1400 データシートの表示（PDF） - Linear Technology
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1400 Datasheet PDF : 20 Pages
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LTC1400
APPLICATIO S I FOR ATIO
0
–10
–20
fSAMPLE = 400kHz
fIN = 199.121kHz
SINAD = 72.1dB
–30 THD = – 80dB
–40
–50
– 60
–70
–80
–90
–100
–110
–120
0 20 40 60 80 100 120 140 160 180 200
FREQUENCY (kHz)
1400 F02b
Figure 2b. LTC1400 Nonaveraged, 4096 Point FFT
Plot with 200kHz Input Frequency in Bipolar Mode
where N is the effective number of bits of resolution and
S/(N + D) is expressed in dB. At the maximum sampling
rate of 400kHz, the LTC1400 maintains very good ENOBs
up to the Nyquist input frequency of 200kHz (refer to
Figure 3).
12
11
10
NYQUIST
FREQUENCY
9
8
7
6
5
4
3
2
1 fSAMPLE = 400kHz
0
10k
100k
INPUT FREQUENCY (Hz)
74
68
62
56
50
1M
1400 F03
Figure 3. Effective Bits and Signal-to-Noise +
Distortion vs Input Frequency in Bipolar Mode
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 of the sampling frequency. THD
is expressed as:
8
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 LTC1400 has good distortion performance
up to the Nyquist frequency and beyond.
0
–10 fSAMPLE = 400kHz
–20
–30
–40
–50
–60
–70
3RD HARMONIC
THD
–80
–90
–100
10k
2ND HARMONIC
100k
INPUT FREQUENCY (Hz)
1M
1400 F04
Figure 4. Distortion vs Input Frequency in Bipolar Mode
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
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 sum and difference
frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (fa + fb)
and (fa – fb) while the 3rd order IMD terms includes (2fa
+ fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). If the two input
sine waves are equal in magnitude, the value (in decibels)
of the 2nd order IMD products can be expressed by the
following formula.
IMD(fa
±
fb)
=
20log
Amplitude at (fa
Amplitude at
±
fa
fb)
1400fa
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