Datasheet

11
LTC1418
APPLICATIONS INFORMATION
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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 LTC1418 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 LTC1418 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 LTC1418
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 must 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 —
1µs for full throughput rate.
Choosing an Input Amplifier
Choosing an input amplifier is easy if a few requirements
are taken into consideration. First, choose an amplifier that
has a low output impedance (<100Ω) at the closed-loop
bandwidth frequency. For example, if an amplifier is used
in a gain of 1 and has a closed-loop bandwidth of 10MHz,
then the output impedance at 10MHz must be less than
100Ω. The second requirement is that the closed-loop
bandwidth must be greater than 5MHz to ensure adequate
small-signal settling for full throughput rate. If slower op
amps are used, more settling time can be provided by
increasing the time between conversions.
The best choice for an op amp to drive the LTC1418 will
depend on the application. Generally, applications fall into
two categories: AC applications where dynamic specifica-
tions are most critical and time domain applications where
DC accuracy and settling time are most critical. The
following list is a summary of the op amps that are suitable
for driving the LTC1418. More detailed information is
available in the Linear Technology Databooks and on the
LinearView
TM
CD-ROM.
LT
®
1354: 12MHz, 400V/µs Op Amp. 1.25mA maximum
supply current. Good AC and DC specifications. Suitable
for dual supply application.
LT1357: 25MHz, 600V/µs Op Amp. 2.5mA maximum
supply current. Good AC and DC specifications. Suitable
for dual supply application.
LT1366/LT1367: Dual/Quad Precision Rail-to-Rail Input
and Output Op Amps. 375µA supply current per amplifier.
1.8V to ±15V supplies. Low input offset voltage: 150µV.
Good for low power and single supply applications with
sampling rates of 20ksps and under.
LT1498/LT1499: 10MHz, 6V/µs, Dual/Quad Rail-to-Rail
Input and Output Op Amps. 1.7mA supply current per
Figure 6. t
ACQ
vs Source Resistance
SOURCE RESISTANCE (Ω)
1
ACQUISITION TIME (µs)
10
1 100 1k 10k
1418 F06
0.1
10
100
100k
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