ADA4084-2ARZ-R7 datasheet(31/35 Pages) AD

31 / 35 page

Data Sheet

ADA4084-1/ADA4084-2/ADA4084-4

Rev. H | Page 31 of 35

DESIGNING LOW NOISE CIRCUITS IN SINGLE-

SUPPLY APPLICATIONS

In single-supply applications, devices like the ADA4084-1/

ADA4084-2/ADA4084-4 extend the dynamic range of the

application through the use of rail-to-rail operation. Referring to

the op amp noise model circuit configuration illustrated in

Figure 109, the expression for the total equivalent input noise

voltage of an amplifier for a source resistance level, RS, is given by

[

]

)

(

)

(

)

(

nOA

, units in

where:

(enR)2 is the source resistance thermal noise voltage power (4kTR).

k is the Boltzmann’s constant, 1.38 × 10–23 J/K.

T is the ambient temperature in Kelvin of the circuit, 273.15 +

TA (°C).

(inOA)2 is the op amp equivalent input noise current spectral

power (1 Hz bandwidth).

RS = 2R, the effective, or equivalent, circuit source resistance.

(enOA)2 is the op amp equivalent input noise voltage spectral

power (1 Hz bandwidth).

enR

enOA

inOA

NOISELESS

IDEAL

NOISELESS

OP AMP

RS = 2R

Figure 109. Op Amp Noise Circuit Model Used to Determine Total Circuit

Equivalent Input Noise Voltage and Noise Figure

As a design aid, Figure 110 shows the equivalent thermal noise

of the ADA4084-1/ADA4084-2/ADA4084-4 vs. the total source

resistance. Note that for source resistance less than 1 kΩ, the

equivalent input noise voltage of the ADA4084-1/ADA4084-2/

ADA4084-4 is dominant.

TOTAL SOURCE RESISTANCE, RS (Ω)

100

10k

ADA4084-1/ADA4084-2/ADA4084-4

TOTAL EQUIVALENT NOISE

RESISTOR THERMAL

NOISE ONLY

100

100k

FREQUENCY = 1kHz

TA = 25°C

Figure 110. Equivalent Thermal Noise vs. Total Source Resistance

Because circuit SNR is the critical parameter in the final analysis,

the noise behavior of a circuit is sometimes expressed in terms

of its noise figure (NF). The noise figure is defined as the ratio

of the signal-to-noise output of a circuit to its signal-to-noise

input.

Noise figure is generally used for RF and microwave circuit analysis

in a 50 Ω system. This is not very useful for op amp circuits where

the input and output impedances can vary greatly. For a more

complete description of noise figure, see the MT-052 Tutorial, Op

Amp Noise Figure: Don’t be Misled.

Signal levels in the application invariably increase to maximize

circuit SNR, which is not an option in low voltage, single-supply

applications.

Therefore, to achieve optimum circuit SNR in single-supply

applications, choose an operational amplifier with the lowest

equivalent input noise voltage, along with source resistance

levels that are consistent with maintaining low total circuit noise.

COMPARATOR OPERATION

Although op amps are quite different from comparators,

occasionally an unused section of a dual or a quad op amp can

be used as a comparator; however, this is not recommended for

any rail-to-rail output op amps. For rail-to-rail output op amps,

the output stage is generally a ratioed current mirror with bipolar

or MOSFET transistors. With the device operating open loop,

the second stage increases the current drive to the ratioed mirror

to close the loop. However, the loop cannot close, which results in

an increase in supply current. With the op amp configured as a

comparator, the supply current can be significantly higher (see

Figure 111). Configure an unused section as a voltage follower

with the noninverting input connected to a voltage within the

input voltage range. The ADA4084-1/ADA4084-2/ADA4084-4

have unique second stage and output stage designs that greatly

reduce the excess supply current when the op amp is operating

open loop.

800

VSY (V)

100

200

300

400

500

600

700

TA = 25°C

RL = ∞

COMPARATOR

OUTPUT LOW

COMPARATOR

OUTPUT HIGH

BUFFER

Figure 111. Supply Current vs. Supply Voltage (VSY)

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