LTC1406 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1406' PDF : 16 Pages View PDF
LTC1406
APPLICATIONS INFORMATION
0
–10
fSAMPLE = 20MHz
fIN1 = 1.000977MHz
–20
SFDR = 64.8dB
SINAD = 48.6dB
–30
–40
–50
–60
–70
–80
–90
–100
0 1 2 3 4 5 6 7 8 9 10
FREQUENCY (Hz)
1406 F02a
Figure 2a. Nonaveraged, 4096 Point FFT
Input Frequency = 1MHz
0
–10
fSAMPLE = 20MHz
fIN1 = 28.99902MHz
–20 SFDR = 54.9dB
SINAD = 47.0dB
–30
–40
–50
–60
–70
–80
–90
–100
0 1 2 3 4 5 6 7 8 9 10
FREQUENCY (Hz)
1406 F02b
Figure 2b. Nonaveraged, 4096 Point FFT
Input Frequency = 30MHz
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 and the RMS amplitude of all other frequency
components at the ADC output. The output is band limited
to frequencies above DC to below half the sampling fre-
quency. The effective number of bits (ENOBs) is a mea-
surement of the resolution of an ADC and is directly related
to the S/(N + D) by the equation:
ENOB = [S/(N + D) – 1.76]/6.02
where ENOB is the effective number of bits and S/(N + D)
is expressed in dB. At the maximum sampling rate of 20MHz
the LTC1406 maintains near ideal ENOBs up to and be-
yond the Nyquist input frequency of 10MHz (see Figure 3).
8
50
7
44
6
38
5
32
4
3
2
1
0
100k
1M
10M
INPUT FREQUENCY (Hz)
100M
1406 TA02
Figure 3. Effective Bits and Signal-to-(Noise + Distortion)
vs Input Frequency
Total Harmonic Distortion
Total harmonic distortion 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 = 20 log 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 sec-
ond through nth harmonics. THD vs Input Frequency is
shown in Figure 4. The LTC1406 has good distortion per-
formance 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
the presence of another sinusoidal input at a different fre-
quency (see Figure 5).
8
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