LTC1406IGN datasheet(10/16 Pages) LINER

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LTC1406

APPLICATIONS INFORMATION

single input providing a

±1V bipolar input range centered

around AIN–. Likewise, AIN+ can be tied to a fixed voltage

and AIN

– used as the single input. In any configuration the

maximum output code (1111 1111) occurs when [(AIN

– (AIN–)] = 1V and the minimum output code (0000 0000)

occurs when [(AIN

+) – (AIN–)] = –1V.

Each analog input can swing from ground to VDD but not

beyond. Therefore, the input common mode voltage can

range from 0.5V to 4.5V in differential mode and from 1V

to 4V in single-ended mode.

As an example, with AIN– connected to the VREF pin (2.5V)

the input range will be 1.5V to 3.5V (see Figure 8a). To

achieve other ranges the input may be capacitively coupled

to achieve a 2V span with virtually any common mode

voltage (see Figure 8b).

The 2V input span requires a 2.5V external reference be

connected to the VREF pin. The LT1460-2.5 micropower

precision series reference is recommended. To achieve

other input spans, the reference voltage (VREF) can vary

between 2V to 3V. The VREF pin can also be driven with a

DAC or other means. This is useful in applications where

the peak input signal amplitude may vary. The input span

of the ADC can then be adjusted to match the peak input

signal, maximizing the signal-to-noise ratio.

The analog inputs of the LTC1406 are easy to drive. The

inputs draw only one small current spike while charging

the sample-and-hold capacitors following a rising CLK edge.

INPUT FREQUENCY (Hz)

100k

10M

100M

1406 G08

Figure 7. Common Mode Rejection

vs Input Frequency

Figure 8b. AC Coupled

Figure 8a. DC Coupled

AIN

ANALOG INPUT

1.5V TO 3.5V

2.5V

1406 F08a

AIN

–

LTC1406

VREF

AIN

ANALOG INPUT

2V SPAN

2.5V

1406 F08b

AIN

–

LTC1406

VREF

While CLK is low the analog inputs draw only a small leak-

age current. If the source impedance of the driving circuit

is low, then the LTC1406 inputs can be driven directly. As

source impedance increases, so will acquisition time. 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 25ns for full throughput rate).

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 (< 50

Ω) 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 out-

put impedance at 50MHz must be less than 50

Ω. The

second requirement is that the closed-loop bandwidth must

be greater than 70MHz to ensure adequate small-signal

settling for full throughput rate.

The following list is a summary of the op amps that are

suitable for driving the LTC1406. More detailed informa-

tion is available in the Linear Technology Databooks and

on the LinearView

TM CD-ROM.

®1223: 100MHz Video Current Feedback Amplifier. 6mA

supply current.

±5V to ±15V supplies. Low noise.

LT1227: 140MHz Video Current Feedback Amplifier. 10mA

supply current.

±5V to ±15V supplies. Low distortion.

Low noise.

LinearView is a trademark of Linear Technology Corporation.

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