OPA690IDBVT datasheet(14/22 Pages) TI

14 / 22 page

OPA690

SBOS223A

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midband signal gain is set to +4 (12dB) in this case. The

capacitor to ground on the noninverting input is intentionally

set larger to dominate input parasitic terms. At a gain of +4,

the OPA690 on a single supply will show ~80MHz small- and

large-signal bandwidth. The resistor values have been slightly

adjusted to account for this limited bandwidth in the amplifier

stage. Tests of this circuit show a precise 5MHz, –3dB point

with a maximally flat passband (above the 32kHz AC-cou-

pling corner), and a maximum stopband attenuation of 36dB

at the amplifier’s –3dB bandwidth of 80MHz.

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

Several PC boards are available to assist in the initial evalu-

ation of circuit performance using the OPA690 in its three

package styles. All of these are available free as an

unpopulated PC board delivered with descriptive documenta-

tion. The summary information for these boards is shown

below:

BOARD

LITERATURE

PART

REQUEST

PRODUCT

PACKAGE

NUMBER

OPA690ID

SO-8

DEM-OPA68xU

SBOU009

OPA690IDBV

SOT23-6

DEM-OPA6xxN

SBOU010

The board can be requested on Texas Instruments’ web site

(www.ti.com.).

MACROMODELS AND APPLICATIONS SUPPORT

Computer simulation of circuit performance using SPICE is

often useful when analyzing the performance of analog

circuits and systems. This is particularly true for Video and

RF amplifier circuits where parasitic capacitance and induc-

tance can have a major effect on circuit performance. A

SPICE model for the OPA690 is available through the Texas

Instruments internet web page (http://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 or dG/dP characteristics. These models do not

attempt to distinguish between the package types in their

small-signal AC performance.

OPERATING SUGGESTIONS

OPTIMIZING RESISTOR VALUES

Since the OPA690 is a unity-gain stable voltage feedback op

amp, a wide range of resistor values may be used for the

feedback and gain setting resistors. The primary limits on these

values are set by dynamic range (noise and distortion) and

parasitic capacitance considerations. For a noninverting unity-

gain follower application, the feedback connection should be

made with a 25

Ω resistor, not a direct short. This will isolate the

inverting input capacitance from the output pin and improve the

frequency response flatness. Usually, for G > 1 application, the

feedback resistor value should be between 200

Ω and 1.5kΩ.

Below 200

Ω, the feedback network will present additional

output loading which can degrade the harmonic distortion

performance of the OPA690. Above 1.5k

Ω, the typical parasitic

capacitance (approximately 0.2pF) across the feedback resistor

may cause unintentional band-limiting in the amplifier response.

A good rule of thumb is to target the parallel combination of RF

and RG (see Figure 1) to be less than approximately 300Ω. The

combined impedance RF || RG interacts with the inverting input

capacitance, placing an additional pole in the feedback network

and thus, a zero in the forward response. Assuming a 2pF total

parasitic on the inverting node, holding RF || RG < 300Ω will keep

this pole above 250MHz. By itself, this constraint implies that the

feedback resistor RF can increase to several kΩ at high gains.

This is acceptable as long as the pole formed by RF and any

parasitic capacitance appearing in parallel is kept out of the

frequency range of interest.

BANDWIDTH VERSUS GAIN: NONINVERTING

OPERATION

Voltage feedback op amps 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 specifications. Ideally, dividing GBP by

the noninverting signal gain (also called the Noise Gain, or

NG) will predict the closed-loop bandwidth. In practice, this

only holds true when the phase margin approaches 90

°, as

it does in high gain configurations. At low gains (increased

feedback factors), most amplifiers will exhibit a more com-

plex response with lower phase margin. The OPA690 is

compensated to give a slightly peaked response in a

noninverting gain of 2 (see Figure 1). This results in a typical

gain of +2 bandwidth of 220MHz, far exceeding that pre-

dicted by dividing the 300MHz GBP by 2. Increasing the gain

will cause the phase margin to approach 90

° and the band-

width to more closely approach the predicted value of (GBP/

NG). At a gain of +10, the 30MHz bandwidth shown in the

Electrical Characteristics agrees with that predicted using the

simple formula and the typical GBP of 300MHz.

Frequency response in a gain of +2 may be modified to

achieve exceptional flatness simply by increasing the noise

gain to 2.5. One way to do this, without affecting the +2 signal

gain, is to add an 804

Ω resistor across the two inputs in the

circuit of Figure 1. A similar technique may be used to reduce

peaking in unity-gain (voltage follower) applications. For

example, by using a 402

Ω feedback resistor along with a

402

Ω resistor across the two op amp inputs, the voltage

follower response will be similar to the gain of +2 response

of Figure 2. Further reducing the value of the resistor across

the op amp inputs will further dampen the frequency re-

sponse due to increased noise gain.

The OPA690 exhibits minimal bandwidth reduction going to

single-supply (+5V) operation as compared with

±5V. This is

because the internal bias control circuitry retains nearly

constant quiescent current as the total supply voltage be-

tween the supply pins is changed.

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