Datasheet

11
LTC1274/LTC1277
APPLICATIONS INFORMATION
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using an FFT algorithm, the ADCs’ spectral content can be
examined for frequencies outside the fundamental. Figures
2a and 2b show typical LTC1274 FFT plots.
Signal-to-Noise Ratio
The Signal-to-Noise plus Distortion Ratio [S/(N + D)] 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 above DC and below half the sam-
pling frequency. Figure 2a shows a typical spectral content
with a 100kHz sampling rate and a 48.85kHz input. The
dynamic performance is excellent for input frequencies well
beyond Nyquist as shown in Figure 2b and Figure 3.
INPUT FREQUENCY (kHz)
0
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
10 20 30 40
LTC1274/77 • F02a
50
f
SAMPLE
= 100kHz
f
IN
= 48.85kHz
Figure 2a. LTC1274 Nonaveraged, 4096 Point
FFT Plot with 50kHz Input Frequency
INPUT FREQUENCY (kHz)
0
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
10 20 30 40
LTC1274/77 • F02b
50
f
SAMPLE
= 100kHz
f
IN
= 97.68kHz
Figure 2b. LTC1274 Nonaveraged, 4096 Point
FFT Plot with 100kHz Input Frequency
Effective Number of Bits
The Effective Number of Bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to the
S/(N + D) by the equation:
N = [S/(N + D) – 1.76]/6.02
where N is the Effective Number of Bits of resolution and
S/(N + D) is expressed in dB. At the maximum sampling
rate of 100kHz, the LTC1274/LTC1277 maintain very
good ENOBs over 300kHz. Refer to Figure 3.
Total Harmonic Distortion
Total Harmonic Distortion (THD) is the ratio of the RMS
sum of all harmonics of the input signal to the fundamen-
tal 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 fre-
quency and V
2
through V
N
are the amplitudes of the
second through Nth harmonics. THD versus input fre-
INPUT FREQUENCY (Hz)
10k
EFFECTIVE NUMBER OF BITS (ENOBs)
12
11
10
9
8
7
6
5
4
3
2
1
0
S/(N + D)(dB)
74
68
62
56
50
100k 1M 2M
LTC1274/77 • F03
f
SAMPLE
= 100kHz
NYQUIST
FREQUENCY
Figure 3. ENOBs and S/(N + D) vs Input Frequency