OPA683IDBVT datasheet(18/31 Pages) TI

18 / 31 page

OPA683

SBOS221E

www.ti.com

cludes the feedback network—in the noninverting configura-

tion (see Figure 1) this is the sum of RF + RG, while in the

inverting configuration it is just RF. Also, providing an addi-

tional supply decoupling capacitor (0.1

µF) between the sup-

ply pins (for bipolar operation) improves the 2nd-order distor-

tion slightly (3dB to 6dB).

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

creases harmonic distortion directly. A low-power part like the

OPA683 includes quiescent boost circuits to provide the full-

power bandwidth shown. These act to increase the bias in a

very linear fashion only when high slew rate or output power

are required. The Typical Characteristics show the 2nd-har-

monic increasing slightly from 500mVPP to 5VPP outputs while

the 3rd-harmonics also increase with output power.

The OPA683 has an extremely low 3rd-order harmonic distor-

tion—particularly for light loads and at lower frequencies. This

also gives low 2-tone, 3rd-order intermodulation distortion as

shown in the Typical Characteristics. Since the OPA683

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 charac-

teristic. Instead, it holds relatively low and constant 3rd-order

intermodulation spurious levels over power. The Typical Char-

acteristics show this spurious level as a dBc below the carrier

at fixed center frequencies swept over single-tone voltage

swing at a 1k

Ω load. Very light loads such as ADC inputs for

will see < –85dBc 3rd-order spurious to 1MHz for full-scale

inputs. For much lower 3rd-order intermodulation distortion

through 200MHz, consider the OPA685.

NOISE PERFORMANCE

Wideband current-feedback op amps generally have a higher

output noise than comparable voltage feedback op amps. The

OPA683 offers an excellent balance between voltage and

current noise terms to achieve low output noise in a low- power

amplifier. The inverting current noise (11.6pA/

√Hz) is lower

than most other current feedback op amps while the input

voltage noise (4.4nV/

√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 noninverting

input current noise (5.1pA/

√Hz). As long as the AC source

impedance looking out of the noninverting node is less than

300

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.

The total output spot noise voltage can be computed as the

square root of the sum of all squared output noise voltage

contributors. Equation 3 shows the general form for the

output noise voltage using the terms shown in Figure 13.

(3)

kTR

I R

kTR G

BN SS

NBI F

(

) +









(

) +

4kT

OPA683

4kT = 1.6E –20J

at 290

°K

4kTR

√

4kTR

√

Dividing this expression by the noise gain (NG = (1 + RF/RG))

will give the equivalent input referred spot noise voltage at the

noninverting input, as shown in Equation 4.

(4)

kTR

NNI

BN SS

BI F

(

) ++









Evaluating these two equations for the OPA683 circuit and

component values (see Figure 1) will give a total output spot

noise voltage of 17.6nV/

√Hz and a total equivalent input spot

noise voltage of 8.8nV/

√Hz. This total input referred spot

noise voltage is higher than the 4.4nV/

√Hz specification for

the op amp voltage noise alone. This reflects the noise

added to the output by the inverting current noise times the

feedback resistor. As the gain is increased, this fixed output

noise power term contributes less to the total output noise

and the total input referred voltage noise given by Equation

3 will approach just the 4.4nV/

√Hz of the op amp itself. For

example, going to a gain of +20 in the circuit of Figure 1,

adjusting only the gain resistor to 63.2

Ω, will give a total input

referred noise of 4.6nV/

√Hz. A more complete description of

op amp noise analysis can be found in the TI application note

AB-103 (SBOA066). Refer to Texas Instruments’ web site at

www.ti.com.

DC ACCURACY AND OFFSET CONTROL

A current-feedback op amp like the OPA683 provides excep-

tional bandwidth in high gains, giving fast pulse settling but

only moderate DC accuracy. The Electrical Characteristics

show an input offset voltage comparable to high slew rate

voltage-feedback amplifiers. However, the two input bias

currents are somewhat higher and are unmatched. Whereas

bias current cancellation techniques are very effective with

most voltage feedback op amps, they do not generally

reduce the output DC offset for wideband current-feedback

op amps. Since the two input bias currents are unrelated in

both magnitude and polarity, matching the source imped-

ance looking out of each input to reduce their error contribu-

tion to the output is ineffective. Evaluating the configuration

of Figure 1, using worst case +25

°C input offset voltage and

FIGURE 13. Op Amp Noise Analysis Model.

OPA683IDBVT Datasheet(PDF) 18 Page - Texas Instruments

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