9
LTC1419
1419fb
APPLICATIONS INFORMATION
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Figure 4. Distortion vs Input Frequency
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.
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at the sum and differ-
ence frequencies of mfa ±nfb, where m and n = 0, 1, 2, 3,
etc. For example, the 2nd order IMD terms include
(fa + fb). If the two input sine waves are equal in magni-
tude, the value (in decibels) of the 2nd order IMD products
can be expressed by the following formula:
IMD fa fb Log+
()
= 20
Amplitude at (fa + fb)
Amplitude at fa
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec-
tral 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 the reconstructed fundamental is
reduced by 3dB for a full-scale input signal.
The full-linear bandwidth is the input frequency at which
the S/(N + D) has dropped to 77dB (12.5 effective bits).
The LTC1419 has been designed to optimize input band-
width, allowing the ADC to undersample input signals with
frequencies above the converter’s Nyquist Frequency. The
noise floor stays very low at high frequencies; S/(N + D)
becomes dominated by distortion at frequencies far
beyond Nyquist.
Driving the Analog Input
The differential analog inputs of the LTC1419 are easy to
drive. The inputs may be driven differentially or as a single-
ended input (i.e., the –A
IN
input is grounded). The +A
IN
and – A
IN
inputs are sampled at the same instant. Any
unwanted signal that is common mode to both inputs will
be reduced by the common mode rejection of the sample-
and-hold circuit. The inputs draw only one small current
spike while charging the sample-and-hold capacitors at
the end of conversion. During conversion, the analog
inputs draw only a small leakage current. If the source
impedance of the driving circuit is low, then the LTC1419
inputs can be driven directly. As source impedance in-
creases so will acquisition time (see Figure 6). For mini-
mum acquisition time with high source impedance, a
buffer amplifier should be used. The only requirement is
Figure 5. Intermodulation Distortion Plot
FREQUENCY (kHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
100 200
300
400
1419 G05
–20
0
50 150
250
350
f
SAMPLE
= 800kHz
f
IN1
= 95.8984375kHz
f
IN2
= 104.1015625kHz
INPUT FREQUENCY (Hz)
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
1419 G03
THD
2ND
3RD
1k 100k 1M 2M10k
