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LTC1404CS8 Datasheet PDF : 24 Pages
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LTC1404
APPLICATIONS INFORMATION
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 frequency
components at the A/D output. The output is band limited
to frequencies from DC to half the sampling frequency.
Figure 2a shows a typical spectral content with a 600kHz
sampling rate and a 100kHz input. The dynamic perfor-
mance is excellent for input frequencies up to the Nyquist
limit of 300kHz as shown in Figure 2b.
Effective Number of Bits
The effective number of bits (ENOBs) is a measurement of
the effective 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 600kHz, the LTC1404 maintains very good ENOBs
up to the Nyquist input frequency of 300kHz (refer to
Figure 3).
12
11
10
NYQUIST
FREQUENCY
9
8
7
6
5
4
3
2
1 fSAMPLE = 600kHz
0
10k
100k
INPUT FREQUENCY (Hz)
74
68
62
56
50
1M
1404 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:
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 LTC1404 has good distortion
performance up to the Nyquist frequency and beyond.
0
–10 fSAMPLE = 600kHz
–20
–30
–40
–50
–60
–70
3RD HARMONIC
–80
2ND HARMONIC
–90
– 100
10k
100k
INPUT FREQUENCY (Hz)
THD
1M
1404 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.
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