10
LTC1279
APPLICATIONS INFORMATION
WUU
U
THD = 20log
√V
2
2
+ V
3
2
+ V
4
2
... + V
N
2
V
1
where V
1
is the RMS amplitude of the fundamental fre-
quency and V
2
through V
N
are the amplitudes of the
second through Nth harmonics. THD versus input fre-
quency is shown in Figure 4. The LTC1279 has good
distortion performance 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
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 sum and difference
frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (fa + fb) and
(fa – fb) while the 3rd order IMD terms include (2fa + fb),
(2fa – fb), (fa + 2fb), and (fa – 2fb). 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) = 20log
Amplitude at (fa ± fb)
Amplitude at fa
INPUT FREQUENCY (Hz)
10k
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
100k 1M
1279 G06
2M
THD
2ND HARMONIC
3RD HARMONIC
f
SAMPLE
= 600kHz
Figure 4. Distortion vs Input Frequency
Figure 5 shows the IMD performance at a 100kHz input.
Figure 5. Intermodulation Distortion Plot
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 re-
duced 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 68dB (11 effective bits). The
LTC1279 has been designed to optimize input bandwidth,
allowing ADC to undersample input signals with frequen-
cies above the converter’s Nyquist Frequency. The noise
floor stays very low at high frequencies; S/(N + D) be-
comes dominated by distortion at frequencies far beyond
Nyquist.
Driving the Analog Input
The LTC1279’s analog input is easy to drive. It draws only
one small current spike while charging the sample-and-
hold capacitor at the end of conversion. During conversion
the analog input draws no current. The only requirement
is that the amplifier driving the analog input must settle
after the small current spike before the next conversion
starts. Any op amp that settles in 160ns to small current
transients will allow maximum speed operation. If slower
FREQUENCY (kHz)
0
AMPLITUDE (dB)
50
100 150 200
1279 G08
250 300
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
SAMPLE
= 600kHz
fa = 94.189kHz
fb = 97.705kHz
(fb – fa)
(2fa – fb)
(2fa)
(2fb)
(3fa)
(2fb – fa)
(fa) (fb)
(fa + fb)
(2fa + fb)
(fa + 2fb)
(3fb)
