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DISTORTION PERFORMANCE

4kT

1/4

OPA4830

4kT=1.6E

20J

at290 K

4kTR

NOISE PERFORMANCE

E =

+(I

R ) +4kTR )NG +(I R ) +4kTR NG

E =

+(I

R ) +4kTR +

I R

4kTR

OPA4830

SBOS350A – DECEMBER 2006 – REVISED MAY 2008.................................................................................................................................................... www.ti.com

The OPA4830 provides good distortion performance

into a 150

Ω load. Relative to alternative solutions, it

provides exceptional performance into lighter loads

and/or operating on a single +3V supply. Generally,

until the fundamental signal reaches very high

frequency

power

levels,

the

2nd-harmonic

dominates

the

distortion

with

negligible

3rd-harmonic component. Focusing then on the

2nd-harmonic,

increasing

the

load

impedance

improves distortion directly. Remember that the total

load

includes

the

feedback

network;

the

noninverting configuration (see Figure 74) this is sum

of RF + RG, while in the inverting configuration, only

RF needs to be included in parallel with the actual

Figure 89. Noise Analysis Model

load.

Running

differential

suppresses

the

2nd-harmonic, as shown in the Differential Typical

The total output spot noise voltage can be computed

Characteristics.

as the square root of the sum of all squared output

noise voltage contributors. Equation 8 shows the

general form for the output noise voltage using the

High slew rate, unity-gain stable, voltage-feedback op

terms shown in Figure 89:

amps usually achieve their slew rate at the expense

of a higher input noise voltage. The 9.2nV/

√Hz input

voltage noise for the OPA4830 however, is much

(8)

lower than comparable amplifiers. The input-referred

voltage noise and the two input-referred current noise

terms (2.8pA/

√Hz) combine to give low output noise

Dividing this expression by the noise gain [ NG = (1 +

under

wide

variety

operating

conditions.

RF/RG) ] gives the equivalent input-referred spot noise

Figure 89 shows the op amp noise analysis model

voltage at the noninverting input; this result is shown

with all the noise terms included. In this model, all

in Equation 9:

noise terms are taken to be noise voltage or current

density terms in either nV/

√Hz or pA/√Hz.

(9)

Evaluating these two equations for the circuit and

component values shown in Figure 72 gives a total

output spot noise voltage of 19.3nV/

√Hz and a total

equivalent input spot noise voltage of 9.65nV/

√Hz.

This value is including the noise added by the

resistors. This total input-referred spot noise voltage

is not much higher than the 9.2nV/

√Hz specification

for the op amp voltage noise alone.

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