11
LTC1417
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, 2nd order IMD terms include (fa ± fb). 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 fa fb Log
Amplitude
+
()
=
±
()
20
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 the input frequency at which
the amplitude of the reconstructed fundamental is
reduced by 3dB from 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 LTC1417 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 LTC1417 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 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 LTC1417
inputs can be driven directly. As source impedance
increases, so will acquisition time (see Figure 7). For
minimum acquisition time, with high source impedance, a
buffer amplifier must be used. The only requirement is that
the amplifier driving the analog input(s) must settle after
the small current spike before the next conversion starts —
500ns for full throughput rate.
APPLICATIONS INFORMATION
WUU
U
Figure 5. Distortion vs Input Frequency
INPUT FREQUENCY (kHz)
1
–120
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
–100
–80
–60
–40
0
10 100
THD
2ND
3RD
1417 G05
1000
–20
Figure 6. Intermodulation Distortion Plot
FREQUENCY (kHz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
40 100
140
200
1417 G09
–20
0
20 60 80
120
160 180
f
SAMPLE
= 400kHz
f
IN1
= 97.303466kHz
f
IN2
= 104.632568kHz
V
IN
= 4.096V
P-P
