OPA690IDBVT datasheet(19/37 Pages) TI1

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OPA690

www.ti.com

SBOS223F – DECEMBER 2001 – REVISED FEBRUARY 2010

OPERATING SUGGESTIONS

OPTIMIZING RESISTOR VALUES

BANDWIDTH VERSUS GAIN: NONINVERTING

OPERATION

Since

the

OPA690

unity-gain

stable,

voltage-feedback op amp, a wide range of resistor

Voltage-feedback

amps

exhibit

decreasing

values may be used for the feedback and gain setting

closed-loop

bandwidth

the

signal

gain

resistors. The primary limits on these values are set

increased. In theory, this relationship is described by

by dynamic range (noise and distortion) and parasitic

the Gain Bandwidth Product (GBP) shown in the

capacitance

considerations.

For

noninverting

Electrical Characteristics. Ideally, dividing GBP by the

unity-gain

follower

application,

the

feedback

noninverting signal gain (also called the Noise Gain,

connection should be made with a 25

Ω resistor, not a

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

direct short. This will isolate the inverting input

practice, this only holds true when the phase margin

capacitance from the output pin and improve the

approaches

90°,

does

high

gain

frequency response flatness. Usually, for G > 1

configurations. At low gains (increased feedback

applications, the feedback resistor value should be

factors), most amplifiers will exhibit a more complex

between 200

Ω and 1.5kΩ. Below 200Ω, the feedback

response with lower phase margin. The OPA690 is

network will present additional output loading which

compensated to give a slightly peaked response in a

can degrade the harmonic distortion performance of

noninverting gain of 2 (see Figure 36). This results in

the OPA690. Above 1.5k

Ω, the typical parasitic

a typical gain of +2 bandwidth of 220MHz, far

capacitance

(approximately

0.2pF)

across

the

exceeding that predicted by dividing the 300MHz

feedback

resistor

may

cause

unintentional

GBP by 2. Increasing the gain will cause the phase

band-limiting in the amplifier response.

margin to approach 90° and the bandwidth to more

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

A good rule of thumb is to target the parallel

a gain of +10, the 30MHz bandwidth shown in the

combination of RF and RG (see Figure 36) to be less

Electrical Characteristics agrees with that predicted

than approximately 300

Ω. The combined impedance

using the simple formula and the typical GBP of

RF || RG interacts with the inverting input capacitance,

300MHz.

placing an additional pole in the feedback network

and thus, a zero in the forward response. Assuming a

The frequency response in a gain of +2 may be

2pF total parasitic on the inverting node, holding RF ||

modified to achieve exceptional flatness simply by

RG < 300Ω will keep this pole above 250MHz. By

increasing the noise gain to 2.5. One way to do this,

itself, this constraint implies that the feedback resistor

without affecting the +2 signal gain, is to add an

RF can increase to several kΩ at high gains. This is

804

Ω resistor across the two inputs in the circuit of

acceptable as long as the pole formed by RF and any

Figure 36. A similar technique may be used to reduce

parasitic capacitance appearing in parallel is kept out

peaking in unity-gain (voltage follower) applications.

of the frequency range of interest.

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 37. Reducing the

value of the resistor across the op amp inputs will

further limit the frequency response 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 between the supply pins is

changed.

Product Folder Link(s): OPA690

OPA690IDBVT Datasheet(PDF) 19 Page - Texas Instruments

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