ADA4097-1BUJZ-R5 datasheet(24/31 Pages) AD

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Data Sheet

ADA4097-1/ADA4097-2

APPLICATIONS INFORMATION

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Rev. A | 24 of 31

NOISE

To analyze the noise performance of an amplifier circuit, identify the

noise sources, and then determine if each source has a significant

contribution to the overall noise performance of the amplifier. To

simplify the noise calculations, noise spectral densities (NSDs)

are used rather than actual voltages, to leave bandwidth out of

the expressions. NSD is generally expressed in nV/√Hz and is

equivalent to the noise in a 1 Hz bandwidth.

The noise model shown in Figure 61 has six individual noise

sources: the Johnson noise of the three resistors (R1 to R3), the op

amp voltage noise, and the current noise (IN±) in each input of the

amplifier. Each noise source has its own contribution to the noise at

the output. Noise is generally specified as referring to input (RTI),

but it is often simpler to calculate the noise referred to the output

(RTO), and then divide by the noise gain to obtain the RTI noise.

Figure 61. Op Amp Noise Analysis Model

Assuming IN+ = IN− = IN, the equation for RTI noise can be simpli-

fied to the following form:

RTI Noise =

en2+en,R2+INREQ2

en, R 4KTREQ

REQ = R3 + R1||R2

where:

en is the op amp voltage noise.

en,R is the thermal noise contribution of the surrounding R1 to R3

resistors.

REQ is the equivalent input resistance.

k is Boltzmann’s constant (1.38 × 10−23 J/K).

T is the absolute temperature in Kelvin.

A 50 Ω resistor generates a Johnson noise of 1 nV/√Hz at 25°C.

For optimal performance, the lower bound of resistance in a feed-

back network is determined by the amount of quiescent power and

distortion that can be tolerated. The upper bound is determined by

the resistor and current noise density. The ADA4097-1/ADA4097-2

has an en of 53 nV/√Hz.

If resistor and current noise contributions are less than half this

value, the en introduced by the op amp dominates and provides

optimal noise performance of the device.

Figure 62. Noise Contributions vs. Equivalent Input Resistance

For the ADA4097-1/ADA4097-2, this lower bound of resistance in

the feedback network is about 40 kΩ. For the amplifier configuration

shown in Figure 61, REQ < 40 kΩ provides stable noise perform-

ance. If noise performance is not important, en is typically fixed for

a given TA, en,R increases with the square root of the resistor value,

and the IN × REQ resistance increases linearly, but does not impact

total noise until it approaches the value of en,R. With REQ < ~6 MΩ,

en,R is larger than IN × REQ. A safe value for REQ < 2 MΩ to ensure

that IN is not the majority contributor to total noise seen by the input.

Figure 62 shows the noise contributions for the range of resistance

values discussed in this section.

DISTORTION

There are two main contributors of distortion in op amps: output

crossover distortion as the output transitions from sourcing to sink-

ing, and distortion caused by nonlinear common-mode rejection. If

the op amp is operating in an inverting configuration, there is no

common-mode induced distortion. If the op amp is operating in the

noninverting configurations within the normal input common-mode

range (−VS to +VS − 1 V), distortion is acceptable. When the inputs

transition from normal to Over-The-Top operation or vice versa, a

significant degradation occurs in linearity due to the change of input

circuitry.

As RLOAD decreases, distortion increases due to a net decrease in

loop gain and greater signal swings internal to the amplifier that are

necessary to drive the load. The lowest distortion can be achieved

with the ADA4097-1/ADA4097-2 operating in Class A operation in

an inverting configuration, with the input common-mode biased at

midsupply.

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