LT1994CDD datasheet(13/20 Pages) LINER

13 / 20 page

LT1994

1994fa

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 sets this average by an internal common mode

feedback loop which internally forces VOUT

+ = –V

OUT

–. The

output common mode range extends from approximately

1.1V above V– to approximately 0.8V below V+. The VOCM

pin sits in the middle of an 80kΩ to 80kΩ voltage divider

that sets the default mid-supply open circuit potential.

In single supply applications, where the LT1994 is used

to interface to an ADC, the optimal common mode input

range to the ADC is often determined by the ADC’s refer-

ence. If the ADC makes a reference available for setting

the input common mode voltage, it can be directly tied

to the VOCM pin, but must be capable of driving a 40kΩ

equivalent resistance that is tied to a mid-supply potential.

If an external reference drives the VOCM pin, it should still

be bypassed with a high quality 0.1μF capacitor to a low

impedance ground plane to ﬁlter any thermal noise and

to prevent common mode signals on this pin from being

inadvertently converted to differential signals.

Noise Considerations

The LT1994’s input referred voltage noise is on the order

of 3nV/√Hz. Its input referred current noise is on the

order of 2.5pA/√Hz. In addition to the noise generated

by the ampliﬁer, the surrounding feedback resistors also

contribute noise. The output noise generated by both the

ampliﬁer and the feedback components is given by the

equation:

nRI

nRF

⎡

⎣⎢

⎤

⎦⎥

⎛

⎝⎜

⎞

⎠⎟

() +

⎡

⎣⎢

⎤

⎦⎥

⎛

⎝⎜

⎞

⎠⎟

••

•

••

•

A plot of this equation and a plot of the noise generated

by the feedback components are shown in Figure 6.

The LT1994’s input referred voltage noise contributes the

equivalent noise of a 560Ω resistor. When the feedback

APPLICATIO S I FOR ATIO

Figure 4. Circuit for Common Mode Range

VCM

1994 F04

VSHDNB

VINM

VINP

–

LT1994

VOCM

VOUT

VIN

–

VOUT

–

VIN

0.1

µF

SHDN

Figure 5. Noise Analysis

1994 F05

RI2

RF2

VS/2

–VS/2

–

LT1994

VOCM

enRI2

RI1

RF1

encm

eno

in–

enRI1

eni

enRF2

enRF1

in+

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