ADA4084-2

Data Sheet

Rev. A | Page 22 of 24

As a design aid, Figure 77 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.

100

1

10

10k

ADA4084-2 TOTAL

EQUIVALENT NOISE

RESISTOR THERMAL

NOISE ONLY

100

1k

100k

FREQUENCY = 1kHz

Figure 77. 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 76). 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

0

0

36

100

200

300

400

500

600

700

4

8

12

16

20

24

28

32

ADA4084-2

COMPARATOR

OUTPUT LOW

COMPARATOR

OUTPUT HIGH

BUFFER

Figure 78. Supply Current vs. Supply Voltage