OPA684IDBVT datasheet(18/25 Pages) TI

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OPA684

SBOS219A

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The Typical Characteristics show the recommended “RS vs

CLOAD” and the resulting frequency response at the load. To

reduce the required value of RS, those curves show a slight

increase in the feedback resistor value and an added load of

250

Ω to ground. The 1kΩ resistor shown in parallel with the

load capacitor is a measurement path and may be omitted.

Parasitic capacitive loads greater than 5pF can begin to

degrade the performance of the OPA684. Long PC board

traces, unmatched cables, and connections to multiple de-

vices can easily cause this value to be exceeded. Always

consider this effect carefully, and add the recommended

series resistor as close as possible to the OPA684 output pin

(see Board Layout Guidelines).

DISTORTION PERFORMANCE

The OPA684 provides very low distortion in a low-power part.

The CFBplus architecture also gives two significant areas of

distortion improvement. First, in operating regions where the

2nd-harmonic distortion due to output stage nonlinearities is

very low (frequencies < 1MHz, low output swings into light

loads), the linearization at the inverting node provided by the

CFBplus design gives 2nd-harmonic distortions that extend

into the –90dBc region. Previous current-feedback amplifiers

have been limited to approximately –85dBc due to the

nonlinearities at the inverting input. The 2nd-area of distor-

tion improvement comes in a distortion performance that is

largely gain independent. To the extent that the distortion at

a particular output power is output stage dependent, 3rd-

harmonics particularly, and to a lesser extent 2nd-harmonic

distortion, are constant as the gain is increased. This is due

to the constant loop gain versus signal gain provided by the

CFBplus design. As shown in the Typical Characteristics,

while the 3rd-harmonic is constant with gain, the 2nd-har-

monic degrades at higher gains. This is largely due to board

parasitic issues. Slightly imbalanced load return currents will

couple into the gain resistor to cause a portion of the 2nd-

harmonic distortion. At high gains, this imbalance has more

gain to the output giving increased 2nd-harmonic distortion.

Relative to alternative amplifiers with < 2mA supply current,

the OPA684 holds much lower distortion at higher frequen-

cies (> 5MHz) and to higher gains. Generally, until the

fundamental signal reaches very high frequency or power

levels, the 2nd-harmonic will dominate the distortion with a

lower 3rd-harmonic component. Focusing then on the 2nd

harmonic, increasing the load impedance improves distortion

directly. Remember that the total load includes the feedback

network—in the non-inverting configuration (see Figure 1)

this is the sum of RF + RG, while in the inverting configuration

it is just RF. Also, providing an additional supply decoupling

capacitor (0.1

µF) between the supply pins (for bipolar opera-

tion) improves the 2nd-order distortion slightly (3dB to 6dB).

In most op amps, increasing the output voltage swing in-

creases harmonic distortion directly. A low-power part like

the OPA684 includes quiescent boost circuits to provide the

full-power bandwidth shown in the Typical Characteristics.

These act to increase the bias in a very linear fashion only

when high slew rate or output power are required. This also

acts to actually reduce the distortion slightly at higher output

power levels. The Typical Characteristics show the 2nd-

harmonic holding constant from 500mVp-p to 5Vp-p outputs,

while the 3rd harmonics actually decrease with increasing

output power.

The OPA684 has an extremely low 3rd-order harmonic

distortion, particularly for light loads and at lower frequen-

cies. This also gives low 2-tone 3rd-order intermodulation

distortion, as shown in the Typical Characteristics. Since the

OPA684 includes internal power boost circuits to retain good

full-power performance at high frequencies and outputs, it

does not show a classical 2-tone, 3rd-order intermodulation

intercept characteristic. Instead, it holds relatively low and

constant 3rd-order intermodulation spurious levels over power.

The Typical Characteristics show this spurious level as a dBc

below the carrier at fixed center frequencies swept over

single-tone power at a matched 50

Ω load. These spurious

levels drop significantly (> 12dB) for lighter loads than the

100

Ω used in the 2-tone 3rd-order intermodulation plot.

Converter inputs for instance will see < –82dBc 3rd-order

spurious to 10MHz for full-scale inputs. For even lower 3rd-

order intermodulation distortion to much higher frequencies,

consider the OPA685.

NOISE PERFORMANCE

Wideband current-feedback op amps generally have a higher

output noise than comparable voltage-feedback op amps.

The OPA684 offers an excellent balance between voltage

and current noise terms to achieve low output noise in a low

power amplifier. The inverting current noise (17pA/

√Hz) is

lower than most other current-feedback op amps, while the

input voltage noise (3.7nV/

√Hz) is lower than any unity-gain

stable, comparable slew rate, voltage-feedback op amp. This

low input voltage noise was achieved at the price of higher

non-inverting input current noise (9.4pA/

√Hz). As long as the

AC source impedance looking out of the non-inverting node

is less than 200

Ω, this current noise will not contribute

significantly to the total output noise. The op amp input

voltage noise and the two input current noise terms combine

to give low output noise under a wide variety of operating

conditions. Figure 13 shows the op amp noise analysis

model with all the noise terms included. In this model, all

noise terms are taken to be noise voltage or current density

terms in either nV/

√Hz or pA/√Hz.

4kT

OPA684

4kT = 1.6E –20J

at 290

°K

4kTR

√

4kTR

√

FIGURE 13. Op Amp Noise Analysis Model.

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