the sampling frequency (Nyquist).
Total Harmonic Distortion
Total Harmonic distortion is the ratio of the RMS sum of the amplitude of the harmonics to the fundamental input
frequency.
THD = 20 log [(V
2
2
+ V
3
2
+ V
4
2
+ V
5
2
+ V
6
2
)
1/2
/V
1
]
where
• V
1
is the RMS amplitude of the fundamental
• V
2
,V
3
, V
4
, V
5
, V
6
are the RMS amplitudes of the individual harmonics. (3)
In theory, all harmonics are included in THD calculations, but in practice only about the first 6 make significant
contributions and require measurement.
For under-sampling applications, the input signal should be band pass filtered (BPF) to prevent out of band
signals, or their harmonics, to appear in the spectral response.
The DC Linearity transfer function of an Analog-to-Digital Converter tends to influence the dominant harmonics.
A parabolic Linearity curve would tend to create 2
nd
(and even) order harmonics, while an S-curve would tend to
create 3
rd
(or odd) order harmonics. The magnitude of an DC linearity error correlates to the magnitude of the
harmonics.
Signal-to-Noise and Distortion
Signal-to-Noise And Distortion ratio (SINAD) is the ratio of RMS magnitude of the fundamental to the RMS sum
of all the non-fundamental signals, including the noise and harmonics, up to 1/2 of the sampling frequency
(Nyquist), excluding DC.
SINAD is also dependent on the number of quantization levels in the A/D Converter used in the waveform
sampling process. The more quantization levels, the smaller the quantization noise and theoretical noise
performance. The theoretical SINAD for a N-Bit Analog-to-Digital Converter is given by:
SINAD = (6.02 N + 1.76) dB
Thus, for an 8-bit converter, the ideal SINAD = 49.92 dB
Effective Number of Bits
Effective Number Of Bits (ENOB) is another specification to quantify dynamic performance. The equation for
ENOB is given by:
ENOB = [(SINAD - 1.76)] / 6.02]
The Effective Number Of Bits portrays the cumulative effect of several errors, including quantization, non-
linearities, noise, and distortion.
Spurious Free Dynamic Range
Spurious Free Dynamic Range (SFDR) is the ratio of the signal amplitude to the amplitude of the highest
harmonic or spurious noise component. If the amplitude is at full scale, the specification is simply the reciprocal
of the peak harmonic or spurious noise.
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