LTC1278-4CSW#TRPBF Linear Technology, LTC1278-4CSW#TRPBF Datasheet - Page 10

• Manufacturer Part Number: LTC1278-4CSW#TRPBF
• Description: IC A/DCONV SAMPLNG W/SHTDN24SOIC
• Manufacturer: Linear Technology

Specifications of LTC1278-4CSW#TRPBF

• Number Of Bits: 12
• Sampling Rate (per Second): 400k
• Data Interface: Parallel
• Number Of Converters: 1
• Power Dissipation (max): 150mW
• Voltage Supply Source: Dual ±
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 24-SOIC (0.300", 7.50mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS
frequency is shown in Figure 4. The LTC1278 has good
distortion performance up to the Nyquist frequency and
beyond.
LTC1278
10
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:
Figure 5 shows the IMD performance at a 100kHz input.
IMD (fa ± fb) = 20log
–100
Figure 4. Distortion vs Input Frequency
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
10k
f
SAMPLE
U
THD
2ND HARMONIC
= 500kHz
INPUT FREQUENCY (Hz)
INFORMATION
U
Amplitude at (fa ± fb)
100k
Amplitude at fa
3RD HARMONIC
W
1M
LT1278 G6
2M
U
Driving the Analog Input
The analog input of the LTC1278 is easy to drive. It draws
only one small current spike while charging the sample-
and-hold capacitor at the end of conversion. During con-
version 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
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
LTC1278 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.
–100
–120
Figure 5. Intermodulation Distortion Plot
–20
–40
–60
–80
0
0
50k
FREQUENCY (Hz)
100k
150k
f
f
f
SAMPLE
IN1
IN2
= 96.80kHz
= 101.68kHz
200k
= 500kHz
LTC1278 G8
250k