LTC6404 datasheet(20/28 Pages) LINER

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LTC6404

6404f

APPLICATIONS INFORMATION

if voltage transients at the input exceed 1.4V. The inputs

also have steering diodes to either supply. The turn-on and

turn-off time between the shutdown and active states is

typically less than 1μs.

General Ampliﬁer Applications

As levels of integration have increased and correspond-

ingly, system supply voltages decreased, there has been

a need for ADCs to process signals differentially in order

to maintain good signal to noise ratios. These ADCs are

typically supplied from a single supply voltage which

can be as low as 3V (2.7V min), and will have an optimal

common mode input range near mid-supply. The LTC6404

makes interfacing to these ADCs easy, by providing both

single-ended to differential conversion as well as com-

mon mode level shifting. The front page of this data sheet

shows a typical application. Referring to Figure 1, the gain

to VOUTDIFF from VINM and VINP is:

OUTDIFF

OUT

INP

INM

=≈

()

+ –•

–

Note from the above equation, the differential output volt-

age (VOUT+ – VOUT–) is completely independent of input

and output common mode voltages, or the voltage at

the common mode pin. This makes the LTC6404 ideally

suited for pre-ampliﬁcation, level shifting and conversion

of single ended signals to differential output signals to

drive differential input ADCs.

Effects of Resistor Pair Mismatch

In the circuit of Figure 3, it is possible the gain setting

resistors will not perfectly match. Assuming inﬁnite open

loop gain, the differential output relationship is given by

the equation:

OUTDIFF

OUT

INDIFF

AVG

=≅

+ –•

•

–

NCM

AVG

OCM

–•

Δβ

where:

βAVG

⎛

⎝⎜

⎞

⎠⎟

•

RF is the average of RF1, and RF2, and RI is the average

of RI1, and RI2.

βAVG is deﬁned as the average feedback factor (or gain)

from the outputs to their respective inputs:

Δβ is deﬁned as the difference in feedback factors:

Δ=

–

Figure 3. Basic Differential Ampliﬁer with Feedback Resistor Pair Mismatch

V–

V+

0.1μF

–

SHDN

IN–

OUT+

OUTF+

IN+

OUT–

OUTF–

VOUTF

–

RF2

VOUTF

VOUT

–

VOUT

V+

V–

V+

V–

V+

V–

VOCM

VSHDN

VVOCM

VOCM

V–

V+

V–

6404 F03

LTC6404

SHDN

0.1μF

0.01μF

RF1

RI2

RI1

–

VINP

–

VINM

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