OPA690 datasheet(19/33 Pages) TI

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OPA2822

SBOS188E

www.ti.com

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

Two printed circuit boards (PCBs) are available to assist in

the initial evaluation of circuit performance using the OPA2822

in its two package options. Both of these are offered free of

charge as unpopulated PCBs, delivered with a user’s guide.

The summary information for these fixtures is shown in

Table III.

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

will give the total equivalent spot noise voltage referred to the

noninverting input, as shown in Equation 3:

kTR

(

) ++ 







(3)

Inserting high resistor values into Equation 3 can quickly

dominate the total equivalent input referred voltage noise. A

250

Ω source impedance on the noninverting input will add as

much noise as the amplifier itself. If the noninverting input is

a DC bias path (as in inverting or in some single-supply

applications), it is critical to include a noise shunting capaci-

tor with that resistor to limit the added noise impact of those

resistors (see the example in Figure 2).

FREQUENCY RESPONSE CONTROL

Voltage-feedback op amps such as the OPA2822 exhibit

decreasing closed-loop bandwidth as the signal gain is

increased. In theory, this relationship is described by the

Gain Bandwidth Product (GBP) shown in the Electrical Char-

acteristics. Ideally, dividing GBP by the noninverting signal

gain (also called the Noise Gain, NG) will predict the closed-

loop bandwidth. In practice, this principle holds true only

when the phase margin approaches 90

°, as it does in higher

gain configurations. At low gains, most high-speed amplifiers

will show a more complex response with lower phase margin

and higher bandwidth than predicted by the GBP. The

OPA2822 is compensated to give a slightly peaked fre-

quency response at a gain of +2 (see the circuit in Figure 1).

The 200MHz typical bandwidth at a gain of +2 far exceeds

that predicted by dividing the GBP of 240MHz by a gain of 2.

The bandwidth predicted by the GBP is more closely correct

as the gain increases. As shown in the Typical Characteris-

tics, at a gain of +10, the –3dB bandwidth of 24MHz matches

that predicted by dividing the GBP by 10.

The demonstration fixtures can be requested at the Texas

Instruments web site (www.ti.com) through the OPA2822

product folder.

MACROMODELS AND APPLICATIONS SUPPORT

Computer simulation of circuit performance using SPICE is

often a quick way to analyze the performance of the OPA2822

in its intended application. This is particularly true for video

and RF amplifier circuits where parasitic capacitance and

inductance can play a major role in circuit performance. A

SPICE model for the OPA2822 is available through the TI

web site (www.ti.com). These models do a good job of

predicting small-signal AC and transient performance under

a wide variety of operating conditions. They do not do as well

in predicting the harmonic distortion characteristics. These

models do not attempt to distinguish between the package

types in their small-signal AC performance.

OPERATING SUGGESTIONS

SETTING RESISTOR VALUES TO MINIMIZE NOISE

Getting the full advantage of the OPA2822’s low input noise

requires careful attention to the external gain setting and DC

biasing networks. The feedback resistor is part of the overall

output load (which can begin to degrade distortion if set too

low). With this in mind, a good starting point for design is to

select the feedback resistor as low as possible (consistent

with loading distortion concerns), then continue with the

design, and set the other resistors as needed. To retain full

performance, setting the feedback resistor in the range of

200

Ω to 750Ω can provide a good start to the design.

Figure 11 shows the full output noise analysis model for any

op amp.

The total output spot noise voltage can be computed as the

square root of the sum of all squared output noise voltage

terms. Equation 2 shows the general form of this output noise

voltage expression using the terms shown in Figure 11.

kTR NG

I R

kTR

(

) +

(

) +( ) +

(2)

ORDERING

LITERATURE

PRODUCT

PACKAGE

NUMBER

OPA2822U

SO-8

DEM-OPA-SO-2A

SBOU003

OPA2822E

MSOP-8

DEM-OPA-MSOP-2A

SBOU004

TABLE III. Demonstration Fixtures by Package.

FIGURE 11. Op Amp Noise Analysis Model.

4kT

1/2

OPA2822

4kT = 1.6E –20J

at 290

°K

4kTR

√

4kTR

√

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