LTC2366 Linear Technology, LTC2366 Datasheet - Page 13

• Manufacturer Part Number: LTC2366
• Description: (LTC2365 / LTC2366) 12-Bit Serial Sampling ADCs
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
produce intermoduation 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 f
to the ADC input, nonlinearities in the ADC transfer function
can create distortion products at the sum and difference
frequencies of mf
For example, the 2nd order IMD terms include (f
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:
The LTC2365/LTC2366 have good IMD as shown in Figure
9a and Figure 9b respectively.
IMD(f
Figure 9a. LTC2365 Intermodulation Distortion Plot
a
–100
–140
–120
± f
–20
–60
–40
–80
0
b
0
) = 20log
V
f
f
f
IMD = –73.5dB
SMPL
b
b
DD
= 396kHz
= 424kHz
50
= 3V
a
±nf
= 1Msps
100 150
INPUT FREQUENCY (kHz)
b
, where m and n = 0, 1, 2, 3, etc.
Amplitude at (f
200
Amplitude at f
250
300
350
400
a
a
23656 F09a
and f
450 500
± f
a
b
)
b
are applied
a
± f
b
).
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spectral
component excluding the input signal and DC. This value
is expressed in decibels relative to the RMS value of a
full-scale input signal.
Full-Power and Full-Linear Bandwidth
The full-power bandwidth is that input frequency at which
the amplitude of reconstructed fundamental is reduced by
3dB for full-scale input signal.
The full-linear bandwidth is the input frequency at which
the SINAD has dropped to 68dB (11 effective bits). The
LTC2365/LTC2366 have been designed to optimize input
bandwidth, allowing the ADC to undersample input sig-
nals with frequencies above the converter’s Nyquist Fre-
quency. The noise fl oor stays very low at high frequencies;
SINAD becomes dominated by distortion at frequencies
far beyond Nyquist.
Figure 9b. LTC2366 Intermodulation Distortion Plot
–100
–120
–140
–20
–40
–60
–80
0
0
V
f
f
f
IMD = –71.5dB
SMPL
a
b
DD
= 935kHz
= 1.045kHz
250
= 3V
= 3Msps
LTC2365/LTC2366
INPUT FREQUENCY (kHz)
500
750
1000
1250
23656 F09b
1500
13
23656f