ADA4098-1BUJZ-R5 datasheet(24/33 Pages) AD

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Data Sheet

ADA4098-1/ADA4098-2

APPLICATIONS INFORMATION

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Rev. B | 24 of 33

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 60 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

amplifiers. 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 60. 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.

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 ADA4098-1 and

ADA4098-2 have an en of 17 nV/√Hz.

If resistor and current noise contributions are less than half this

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

optimal noise performance of the devices.

For the ADA4098-1 and ADA4098-2, this lower bound of resistance

in the feedback network is about 4.5 kΩ. For the amplifier configu-

ration shown in Figure 60, REQ < 4.5 kΩ provides stable noise

performance. 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 < ~700 kΩ, en,R is larger than IN × REQ. A safe value for

REQ is ~350 kΩ to ensure that IN is not the majority contributor to

total noise seen by the input.

Figure 61 shows the noise contributions for the range of resistance

values discussed in this section.

Figure 61. Noise Contributions vs. Equivalent Input Resistance

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 amps are operating in an inverting configuration, there is

no common-mode induced distortion. If the op amps are operating

in the noninverting configurations within the normal input common-

mode range (−VS to +VS − 1.25 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 RL decreases, distortion increases due to a net decrease in loop

gain and greater signal swings internal to the amplifiers that are

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

with the ADA4098-1 and ADA4098-2 operating in Class A operation

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

midsupply.

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