LTC1418AIN datasheet(11/28 Pages) LINER

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LTC1418

APPLICATIONS INFORMATION

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 AIN

– input is grounded). The AIN+ and

AIN– 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 —

µs 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 (<100

Ω) 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

100

Ω. 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.

®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. 375

µA 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. tACQ vs Source Resistance

SOURCE RESISTANCE (

Ω)

100

10k

1418 F06

0.1

100

100k

LinearView is a trademark of Linear Technology Corporation.

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