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LTC6404

6404f

APPLICATIONS INFORMATION

According to Figure 4, the input impedance looking into

the differential amp (RINM) reﬂects the single ended source

case, thus:

INM

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

–•

R2 is chosen to balance R1 || RS:

2 =

•

Input Common Mode Voltage Range

The LTC6404’s input common mode voltage (VICM) is

deﬁned as the average of the two input voltages, VIN+, and

VIN–. It extends from V– to 1.4V below V+. The operating

input common mode range depends on the circuit con-

ﬁguration (gain), VOCM and VCM (Refer to Figure 5). For

fully differential input applications, where VINP = –VINM,

the common mode input voltage is approximately:

ICM

OCM

≈

⎛

⎝⎜

⎞

⎠⎟

–

•

⎛

⎝⎜

⎞

⎠⎟

With singled ended inputs, there is an input signal com-

ponent to the input common mode voltage. Applying only

VINP (setting VINM to zero), the input common voltage is

approximately:

ICM

OCM

≈

⎛

⎝⎜

⎞

⎠⎟

–

•

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

INP

•

Output Common Mode Voltage Range

The output common mode voltage is deﬁned as the aver-

age of the two outputs:

OUTCM

OCM

OUT

–

The VOCM pin sets this average by an internal common

mode feedback loop which internally forces VOUT+=–VOUT–.

The output common mode range extends from 1.1V above

V– to 1V below V+ (see the Electrical Characteristics table

for the LTC6404-4 output common mode voltage range).

The VOCM pin sits in the middle of a voltage divider which

sets the default mid-supply open circuit potential.

Figure 5. Circuit for Common Mode Range

V–

V+

0.1μF

VCM

–

SHDN

IN–

OUT+

OUTF+

IN+

OUT–

OUTF–

VOUTF

–

VOUTF

VOUT

–

VOUT

V+

V–

V+

V–

V+

V–

VOCM

VSHDN

VVOCM

VOCM

V–

V+

V–

6404 F05

LTC6404

SHDN

0.1μF

0.01μF

–

VINP

–

VINM

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