Datasheet

LTC2360/LTC2361/LTC2362
13
236012fa
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
a
and f
b
are applied
to the ADC input, nonlinearities in the ADC transfer function
can create distortion products at the sum and difference
frequencies of mf
a
± nf
b
, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (f
a
± f
b
).
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 f
a
± f
b
()
= 20log
Amplitude at f
a
± f
b
()
Amplitude at f
a
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 LTC2362
has been designed to optimize input bandwidth, allowing the
ADC to undersample input signals with frequencies above
the converter’s Nyquist frequency. The noise fl oor stays
very low at high frequencies; SINAD becomes dominated
by distortion at frequencies far beyond Nyquist.
Figure 9. LTC2362 Intermodulation Distortion Plot
INPUT FREQUENCY (kHz)
0
MAGNITUDE (dB)
0
–20
–60
–100
–40
–80
–120
100 200
236012 F09
25050 150
V
DD
= 3.6V
f
SMPL
= 500ksps
f
a
= 99kHz
f
b
= 101kHz
IMD = –76.5dB