OPA690 datasheet(22/35 Pages) TI

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OPERATING RECOMMENDATIONS

OPTIMIZING RESISTOR VALUES

INVERTING AMPLIFIER OPERATION

BANDWIDTH vs GAIN: NONINVERTING

OPA2890

SBOS364A – DECEMBER 2007 – REVISED DECEMBER 2007

bandwidth to more closely approach the predicted

value of (GBP/NG). At a gain of +10V/V, the 12MHz

bandwidth shown in the Electrical Characteristics

Because

the

OPA2890

unity-gain

stable,

agrees with that predicted using the simple formula

voltage-feedback op amp, a wide range of resistor

and the typical GBP of 120MHz.

values may be used for the feedback and gain setting

The frequency response in a gain of +2V/V may be

resistors. The primary limits on these values are set

modified to achieve exceptional flatness simply by

by dynamic range (noise and distortion) and parasitic

increasing the noise gain to 2.5V/V. One way to

capacitance

considerations.

For

noninverting

modify the response without affecting the +2V/V

unity-gain

follower

application,

the

feedback

signal gain, is to add an 1.5k

Ω resistor across the two

connection should be made with a 25

Ω resistor, not a

inputs, as illustrated in the circuit of Figure 49. A

direct short. This feedback resistor isolates the

similar technique may be used to reduce peaking in

inverting input capacitance from the output pin and

unity-gain

(voltage

follower)

applications.

For

improve the frequency response flatness. Usually, the

example, by using a 750

Ω feedback resistor along

feedback resistor value should be between 200

Ω and

with a 750

Ω resistor across the two op amp inputs,

1.5k

Ω. Below 200Ω, the feedback network presents

the voltage follower response is similar to the gain of

additional output loading that can degrade the

+2V/V response of Figure 50. Reducing the value of

harmonic distortion performance of the OPA2890.

the resistor across the op amp inputs further limits the

Above

1.5k

Ω,

the

typical

parasitic

capacitance

frequency response due to increased noise gain.

(approximately 0.2pF) across the feedback resistor

can cause unintentional band-limiting in the amplifier

The OPA2890 exhibits minimal bandwidth reduction

response.

going to single-supply (+5V) operation as compared

with ±5V. This feature arises because the internal

A good rule of thumb is to target the parallel

bias control circuitry retains nearly constant quiescent

combination of RF and RG (see Figure 49) to be less

current as the total supply voltage between the

than approximately 400

Ω. The combined impedance

supply pins changes.

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 ||

The OPA2890 is a general-purpose, wideband,

RG < 400Ω keeps this pole above 160MHz. By itself,

voltage-feedback op amp; therefore, all of the familiar

this constraint implies that the feedback resistor RF

op amp application circuits are available to the

can increase to several k

Ω at high gains. This

designer. Inverting operation is one of the more

increase in resistor size is acceptable as long as the

common

requirements

and

offers

several

pole formed by RF and any parasitic capacitance

performance benefits. See Figure 59 for a typical

appearing in parallel is kept out of the frequency

inverting configuration where the I/O impedances and

range of interest.

signal gain from Figure 49 are retained in an inverting

circuit configuration.

OPERATION

In the inverting configuration, three key design

considerations must be noted. The first is that the

Voltage-feedback

amps

exhibit

decreasing

gain resistor (RG) becomes part of the signal channel

closed-loop bandwidth as the signal gain increases.

input impedance. If input impedance matching is

In theory, this relationship is described by the Gain

desired (which is beneficial whenever the signal is

Bandwidth Product (GBP) shown in the Electrical

coupled through a cable, twisted-pair, long PCB

Characteristics.

Ideally,

dividing

GBP

the

trace, or other transmission line conductor), RG may

noninverting signal gain (also called the Noise Gain,

be set equal to the required termination value and RF

NG)

predicts

the

closed-loop

bandwidth.

adjusted to give the desired gain. This consideration

practice, this principle only holds true when the phase

is the simplest approach and results in optimum

margin approaches 90

°, as it does in high gain

bandwidth and noise performance. However, at low

configurations. At low gains (increased feedback

inverting gains, the resultant feedback resistor value

factors), most amplifiers exhibit a more complex

can present a significant load to the amplifier output.

response with lower phase margin. The OPA2890 is

For an inverting gain of –2V/V, setting RG to 50Ω for

compensated to give a slightly peaked response in a

input matching eliminates the need for RM but

noninverting gain of 2V/V (see Figure 49). This

requires a 100

Ω feedback resistor. This consideration

compensation results in a typical gain of +2V/V

has the interesting advantage that the noise gain

bandwidth of 100MHz, far exceeding that predicted

becomes

equal

2V/V

for

Ω

source

by dividing the 60MHz GBP by 2. Increasing the gain

impedance—the same as the noninverting circuits

causes the phase margin to approach 90

° and the

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