9

OPA134/2134/4134

®

V

OUT

V

IN

R

1

If R

R

R

2

OPA134

configuration alters the feedback factor or noise gain of the

circuit. The closed-loop gain is unchanged, but the feedback

available for error correction is reduced by a factor of 101,

thus extending the resolution by 101. Note that the input

signal and load applied to the op amp are the same as with

be kept small to minimize its effect on the distortion mea-

surements.

Validity of this technique can be verified by duplicating

measurements at high gain and/or high frequency where the

distortion is within the measurement capability of the test

equipment. Measurements for this data sheet were made

with an Audio Precision distortion/noise analyzer which

greatly simplifies such repetitive measurements. The mea-

surement technique can, however, be performed with manual

distortion measurement instruments.

SOURCE IMPEDANCE AND DISTORTION

For lowest distortion with a source or feedback network

which has an impedance greater than 2k

Ω, the impedance

seen by the positive and negative inputs in noninverting

applications should be matched. The p-channel JFETs in the

FET input stage exhibit a varying input capacitance with

applied common-mode input voltage. In inverting configu-

rations the input does not vary with input voltage since the

inverting input is held at virtual ground. However, in

noninverting applications the inputs do vary, and the gate-

to-source voltage is not constant. The effect is increased

distortion due to the varying capacitance for unmatched

source impedances greater than 2k

Ω.

To maintain low distortion, match unbalanced source im-

pedance with appropriate values in the feedback network as

shown in Figure 3. Of course, the unbalanced impedance

may be from gain-setting resistors in the feedback path. If

matching impedance on the noninverting input should be

used. As always, resistor values should be minimized to

reduce the effects of thermal noise.

FIGURE 3. Impedance Matching for Maintaining Low

Distortion in Non-Inverting Circuits.

NOISE PERFORMANCE

Circuit noise is determined by the thermal noise of external

resistors and op amp noise. Op amp noise is described by

two parameters—noise voltage and noise current. The total

noise is quantified by the equation:

With low source impedance, the current noise term is

insignificant and voltage noise dominates the noise perfor-

mance. At high source impedance, the current noise term

becomes the dominant contributor.

Low noise bipolar op amps such as the OPA27 and OPA37

provide very low voltage noise at the expense of a higher

current noise. However, OPA134 series op amps are unique

in providing very low voltage noise and very low current

noise. This provides optimum noise performance over a

wide range of sources, including reactive source imped-

ances, refer to the typical curve, “Voltage Noise vs Source

Resistance.” Above 2k

Ω source resistance, the op amp

contributes little additional noise—the voltage and current

terms in the total noise equation become insignificant and

the source resistance term dominates. Below 2k

Ω, op amp

voltage noise dominates over the resistor noise, but com-

pares favorably with other audio op amps such as OP176.

PHASE REVERSAL PROTECTION

OPA134 series op amps are free from output phase-reversal

problems. Many audio op amps, such as OP176, exhibit

phase-reversal of the output when the input common-mode

voltage range is exceeded. This can occur in voltage-fol-

lower circuits, causing serious problems in control loop

applications. OPA134 series op amps are free from this

undesirable behavior even with inputs of 10V beyond the

input common-mode range.

POWER DISSIPATION

OPA134 series op amps are capable of driving 600

Ω loads

with power supply voltage up to

±18V. Internal power

dissipation is increased when operating at high supply

voltages. Copper leadframe construction used in OPA134

series op amps improves heat dissipation compared to con-

ventional materials. Circuit board layout can also help

minimize junction temperature rise. Wide copper traces help

dissipate the heat by acting as an additional heat sink.

Temperature rise can be further minimized by soldering the

devices to the circuit board rather than using a socket.

OUTPUT CURRENT LIMIT

Output current is limited by internal circuitry to approxi-

mately

±40mA at 25°C. The limit current decreases with

increasing temperature as shown in the typical performance

curve “Short-Circuit Current vs Temperature.”

V total

i R

e

kTR

nn

S

n

s

()

(

)

=+

+

2

2

4