LTC6419 datasheet(11/24 Pages) AD

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LTC6419

6419fa

For more information www.linear.com/LTC6419

APPLICATIONS INFORMATION

Functional Description

The LTC6419 is a small outline, wideband, high speed, low

noise, and low distortion fully-differential dual amplifier

with accurate output phase balancing. The amplifier is opti-

mized to drive low voltage, single-supply, differential input

analog-to-digital converters (ADCs). The LTC6419 input

common mode range includes ground, which makes it ideal

to DC-couple and convert ground-referenced, single-ended

signals into differential signals that are referenced to the

user-supplied output common mode voltage. This is ideal

for driving these differential ADCs. The balanced differential

nature of the amplifier also provides even-order harmonic

distortion cancellation, and low susceptibility to common

mode noise (like power supply noise). The LTC6419 can

operate with a single-ended input and differential output,

or with a differential input and differential output.

The outputs of the LTC6419 are capable of swinging from

close-to-ground to 1V below V+. They can source or sink

up to approximately 70mA of current. Load capacitances

should be decoupled with at least 10Ω of series resistance

from each output.

Input Pin Protection

The LTC6419 input stage is protected against differential

input voltages which exceed 1.4V by two pairs of series

diodes connected back to back between +INA and –INA

and between +INB and –INB. Moreover, the input pins,

as well as VOCMA, VOCMB, SHDNA and SHDNB pins, have

clamping diodes to either power supply. If these pins are

driven to voltages which exceed either supply, the current

should be limited to 10mA to prevent damage to the IC.

SHDN

Pins

The SHDNA/SHDNB pins are CMOS logic inputs with a

150k internal pull-up resistor. If the pin is driven low, the

LTC6419 powers down. If the pin is left unconnected or

driven high, the part is in normal active operation. Some

care should be taken to control leakage currents at this

pin to prevent inadvertently putting the LTC6419 into

shutdown. The turn-on and turn-off time between the

shutdown and active states is typically less than 200ns.

Keep in mind that each channel of the amplifier has its

own independent SHDN pin.

General Amplifier Applications

In Figure 1, the gain to VOUTDIFF from VINP and VINM is

given by:

VOUTDIFF = V+OUT – V–OUT ≈

• VINP – VINM

(

) (1)

Note from Equation (1), the differential output voltage

(V+OUT – V–OUT) is completely independent of input and

output common mode voltages, or the voltage at the

common mode pin. This makes the LTC6419 ideally

suited for pre-amplification, level shifting and conversion

of single-ended signals to differential output signals for

driving differential input ADCs.

Figure 1. Circuit for Common Mode Range

–

V–OUT

V+OUT

VVOCM

VOCM

6419 F01

–

VINP

–

VCM

–

VINM

V–IN

V+IN

Output Common Mode and VOCM Pin

The output common mode voltage is defined as the aver-

age of the two outputs:

VOUTCM = VOCM =

V+OUT + V–OUT

As the equation shows, the output common mode voltage

is independent of the input common mode voltage, and

is instead determined by the voltage on the VOCM pin, by

means of an internal common mode feedback loop.

If the VOCM pin is left open, an internal resistor divider

develops a default voltage of 1.25V with a 5V supply. The

VOCM pin can be overdriven to another voltage if desired.

For example, when driving an ADC, if the ADC makes a

reference available for setting the common mode voltage, it

can be directly tied to the VOCM pin, as long as the ADC is

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