ADA4922-1

Rev. 0 | Page 16 of 20

APPLICATIONS

The ADA4922-1 is a fixed-gain, single-ended-to-differential

voltage amplifier, optimized for driving high resolution ADCs

in high voltage applications. There are no gain adjustments

available to the user.

ADA4922-1 DIFFERENTIAL OUTPUT NOISE MODEL

The principal noise sources in a typical ADA4922-1 application

circuit are shown in Figure 49.

OUT–

OUT+

REF

Figure 49. ADA4922-1 Differential Output Noise Model

Using the traditional approach, a noise source is applied in

series with one of the inputs of each op amp to model input-

referred voltage noise. The input current noise that matters the

most is present at the input pin. The output voltage noise due to

this noise current depends on the source resistance feeding the

input, as well as the downstream gain in the amplifier. Resistor

noise is modeled by placing a noise voltage source in series with

the same noise voltage density.

At room temperature,

()

Hz

nV/

3.2

Ω

600

kT

4

≈

=

=

nRf

nRg

V

V

(2)

The noise at OUT+ is due to the input-referred current and

voltage noise sources of the noninverting amplifier and the

noise of the source resistance, all reflected to the output with a

noise gain of 1, and is equal to:

(3)

the source resistance noise.

The noise at OUT− originates from a number of sources:

n1

g

f

n1

V

R

R

V

−

=

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −

(4)

Voltage Noise

()

()

1

1

n

S

g

f

n

S

I

R

R

R

I

R

−

=

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −

(5)

nRs

g

f

nRs

V

R

R

V

−

=

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −

(6)

nRg

g

f

nRg

V

R

R

V

−

=

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ −

(7)

(8)

2

2

1

n

g

f

n2

V

R

R

V

=

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

(9)

When looking at OUT− by itself, the contributing noise sources

are uncorrelated, and therefore, the total output noise is

calculated as the root-sum-square (rss) of the individual

contributors. When looking at the differential output noise, the

can be seen from the previous results that the output noise due

and at OUT− with a gain of −1. This produces a gain of 2 for each

of these three sources at the differential output.

The total differential output noise density is calculated as

(

)

(

) ()

2

2

2

4

Hz

nV/

3.2

2

)

Hz

pA/

(1.4

2

n

nRs

s

n

V

V

R

V

+

+

+

+

(10)

noise of each amplifier is the same.

The output noise due to the amplifier alone is calculated by

(11)

Clearly, the output noise is not balanced between the outputs,

but this is not an issue in most applications.

USING THE REF PIN

The REF pin sets the output baseline in the inverting path and

is used as a reference for the input signal. In most applications,

the REF pin is set to the input signal midswing level, which in

many cases is also midsupply. For bipolar signals and power

supplies, REF is generally set to ground. In single-supply

applications, setting REF to the input signal midswing level

provides optimal output dynamic range performance with

minimum differential offset. Note that the REF input only

affects the inverting signal path, or OUT−.

Most applications require a differential output signal with the

same dc common-mode level on each output. It is possible for

the signal measured across OUT+ and OUT− to have a common-

mode voltage that is of the desired level but has different dc

levels at both outputs. Typically, this situation is avoided,

because it wastes the amplifier’s output dynamic range.