LTC2360/LTC2361/LTC2362
12
236012fa
APPLICATIONS INFORMATION
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 fre-
quency 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
500kHz sampling rate and a 248kHz input. The dynamic
performance is excellent for input frequencies up to and
beyond the Nyquist frequency of 250kHz.
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 resolution
and SINAD is expressed in dB. At the maximum sampling
rate of 500kHz, the LTC2362 maintains ENOB above 11
bits up to the Nyquist input frequency of 250kHz (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
1
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 vs Input Frequency
is shown in Figure 8. The LTC2362 has excellent distortion
performance up to the Nyquist frequency and beyond.
Figure 7. LTC2362 ENOB and SINAD vs Input Frequency
Figure 8. LTC2362 THD vs Input Frequency
INPUT FREQUENCY (kHz)
SINAD (dB)
ENOB
2306012 F07
74
72
73
70
71
12
67
69
68
11.67
11.34
11
1 100 100010
V
DD
= 3.6V
V
DD
= 3.0V
V
DD
= 2.35V
INPUT FREQUENCY (kHz)
THD (dB)
2306012 F08
–67
–75
–71
–87
–79
–83
–91
1 100 100010
V
DD
= 3.6V
V
DD
= 3.0V
V
DD
= 2.35V
