Datasheet

MAX11634–MAX11637
Signal-to-Noise Ratio
For a waveform perfectly reconstructed from digital
samples, signal-to-noise ratio (SNR) is the ratio of the
full-scale analog input (RMS value) to the RMS quanti-
zation error (residual error). The ideal, theoretical mini-
mum analog-to-digital noise is caused by quantization
error only and results directly from the ADC’s resolution
(N bits):
SNR = (6.02 x N + 1.76)dB
In reality, there are other noise sources besides quanti-
zation noise, including thermal noise, reference noise,
clock jitter, etc. Therefore, SNR is calculated by taking
the ratio of the RMS signal to the RMS noise, which
includes all spectral components minus the fundamen-
tal, the first five harmonics, 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:
SINAD (dB) = 20 x log (Signal
RMS
/Noise
RMS
)
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 error consists of quantiza-
tion noise only. With an input range equal to the full-
scale range of the ADC, calculate the effective number
of bits as follows:
ENOB = (SINAD - 1.76)/6.02
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sum of the first five harmonics of the input signal to the
fundamental itself. This is expressed as:
where V
1
is the fundamental amplitude, and V
2
–V
5
are
the amplitudes of the 2nd-order to 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.
logTHD x VVVV V= +++
()
⎡
⎣
20
2
2
3
2
4
2
5
2
1
⎢⎢
⎤
⎦
⎥
12-Bit, 300ksps ADCs with Differential
Track/Hold, and Internal Reference
22 ______________________________________________________________________________________
Figure 9. Bipolar Transfer Function, Full Scale (±FS) = ±V
REF
/2
OUTPUT CODE
FULL-SCALE
TRANSITION
11. . .111
11. . .110
11. . .101
00. . .011
00. . .010
00. . . 001
00. . . 000
123
0
(COM)
FS
FS - 3/2 LSB
FS = V
REF
+ V
COM
ZS = V
COM
INPUT VOLTAGE (LSB)
1 LSB =
V
REF
4096
011. . . 111
011. . .110
000. . . 010
000. . .001
000. . .000
111 . . .111
111 . . . 110
111 . . . 101
100 . . . 001
100. . . 000
- FS
COM*
INPUT VOLTAGE (LSB)
OUTPUT CODE
ZS = COM
+FS - 1 LSB
*V
COM
≥ V
REF
/ 2
+
V
COM
+ V
COM
FS
=
V
REF
2
-FS =
-V
REF
2
1 LSB =
V
REF
4096
Figure 8. Unipolar Transfer Function, Full Scale (FS) = V
REF