Low-power, 1-/2-channel, i – Rainbow Electronics MAX11645 User Manual
Page 20
MAX11644/MAX11645
20
______________________________________________________________________________________
Signal-to-Noise Ratio
For a waveform perfectly reconstructed from digital
samples, the theoretical maximum SNR is the ratio of
the full-scale analog input (RMS value) to the RMS
quantization error (residual error). The ideal, theoretical
minimum analog-to-digital noise is caused by quantiza-
tion error only and results directly from the ADC’s reso-
lution (N bits):
SNR
MAX[dB]
= 6.02dB x N + 1.76dB
In reality, there are other noise sources besides quanti-
zation noise: thermal noise, reference noise, clock jitter,
etc. SNR is computed by taking the ratio of the RMS
signal to the RMS noise, which includes all spectral
components minus the fundamental, the first five har-
monics, and the DC offset.
Signal-to-Noise Plus Distortion
Signal-to-noise plus distortion (SINAD) is the ratio of the
fundamental input frequency’s RMS amplitude to the
RMS equivalent of all other ADC output signals.
Effective Number of Bits
Effective number of bits (ENOB) indicates the global
accuracy of an ADC at a specific input frequency and
sampling rate. An ideal ADC’s error consists of quanti-
zation noise only. With an input range equal to the
ADC’s full-scale range, calculate the ENOB as follows:
ENOB = (SINAD - 1.76)/6.02
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sum of the input signal’s first five harmonics to the fun-
damental itself. This is expressed as:
where V
1
is the fundamental amplitude, and V
2
through
V
5
are the amplitudes of the 2nd- through 5th-order
harmonics.
Spurious-Free Dynamic Range
Spurious-free dynamic range (SFDR) is the ratio of the
RMS amplitude of the fundamental (maximum signal
component) to the RMS value of the next largest distor-
tion component.
THD
V
V
V
V
V
log
=
×
+
+
+
⎛
⎝
⎜
⎞
⎠
⎟
20
2
2
3
2
4
2
5
2
1
⎛⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
SINAD dB
Signal
Noise
THD
RMS
RMS
RMS
(
)
log
=
×
+
⎡
⎣
⎢
20
⎤⎤
⎦
⎥
GND
V
LOGIC
= 3V/5V
3V OR 5V
SUPPLIES
DGND
3V/5V
GND
*OPTIONAL
4.7
μF
R* = 5
Ω
0.1
μF
V
DD
DIGITAL
CIRCUITRY
MAX11644
MAX11645
Figure 14. Power-Supply Grounding Connection
Low-Power, 1-/2-Channel, I
2
C, 12-Bit ADCs
in Ultra-Tiny 1.9mm x 2.2mm Package