LTC1414 データシートの表示（PDF） - Linear Technology

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LTC1414

14-Bit, 2.2Msps, Sampling A/D Converter

Linear Technology 

LTC1414

APPLICATIONS INFORMATION

Choosing an Input Amplifier

Choosing an input amplifier is easy if a few requirements

are taken into consideration. First, to limit the magnitude

of the voltage spike seen by the amplifier from charging

the sampling capacitor, 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 unity-gain bandwidth of 50MHz, then the

output impedance at 50MHz must be less than 100Ω. The

second requirement is that the closed-loop bandwidth

must be greater than 40MHz 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 LTC1414 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 LTC1414. More detailed information is

available in the Linear Technology Databooks and on the

LinearViewTM CD-ROM.

LT®1223: 100MHz Video Current Feedback Amplifier.

6mA supply current. ±5V to ±15V supplies. Low noise.

Good for AC applications.

LT1227: 140MHz Video Current Feedback Amplifier. 10mA

supply current. ±5V to ±15V supplies. Low noise. Best for

AC applications.

LT1229/LT1230: Dual and Quad 100MHz Current Feed-

back Amplifiers. ±2V to ±15V supplies. Low noise. Good

AC specifications, 6mA supply current each amplifier.

LT1360: 50MHz Voltage Feedback Amplifier. 3.8mA sup-

ply current. Good AC and DC specs. ±5V to ±15V supplies.

70ns settling to 0.5LSB.

LT1363: 70MHz, 1000V/µs Op Amps. 6.3mA supply cur-

rent. Good AC and DC specifications. 60ns settling to

0.5LSB.

LT1364/LT1365: Dual and Quad 70MHz, 1000V/µs Op

Amps. 6.3mA supply current per amplifier. 60ns settling

to 0.5LSB.

LinearView is a trademark of Linear Technology Corporation.

10

AC Coupled Inputs

In applications where only the AC component of the analog

input is important, it may be desirable to AC couple the

input. This is easily accomplished by DC biasing the

LTC1414 analog input with a resistor to ground and using

a coupling capacitor to the input. Figure 7 shows a simple

AC coupled input circuit for the LTC1414 using only two

additional components. C1 is a 10µF ceramic capacitor

and R1 is a 1000Ω resistor to ground. R1 and C1 form a

highpass filter with a lower cut off frequency of 1/2π(C1)R1

or 15.9Hz.

ANALOG INPUT

C1

10µF

R1

1k

1µF

10µF

1 AIN+

2 AIN–

3

LTC1414

VREF

4

REFCOMP

5

AGND

LTC1414 • F07

Figure 7. AC Coupled Input

Differential Drive

In some applications the ADC drive circuitry is differential.

The differential drive can be applied directly to the LTC1414

without any special translation circuitry. Differential drive

can be advantageous at high frequencies (>1MHz) since it

provides improved THD and SFDR. Transformers can be

used to provide AC coupling, input scaling and single

ended to differential conversion as shown in Figure 8. The

resistor RS across the secondary will determine the input

impedance on the primary. The input impedance of the

primary RP will be related to the secondary load resistor RS

by the equation

RP = RS/n2

For example, if a Minicircuits T4-6T transformer is used,

the turns ratio is 2; if RS is 200Ω then RP is equal to 50Ω.

The center tap of the secondary will set the common

mode voltage and should be grounded for optimal AC

performance.

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