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-40

-45

-50

-55

-60

-65

-70

-75

-80

-85

-90

Load Resistance (

Ω)

50

100

1000

V

f

HD2

HD3

G053

Noise Performance

4kT

R

G

R

G

R

F

R

S

OPA698

I

BI

E

O

I

BN

4kT = 1.6E -20J

at 290 K

E

RS

E

NI

4kTR

F

S0049-01

4kTR

S

E

O +

E

NI

2

) IBNRS

2

) 4kTRS NG

2

) 4kTRFNG

(5)

OPA698M

SGLS287A – MAY 2005 – REVISED MARCH 2006

Figure 69. 5-MHz Harmonic Distortion vs Load Resistance

High slew rate, unity-gain stable, voltage feedback op amps usually achieve their slew rate at the expense of a

higher input noise voltage. The 5.6-nV/

√Hz input voltage noise for the OPA698, however, is much lower than

comparable amplifiers. The input-referred voltage noise and the two input-referred current noise terms combine

to give low output noise under a wide variety of operating conditions. Figure 70 shows the op amp noise analysis

model with all the noise terms included. In this model, all noise terms are taken to be noise voltage or current

density terms in either nV/

√Hz or pA/√Hz.

Figure 70. 5-MHz Harmonic Distortion vs Load Resistance

The total output spot noise voltage can be computed as the square root of the sum of all squared output noise

voltage contributors. Equation 5 shows the general form for the output noise voltage using the terms shown in

Figure 70.

voltage at the noninverting input, as shown in Equation 6.

27

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