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LTC1415 Datasheet PDF : 24 Pages
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LTC1415
APPLICATIONS INFORMATION
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 1.25MHz the LTC1415 maintains very good ENOBs
up to the Nyquist input frequency of 625kHz (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
12
11
10
9
8
7
6
5
4
3
2
1
0
1k
74
68
62
56
10k
100k
INPUT FREQUENCY (Hz)
1M 2M
LT1415 • F03
Figure 3. Effective Bits and Signal/(Noise +
Distortion) vs Input 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 LTC1415 has good distortion
performance up to the Nyquist frequency and beyond.
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
0
–10
–20
–30
–40
–50
–60
THD
–70
–80
2ND
–90
3RD
– 100
1k
10k
100k
INPUT FREQUENCY (Hz)
1M 2M
LTC1415 • F04
Figure 4. Distortion vs Input Frequency
0
–20
–40
fb – fa
–60
2fa – fb
–80
2fb – fa
fa + fb
2fa 2fb
3fa
2fa + fb
fa + 2fb
3fb
fSAMPLE = 1.25MHz
fIN1 = 86.97509766kHz
fIN2 = 113.2202148kHz
–100
–120
0
100k
200k
300k
400k
FREQUENCY (Hz)
500k
600k
LTC1415 • F05
Figure 5. Intermodulation Distortion Plot
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