LTC1418C View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC1418C

Low Power, 14-Bit, 200ksps ADC with Serial and Parallel I/O

Linear Technology 

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 effec-

tive bits). The LTC1418 has been designed to optimize

input bandwidth, 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 imped-

ance 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 acquisi-

tion 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.

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 fol-

lowing list is a summary of the op amps that are suitable

for driving the LTC1418. More detailed information is

available at www.linear.com.

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

amplifier. 2.2V to ±15V supplies. Good AC performance,

input noise voltage = 12nV/√Hz (typ).

100

10

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

1

0.1

1

10

100

1k

10k 100k

SOURCE RESISTANCE (Ω)

1418 F06

Figure 6. tACQ vs Source Resistance

1418fa

12

For more information www.linear.com/LTC1418

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