Rainbow Electronics MAX1208 User Manual
Page 25
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. RMS noise includes all spec-
tral components to the Nyquist frequency excluding the
fundamental, the first six harmonics (HD2–HD7), and
the DC offset.
Signal-to-Noise Plus Distortion (SINAD)
SINAD is computed by taking the ratio of the RMS signal
to the RMS noise plus distortion. RMS noise plus distor-
tion includes all spectral components to the Nyquist fre-
quency excluding the fundamental and the DC offset.
Effective Number of Bits (ENOB)
ENOB specifies the dynamic performance of an ADC at
a specific input frequency and sampling rate. An ideal
ADC’s error consists of quantization noise only. ENOB for
a full-scale sinusoidal input waveform is computed from:
Single-Tone Spurious-Free Dynamic Range
(SFDR)
SFDR is the ratio expressed in decibels of the RMS
amplitude of the fundamental (maximum signal compo-
nent) to the RMS amplitude of the next-largest spurious
component, excluding DC offset.
Total Harmonic Distortion (THD)
THD is the ratio of the RMS sum of the first six harmon-
ics of the input signal to the fundamental itself. This is
expressed as:
where V
1
is the fundamental amplitude, and V
2
through
V
7
are the amplitudes of the 2nd- through 7th-order
harmonics (HD2–HD7).
Intermodulation Distortion (IMD)
IMD is the ratio of the RMS sum of the intermodulation
products to the RMS sum of the two fundamental input
tones. This is expressed as:
The fundamental input tone amplitudes (V
1
and V
2
) are
at -7dBFS. Fourteen intermodulation products (V
IM
_)
are used in the MAX1208 IMD calculation. The inter-
modulation products are the amplitudes of the output
spectrum at the following frequencies, where f
IN1
and
f
IN2
are the fundamental input tone frequencies:
• Second-order intermodulation products:
f
IN1
+ f
IN2
, f
IN2
- f
IN1
• Third-order intermodulation products:
2 x f
IN1
- f
IN2
, 2 x f
IN2
- f
IN1
, 2 x f
IN1
+ f
IN2
, 2 x f
IN2
+ f
IN1
• Fourth-order intermodulation products:
3 x f
IN1
- f
IN2
, 3 x f
IN2
- f
IN1
, 3 x f
IN1
+ f
IN2
, 3 x f
IN2
+ f
IN1
• Fifth-order intermodulation products:
3 x f
IN1
- 2 x f
IN2
, 3 x f
IN2
- 2 x f
IN1
, 3 x f
IN1
+ 2 x f
IN2
,
3 x f
IN2
+ 2 x f
IN1
Third-Order Intermodulation (IM3)
IM3 is the total power of the third-order intermodulation
products to the Nyquist frequency relative to the total
input power of the two input tones f
IN1
and f
IN2
. The
individual input tone levels are at -7dBFS. The third-
order intermodulation products are 2 x f
IN1
- f
IN2
, 2 x
f
IN2
- f
IN1
, 2 x f
IN1
+ f
IN2
, 2 x f
IN2
+ f
IN1
.
Two-Tone Spurious-Free Dynamic Range
(SFDR
TT
)
SFDR
TT
represents the ratio, expressed in decibels, of
the RMS amplitude of either input tone to the RMS ampli-
tude of the next-largest spurious component in the spec-
trum, excluding DC offset. This spurious component can
occur anywhere in the spectrum up to Nyquist and is usu-
ally an intermodulation product or a harmonic.
Aperture Delay
The MAX1208 samples data on the falling edge of its
sampling clock. In actuality, there is a small delay
between the falling edge of the sampling clock and the
actual sampling instant. Aperture delay (t
AD
) is the time
defined between the falling edge of the sampling clock
and the instant when an actual sample is taken (Figure 4).
Aperture Jitter
Figure 4 depicts the aperture jitter (t
AJ
), which is the
sample-to-sample variation in the aperture delay.
IMD
V
V
V
V
V
V
IM
IM
IM
IM
log
.......
=
×
+
+
+
+
+
20
1
2
2
2
13
2
14
2
1
2
2
2
THD
V
V
V
V
V
V
V
log
=
×
+
+
+
+
+
20
2
2
3
2
4
2
5
2
6
2
7
2
1
ENOB
SINAD
.
.
=
−
1 76
6 02
MAX1208
12-Bit, 80Msps, 3.3V ADC
______________________________________________________________________________________
25