LTC1274 Linear Technology, LTC1274 Datasheet - Page 11

• Manufacturer Part Number: LTC1274
• Description: 12-Bit/ 10mW/ 100ksps ADCs with 1uA Shutdown
• Manufacturer: Linear Technology

APPLICATIONS
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.
Figure 2a. LTC1274 Nonaveraged, 4096 Point
FFT Plot with 50kHz Input Frequency
Figure 2b. LTC1274 Nonaveraged, 4096 Point
FFT Plot with 100kHz Input Frequency
–100
–120
–100
–120
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
f
f
SAMPLE
IN
= 48.85kHz
10
10
U
= 100kHz
INPUT FREQUENCY (kHz)
INPUT FREQUENCY (kHz)
20
20
INFORMATION
U
f
30
30
SAMPLE
f
IN
= 97.68kHz
W
= 100kHz
LTC1274/77 • F02a
LTC1274/77 • F02b
40
40
50
50
U
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:
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:
where V
quency and V
second through Nth harmonics. THD versus input fre-
N = [S/(N + D) – 1.76]/6.02
THD = 20log
Figure 3. ENOBs and S/(N + D) vs Input Frequency
1
is the RMS amplitude of the fundamental fre-
12
11
10
9
8
7
6
5
4
3
2
1
0
10k
f
2
SAMPLE
through V
V
2
= 100kHz
INPUT FREQUENCY (Hz)
2
+ V
LTC1274/LTC1277
100k
FREQUENCY
3
NYQUIST
2
N
+ V
V
are the amplitudes of the
1
4
2
... + V
LTC1274/77 • F03
1M
N
2M
2
74
68
62
56
50
11