LTC2365/LTC2366
12
23656fb
For more information www.linear.com/LTC2365
Signal-to-Noise Plus Distortion Ratio
The signal-to-noise plus distortion ratio (SINAD) is the
ratio between the RMS amplitude of the fundamental input
frequency to the RMS amplitude of all other frequency
components at the A/D output. The output is band limited
to frequencies from above DC and below half the sampling
frequency. Figure 6 shows a typical FFT with a 3MHz sam-
pling rate and a 1MHz input. The dynamic performance
is excellent for input frequencies up to and beyond the
Nyquist frequency of 1.5MHz.
Effective Number of Bits
The effective number of bits (ENOB) is a measurement
of the resolution of an ADC and is directly related to
SINAD by the equation:
ENOB = (SINAD – 1.76)/6.02
where ENOB is the effective number of bits of resolu-
tion and SINAD is expressed in dB. At the maximum
sampling
rate
of 3MHz, the LTC2366 maintains ENOB
above 11 bits up to the Nyquist input frequency of 1.5MHz
(refer to Figure 7).
Total Harmonic Distortion
The total harmonic distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal to the fundamental
itself. The out-of-band harmonics alias into the frequency
band between DC and half the sampling
frequency. THD
is expressed as:
THD = 20log
V
2
2
+ V
3
2
+ V
4
2
+ ...V
n
2
V
where V
1
is the RMS amplitude of the fundamental
frequency and V
2
through V
n
are the amplitudes of the
second through nth harmonics. THD versus Input Fre-
quency is shown in Figure 8. The LTC2366 has excellent
distortion performance up to the Nyquist frequency and
beyond.
Figure 8. LTC2366 Distortion vs Input Frequency
applications inForMation
Figure 7. LTC2366 ENOB and SINAD vs Input Frequency
INPUT FREQUENCY (kHz)
100
70.0
SINAD (dB)
70.5
71.0
71.5
72.0
72.5
11.34
11.50
11.67
1000 1500
23656 F07
V
DD
= 2.35V
V
DD
= 3.6V
V
DD
= 3V
INPUT FREQUENCY (kHz)
100
THD (dB)
–80
–78
–76
1500
23656 F08
–82
–84
–88
1000
–86
–72
–74
V
DD
= 2.35V
V
DD
= 3.6V
V
DD
= 3V
R
IN
= 10Ω
