LTC6601IUF 데이터 시트보기 (PDF) - Linear Technology

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LTC6601IUF

Low Noise, 0.5% Tolerance, 5MHz to 28MHz, Pin Confi gurable Filter/ADC Driver

Linear Technology 

LTC6601-1

APPLICATIONS INFORMATION

Likewise, substituting the numbers for a single 5V power

supply, (V+ = 5V, V– = 0V) with VOCM = 2.5V, and R1 = R2

= 100Ω, into the above equation, the input common mode

range (VINCMR) is between the two limits:

0V ≤ VINCM ≤ 4.7V

The output common mode voltage is defined as the aver-

age of the two outputs:

VOUTCM

=

VOCM

=

VOUT +

+

2

VOUT –

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

1.1 V above V– to 1V below V+. The VOCM pin sits in the

middle of a voltage divider which sets the default mid-

supply open circuit potential.

In single supply applications, where the LTC6601 is used

to interface to an ADC, the optimal common mode input

to the ADC is often determined by the ADC’s reference. 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 the input impedance of

the VOCM pin (RVOCM). This impedance can be assumed

to be connected to a mid-supply potential. If an external

reference drives the VOCM pin, it should still be bypassed

with a high quality 0.01μF or higher 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

When comparing the LTC6601 noise to other amplifiers,

be sure to compare similar specifications. Competing

devices often specify noise referred to the inputs of the

amplifier. The input referred voltage noise of the LTC6601-1

is 2.1nV/√Hz. This level is one of the lowest available for

amplifiers in this speed and power range.

In addition to the noise generated by the amplifier, the

surrounding feedback resistors also contribute noise. A

noise model is shown in Figure 5. The output spot noise

generated by both the amplifier and the feedback compo-

nents is governed by the equation:

eno =

⎛

⎝⎜

eni

•

⎛

⎝⎜

1+

R2 ⎞

R1⎠⎟

⎞

⎠⎟

2

+

2

•

⎛

⎝⎜⎜ In 2

•

⎛

⎜

⎝

R22

+

R32

•

⎛

⎝⎜

1+

R2 ⎞

R1⎠⎟

2

⎞

⎟

⎠

⎞

⎠⎟⎟

+

2

•

⎛

⎝⎜

enR1

•

⎛

⎝⎜

R2 ⎞

R1⎠⎟

⎞

⎠⎟

2

+

2⎛⎝⎜

enR3

•

⎛

⎝⎜

1+

R2 ⎞

R1⎠⎟

⎞

⎠⎟

2

+

2

•

enR22

Substituting the equation for Johnson noise of a resistor (enR2 = 4kTR), and simplifying:

eno =

⎛

⎝⎜

eni

•

⎛

⎝⎜

1+

R2 ⎞

R1⎠⎟

⎞

⎠⎟

2

+

2

•

⎛

⎝⎜⎜

In2

•

⎛

⎜

⎝

R22

+

R32

•

⎛

⎝⎜

1+

R2 ⎞

R1⎠⎟

2

⎞

⎟

⎠

⎞

⎠⎟⎟

+

8

•

k

•

T

⎛

⎜

⎝

R2

⎛

⎝⎜

1+

R2 ⎞

R1⎠⎟

+

R3

⎛

⎝⎜

1+

R2 ⎞

R1⎠⎟

2

⎞

⎟

⎠

66011f

19

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