AD8137YCP-R2 datasheet(19/24 Pages) AD

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AD8137

Rev. A | Page 19 of 24

The contribution from the input voltage noise spectral density

is computed as

⎟

⎠

⎞

⎜

⎝

⎛ +

Vo_n

, or equivalently, vn/β

(7)

where vn is defined as the input-referred differential voltage

noise. This equation is the same as that of traditional op amps.

The contribution from the input current noise of each input is

computed as

( )

Vo_n =

(8)

where in is defined as the input noise current of one input. Each

input needs to be treated separately since the two input currents

are statistically independent processes.

The contribution from each RG is computed as

⎟

⎠

⎞

⎜

⎝

⎛

Vo_n

(9)

This result can be intuitively viewed as the thermal noise of

each RG multiplied by the magnitude of the differential gain.

The contribution from each RF is computed as

Vo_n

4 =

(10)

Voltage Gain

The behavior of the node voltages of the single-ended-to-

differential output topology can be deduced from the signal

definitions and Figure 63. Referring to Figure 63, (CF = 0) and

setting VIN = 0 one can write:

−

(11)

⎥⎦

⎤

⎢⎣

⎡

(12)

Solving the above two equations and setting VIP to Vi gives the

gain relationship for VO, dm/Vi.

−

(13)

An inverting configuration with the same gain magnitude can

be implemented by simply applying the input signal to VIN and

setting VIP = 0. For a balanced differential input, the gain from

VIN, dm to VO, dm is also equal to RF/RG, where VIN, dm = VIP − VIN.

Feedback Factor Notation

When working with differential drivers, it is convenient to in-

troduce the feedback factor β, which is defined as

≡

(14)

This notation is consistent with conventional feedback analysis

and is very useful, particularly when the two feedback loops are

not matched.

Input Common-Mode Voltage

The linear range of the VAN and VAP terminals extends to within

approximately 1 V of either supply rail. Since VAN and VAP are

essentially equal to each other, they are both equal to the ampli-

fier’s input common-mode voltage. Their range is indicated in

the specifications tables as input common-mode range. The

voltage at VAN and VAP for the connection diagram in Figure 63

can be expressed as

ACM

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

OCM

)

(

(15)

where VACM is the common-mode voltage present at the ampli-

fier input terminals.

Using the β notation, Equation (15) can be written as

( )

ICM

OCM

ACM

−

(16)

or equivalently,

(

)

ICM

OCM

ICM

ACM

−

(17)

where VICM is the common-mode voltage of the input signal, i.e.,

ICM

≡

For proper operation, the voltages at VAN and VAP must stay

within their respective linear ranges.

Calculating Input Impedance

The input impedance of the circuit in Figure 63 will depend on

whether the amplifier is being driven by a single-ended or a

differential signal source. For balanced differential input signals,

the differential input impedance (RIN, dm) is simply

IN,

(18)

For a single-ended signal (for example, when VIN is grounded,

and the input signal drives VIP), the input impedance becomes

)

(

−

(19)

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