OPA690 datasheet(28/43 Pages) TI1

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DESIGN-IN TOOLS

Demonstration Fixtures

Macromodel and Applications Support

BANDWIDTH vs GAIN:

OPERATING SUGGESTIONS

OPA2830

SBOS309D – AUGUST 2004 – REVISED AUGUST 2008.................................................................................................................................................. www.ti.com

A good rule of thumb is to target the parallel

combination of RF and RG (see Figure 72) to be less

than about 400

Ω. The combined impedance RF || RG

interacts with the inverting input capacitance, placing

Two printed circuit boards (PCBs) are available to

an additional pole in the feedback network, and thus

assist in the initial evaluation of circuit performance

a zero in the forward response. Assuming a 2pF total

using the OPA2830 in its two package options. Both

parasitic on the inverting node, holding RF || RG <

of these are offered free of charge as unpopulated

400

Ω will keep this pole above 200MHz. By itself, this

PCBs, delivered with a user's guide. The summary

constraint implies that the feedback resistor RF can

information for these fixtures is shown in Table 1.

increase to several k

Ω at high gains. This is

acceptable as long as the pole formed by RF and any

Table 1. Demonstration Fixtures by Package

parasitic capacitance appearing in parallel is kept out

ORDERING

LITERATURE

of the frequency range of interest.

PRODUCT

PACKAGE

NUMBER

In the inverting configuration, an additional design

OPA2830ID

SO-8

DEM-OPA-SO-2A

SBOU003

consideration must be noted. RG becomes the input

OPA2830IDGK

MSOP-8

DEM-OPA-MSOP-2A

SBOU004

resistor and therefore the load impedance to the

driving source. If impedance matching is desired, RG

The demonstration fixtures can be requested at the

may be set equal to the required termination value.

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

However,

low

inverting

gains,

the

resultant

OPA2830 product folder.

feedback resistor value can present a significant load

to the amplifier output. For example, an inverting gain

of 2 with a 50

Ω input matching resistor (= RG) would

Computer simulation of circuit performance using

require a 100

Ω feedback resistor, which would

SPICE

often

quick

way

analyze

the

contribute to output loading in parallel with the

performance of the OPA2830 and its circuit designs.

external load. In such a case, it would be preferable

This is particularly true for video and RF amplifier

to increase both the RF and RG values, and then

circuits where parasitic capacitance and inductance

achieve the input matching impedance with a third

can play a major role on circuit performance. A

resistor to ground (see Figure 84). The total input

SPICE model for the OPA2830 is available through

impedance becomes the parallel combination of RG

the TI web page (www.ti.com). The applications

and the additional shunt resistor.

department is also available for design assistance.

These models predict typical

small

signal

AC,

transient steps, DC performance, and noise under a

NONINVERTING OPERATION

wide variety of operating conditions. The models

Voltage-feedback

amps

exhibit

decreasing

include the noise terms found in the electrical

closed-loop

bandwidth

the

signal

gain

specifications of the data sheet. These models do not

increased. In theory, this relationship is described by

attempt to distinguish between the package types in

the Gain Bandwidth Product (GBP) shown in the

their small-signal AC performance.

specifications.

Ideally,

dividing

GBP

the

noninverting signal gain (also called the Noise Gain,

or NG) will predict the closed-loop bandwidth. In

OPTIMIZING RESISTOR VALUES

practice, this only holds true when the phase margin

Since

the

OPA2830

unity-gain

stable,

approaches

90°,

does

high-gain

voltage-feedback op amp, a wide range of resistor

configurations. At low gains (increased feedback

values may be used for the feedback and gain setting

factors), most amplifiers will exhibit a more complex

resistors. The primary limits on these values are set

response with lower phase margin. The OPA2830 is

by dynamic range (noise and distortion) and parasitic

compensated to give a slightly peaked response in a

capacitance

considerations.

For

noninverting

noninverting gain of 2 (see Figure 72). This results in

unity-gain

follower

application,

the

feedback

a typical gain of +2 bandwidth of 105MHz, far

connection should be made with a direct short.

exceeding that predicted by dividing the 105MHz

GBP by 2. Increasing the gain will cause the phase

Below 200

Ω, the feedback network will present

margin to approach 90° and the bandwidth to more

additional output loading which can degrade the

closely approach the predicted value of (GBP/NG). At

harmonic distortion performance of the OPA2830.

a gain of +10, the 10MHz bandwidth shown in the

Above

Ω,

the

typical

parasitic

capacitance

Electrical Characteristics agrees with that predicted

(approximately 0.2pF) across the feedback resistor

using the simple formula and the typical GBP of

may cause unintentional band limiting in the amplifier

105MHz.

response.

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