LTC1279 LINER [Linear Technology], LTC1279 Datasheet - Page 10

• Manufacturer Part Number: LTC1279
• Description: 12-Bit, 600ksps Sampling A/D Converter with Shutdown
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
LTC1279
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:
10
where V
quency and V
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.
IMD (fa ± fb) = 20log
THD = 20log
1
is the RMS amplitude of the fundamental fre-
Figure 4. Distortion vs Input Frequency
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
2
10k
through V
f
V
SAMPLE
THD
U
2
2
2ND HARMONIC
+ V
= 600kHz
INPUT FREQUENCY (Hz)
INFORMATION
3
U
Amplitude at (fa ± fb)
2
100k
N
+ V
V
Amplitude at fa
are the amplitudes of the
1
4
2
3RD HARMONIC
... + V
W
1M
N
1279 G06
2
2M
U
Figure 5 shows the IMD performance at a 100kHz input.
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
Figure 5. Intermodulation Distortion Plot
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
(fb – fa)
(2fa – fb)
50
(fa) (fb)
100
FREQUENCY (kHz)
(2fb – fa)
150
(2fa)
f
fa = 94.189kHz
fb = 97.705kHz
(fa + fb)(2fa + fb)
SAMPLE
200
(2fb)
(fa + 2fb)
= 600kHz
(3fa)
250
(3fb)
1279 G08
300