ADA4084-2ARZ-R7 datasheet(24/28 Pages) AD

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ADA4084-2

Data Sheet

Rev. C | Page 24 of 28

DESIGNING LOW NOISE CIRCUITS IN SINGLE-

SUPPLY APPLICATIONS

In single-supply applications, devices like the ADA4084-2 extend

the dynamic range of the application through the use of rail-to-rail

operation. Referring to the op amp noise model circuit configu-

ration illustrated in Figure 82, the expression for an amplifier’s

total equivalent input noise voltage for a source resistance level,

RS, is given by





)

(

)

(

)

(

nOA









, units in

where:

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

(enR)2

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

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

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).

(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 82. Op Amp Noise Circuit Model Used to Determine Total Circuit

Equivalent Input Noise Voltage and Noise Figure

As a design aid, Figure 83 shows the total equivalent input noise

of the ADA4084-2 and the total thermal noise of a resistor for

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

equivalent input noise voltage of the ADA4084-2 is dominant.

TOTAL SOURCE RESISTANCE, RS (Ω)

100

10k

ADA4084-2 TOTAL

EQUIVALENT NOISE

RESISTOR THERMAL

NOISE ONLY

100

100k

FREQUENCY = 1kHz

TA = 25°C

Figure 83. ADA4084-2 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

a circuit’s output signal-to-noise to its input signal-to-noise.

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 Mislead

, available at www.analog.com.

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, it is recommended that an operational amplifier

with the lowest equivalent input noise voltage be chosen, 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 part operating open

loop, the second stage increases the current drive to the ratioed

mirror to close the loop. However, it cannot, 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 84). An unused section should be configured as a voltage

follower with the noninverting input connected to a voltage within

the input voltage range. The ADA4084-2 has unique second

stage and output stage designs that greatly reduce the excess

supply current when the op amp is operating open loop.

800

036

VSY (V)

100

200

300

400

500

600

700

ADA4084-2

TA = 25°C

RL = ∞

COMPARATOR

OUTPUT LOW

COMPARATOR

OUTPUT HIGH

BUFFER

Figure 84. Supply Current vs. Supply Voltage

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