OPA698 datasheet(23/27 Pages) TI

23 / 27 page

OPA698

SBOS258B

www.ti.com

NOISE PERFORMANCE

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 5.6nV/

√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 24 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.

The total output spot noise voltage can be computed as the

square root of the sum of all squared output noise voltage

contributors. Equation 3 shows the general form for the

output noise voltage using the terms shown in Figure 25.

kTR

I R

kTR NG

BN SS

BI F

(

) +









(

) +

Dividing this expression by the noise gain (NG = (1+RF/RG))

will give the equivalent input-referred spot noise voltage at

the noninverting input, as shown in Equation 4.

kTR

NNI

BN SS

BI F

(

) ++ 







Evaluating these two equations for the OPA698 circuit and

component values (see Figure 1) will give a total output spot

noise voltage of 11.9nV/

√Hz and a total equivalent input spot

noise voltage of 6nV/

√Hz. This total input-referred spot noise

voltage is only slightly higher than the 5.6nV/

√Hz specifica-

tion for the op amp voltage noise alone. This will be the case

as long as the impedances appearing at each op amp input

are limited to a maximum value of 300

Ω. Keeping both

(RF || RG) and the noninverting input source impedance less

than 300

Ω will satisfy both noise and frequency response

flatness considerations. Since the resistor-induced noise is

relatively negligible, additional capacitive decoupling across

the bias current cancellation resistor (RT) for the inverting op

amp configuration of Figure 3 is not required, but is still

desirable.

DC ACCURACY AND OFFSET CONTROL

The balanced input stage of a wideband voltage feedback op

amp allows good output DC accuracy in a large variety of

applications. The power-supply current trim for the OPA698

gives even tighter control than comparable products. Al-

though the high-speed input stage does require relatively

high input bias current (typically

±8µA at each input terminal),

the close matching between them may be used to reduce the

output DC error caused by this current. The total output offset

voltage may be considerably reduced by matching the DC

source resistances appearing at the two inputs. This reduces

the output DC error due to the input bias currents to the offset

current times the feedback resistor. Evaluating the configura-

tion of Figure 1, using worst-case +25

°C input offset voltage

and current specifications, gives a worst-case output offset

voltage equal to: –(NG = noninverting signal gain)

±(NG • V

OS(MAX)) ± (RF • IOS(MAX))

±(2 • 5mV) ± (402Ω • 1.4µA)

±10.6mV

FIGURE 23. 5MHz Harmonic Distortion vs Load Resistance.

4kT

OPA698

4kT = 1.6E –20J

at 290

°K

4kTR

√

4kTR

√

FIGURE 24. Op Amp Noise Analysis Model.

(3)

(4)

–40

–45

–50

–55

–60

–65

–70

–75

–80

–85

–90

Load Resistance (

Ω)

100

1000

O = 2VPP

1 = 5MHz

HD2

HD3

OPA698 Datasheet(PDF) 23 Page - Texas Instruments

OPA698 Datasheet(HTML) 23 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Datasheet Download

Link URL