LTC1414 データシートの表示（PDF） - Linear Technology
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LTC1414 Datasheet PDF : 20 Pages
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LTC1414
APPLICATIONS INFORMATION
0
SINAD = 78dB
–20 SFDR = 84dB
fSAMPLE = 2.2MHz
–40 fIN = 997.949kHz
–60
–80
–100
–120
0
200 400 600 800 1000
FREQUENCY (kHz)
1414 F02b
Figure 2b. LTC1414 2048 Point FFT,
Input Frequency = 1MHz
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:
ENOBS = [S/(N + D) – 1.76]/6.02
where S/(N + D) is expressed in dB. At the maximum
sampling rate of 2.2MHz the LTC1414 maintains near ideal
ENOBs up to the Nyquist input frequency of 1.1MHz. 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 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 V22 + V32 + V42 + …VN2
V1
where V1 is the RMS amplitude of the fundamental fre-
quency and V2 through VN are the amplitudes of the
second through Nth harmonics. THD vs input frequency is
shown in Figure 4. The LTC1414 has good distortion
performance up to the Nyquist frequency and beyond.
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12
11
10
9
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7
6
5
4
3 fSAMPLE = 2.2MHz
2
1k
10k
100k
1M
INPUT FREQUENCY (Hz)
86
80
74
68
10M
1414 TA02
Figure 3. Effective Bits and Signal/(Noise + Distortion)
vs Input Frequency
0
–10
–20
–30
–40
–50
THD
–60
–70
–80
–90
– 100
1
2nd
3rd
10k
100k
1M
INPUT FREQUENCY (Hz)
10M
1414 F04
Figure 4. Distortion vs Input Frequency
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
produce intermodulation distortion (IMD) in addition to
the THD. IMD is the change in one sinusoidal input caused
by the presence of another sinusoidal input at a different
frequency.
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at the sum and differ-
ence frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3
etc. For example, the 2nd order IMD terms include (fa ± fb).
If the two input sine waves are equal in magnitude, the
value (in dB) of the 2nd order IMD products can be
expressed by the following formula:
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