OPA690 datasheet(21/39 Pages) TI1

21 / 39 page

OPA2690

www.ti.com

SBOS238G – JUNE 2002 – REVISED MARCH 2010

OPERATING SUGGESTIONS

OPTIMIZING RESISTOR VALUES

BANDWIDTH vs GAIN: NONINVERTING

OPERATION

the

OPA2690

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, the feedback

configurations. At low gains (increased feedback

resistor value should be between 200

Ω and 1.5kΩ.

factors), most amplifiers will exhibit a more complex

Below 200

Ω, the feedback network will present

response with lower phase margin. The OPA2690 is

additional output loading which can degrade the

compensated to give a slightly peaked response in a

harmonic distortion performance of the OPA2690.

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

Above

1.5k

Ω,

the

typical

parasitic

capacitance

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

(approximately 0.2pF) across the feedback resistor

exceeding that predicted by dividing the 300MHz

can cause unintentional band-limiting in the amplifier

GBP by 2. Increasing the gain will cause the phase

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 feedback resistor RF

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

can increase to several k

Ω 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 OPA2690 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): OPA2690

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