OPA690 datasheet(24/35 Pages) TI

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4kT

1/2

OPA2890

4kT=1.6E

20J

at290 K

4kTR

4kTRF

E =

+(I

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

NIBN

E =

+(I

R ) +4kTR +

NIBN

I R

(

)

4kTR

NOISE PERFORMANCE

OPA2890

SBOS364A – DECEMBER 2007 – REVISED DECEMBER 2007

signal reaches very high frequency or power levels,

the 2nd harmonic dominates the distortion with a

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 49), this value

is the sum of RF + RG, while in the inverting

configuration it is only RF. Also, providing an

additional

supply-decoupling

capacitor

(0.1

µF)

between the supply pins (for bipolar operation)

improves the 2nd-order distortion slightly (3dB to

6dB).

Operating

differentially

also

lowers

2nd-harmonic distortion terms (see the plot on the

front page).

Figure 60. Op Amp Noise Analysis Model

In most op amps, increasing the output voltage swing

increases harmonic distortion directly. The output

The total output spot noise voltage can be computed

stage used in the OPA2890 holds the difference

as the square root of the sum of all squared output

between fundamental power and the 2nd- and

noise voltage contributors. Equation 4 shows the

3rd-harmonic

powers

relatively

constant

with

general form for the output noise voltage using the

increasing output power until very large output swings

terms shown in Figure 60.

are required ( > 4VPP). This also shows up in the

two-tone, 3rd-order intermodulation spurious (IM3)

response curves. The 3rd-order spurious levels are

(4)

extremely low at low output power levels. The output

stage continues to hold them low even as the

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

fundamental power reaches very high levels. As the

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

Typical

Characteristics

show,

the

spurious

voltage at the noninverting input, as shown in

intermodulation powers do not increase as predicted

Equation 5.

by a traditional intercept model. As the fundamental

power level increases, the dynamic range does not

decrease significantly. For two tones centered at

10MHz, with 4dBm/tone into a matched 50

Ω load

(5)

(that is, 1VPP for each tone at the load, which requires

4VPP for the overall two-tone envelope at the output

Evaluating these two equations for the OPA2890

pin), the Typical Characteristics show a 38dBc

circuit and component values (see Figure 49) gives a

difference between the test tone powers and the

total output spot noise voltage of 17.5nV/

√Hz and a

3rd-order

intermodulation

spurious

powers.

This

total equivalent input spot noise voltage of 8.7nV/

√Hz.

exceptional performance for all 22.5mW internal

This result includes the noise added by the bias

power

dissipation

parts

improves

further

when

current

cancellation

resistor

(350

Ω)

the

operating at lower frequencies or powers.

noninverting input. This total input-referred spot noise

voltage is only slightly higher than the 8nV/

√Hz

specification for the op amp voltage noise alone. This

result is the case as long as the impedances

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

appearing at each op amp input are limited to the

amps usually achieve the slew rate at the expense of

previously recommend maximum value of 400

Ω.

a higher input noise voltage. However, the 8nV/

√Hz

Keeping both (RF || RG) and the noninverting input

input voltage noise for the OPA2890 is much lower

source impedance less than 400

Ω satisfies both

than that of comparable amplifiers. The input-referred

noise

and

frequency

response

flatness

voltage noise, and the two input-referred current

considerations. Because the resistor-induced noise is

noise terms, combine to give low output noise under

relatively negligible, additional capacitive decoupling

a wide variety of operating conditions. Figure 60

across the bias current cancellation resistor (RB) for

shows the op amp noise analysis model with all the

the inverting op amp configuration of Figure 59 is not

noise terms included. In this model, all noise terms

required.

are taken to be noise voltage or current density terms

in either nV/

√Hz or pA/√Hz.

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