ADA4097-1BUJZ-R5 datasheet(22/27 Pages) AD

22 / 27 page

ADA4097-1

Data Sheet

Rev. 0 | Page 22 of 27

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 59 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.

4kTR3

IN+

eN, R3

4kTR1

IN–

eN, R1

VOUT

GAIN FROM

A TO OUTPUT

NOISE GAIN =

NG = 1 +

GAIN FROM

B TO OUTPUT

RTI NOISE =

RTO NOISE = NG × RTI NOISE

eN2 + 4kTR3 + 4kTR1

= –

4kTR2

eN, R2

R1 + R2

+IN+ 2R32 + IN–2

R1 × R2

R1 + R2

+ 4kTR2

R1 + R2

Figure 59. Op Amp Noise Analysis Model

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

simplified to the following form:

RTI Noise =

2

nn,R

I R

n,R

ekTR

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

feedback 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 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.

RTI NOISE

VNR

IN × REQ

REQ (Ω)

10k

100k

10M

0.1

100

Figure 60. Noise Contributions vs. Equivalent Input Resistance

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

network is about 40 kΩ. For the amplifier configuration shown

in Figure 59, REQ < 40 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 < ~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 60 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

sinking, 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 RL 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 operating in Class A operation in an inverting

configuration, with the input common-mode biased at midsupply.

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