Datasheet

9
LTC1414
APPLICATIONS INFORMATION
WUU
U
IMD f f
amplitude at f f
amplitude at f
ab
ab
a
±
()
=
±
()








20log
The full-linear bandwidth is the input frequency at which
the S/(N + D) has dropped to 74dB (12 effective bits). The
LTC1414 has been designed to optimize input bandwidth,
allowing the ADC to undersample input signals with fre-
quencies 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 be-
yond Nyquist.
Driving the Analog Input
The differential analog inputs of the LTC1414 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 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 LTC1414
inputs can be driven directly. As source impedance
increases so will acquisition time (see Figure 6). For
minimum acquisition time, with high source impedance, a
buffer amplifier should 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 (settling time must be 70ns for full throughput rate).
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 dB 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.
FREQUENCY (kHz)
0 400 800200 600 1000
AMPLITUDE (dB)
1414 F05a
0
–20
–40
–60
–80
–100
–120
f
SAMPLE
= 2.2MHz
f
IN1
= 80.566kHz
f
IN2
= 97.753kHz
FREQUENCY (kHz)
0 400 800200 600 1000
AMPLITUDE (dB)
1414 F05b
0
–20
–40
–60
–80
–100
–120
f
SAMPLE
= 2.2MHz
f
IN1
= 970.019kHz
f
IN2
= 1.492MHz
Figure 6. Acquisition Time vs Source Resistance
SOURCE RESISTANCE (Ω)
10
0.01
ACQUISITION TIME (µs)
0.1
1
10
100 1k
1414 FO6
10k 100k
Figure 5a. Intermodulation Distortion Plot
with Inputs at 80kHz and 97kHz
Figure 5b. Intermodulation Distortion Plot
with Input Signals of 1MHz and 1.5MHz