LTC1419 LINER [Linear Technology], LTC1419 Datasheet - Page 9

• Manufacturer Part Number: LTC1419
• Description: 14-Bit, 800ksps Sampling A/D Converter with Shutdown
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
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
–100
– 110
–100
–120
– 60
– 20
– 40
–10
–20
–30
–40
–50
–60
–70
–80
–90
– 80
Figure 5. Intermodulation Distortion Plot
Figure 4. Distortion vs Input Frequency
0
0
1k
0
50
U
100
INPUT FREQUENCY (Hz)
10k
FREQUENCY (kHz)
150
INFORMATION
U
200
f
f
f
SAMPLE
IN1
IN2
100k
= 95.8984375kHz
= 104.1015625kHz
250
3RD
= 800kHz
THD
300
W
350
2ND
1M 2M
1419 G05
1419 G03
400
U
(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:
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
and – A
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
IMD fa fb
IN
inputs are sampled at the same instant. Any
20
Log
IN
Amplitude at (fa + fb)
input is grounded). The + A
Amplitude at fa
LTC1419
9
IN