11

®

OPA686

the primary through to the secondary as a 200

Ω source

impedance and likewise, the 200

Ω R

through to the transformer primary as a 50

Ω input match-

ing impedance. The noise gain (NG) to the amplifier

output is then 1+ 1000/400 = 3.5V/V. Taking the op amp’s

1.3nV/

√Hz input voltage noise times this noise gain to the

output, then reflecting this noise term to the input side of

for the non-inverting input voltage noise when reflected to

the input point for the op amp circuit. This is further

reduced when referred back to the transformer primary.

The 14dB gain to the matched load for the circuit of Figure

6 is precisely controlled (

±0.2dB) and gives a 6dB noise

figure at the input of the transformer. The DC noise gain for

this circuit (3.5) is below the specified minimum stable gain.

This will improve the distortion performance at frequencies

below 20MHz from those shown in the Typical Performance

Curves. Adding the inverting compensation capacitors holds

this configuration stable as described in the previous section.

Measured results show 140MHz small-signal bandwidth for

the circuit of Figure 6 with

±0.1dB flatness through 50MHz.

The OPA686 will easily deliver a 2Vp-p ADC full-scale

input at the matched 50

Ω load. Two-tone testing at 20MHz

for the circuit of Figure 6 (1Vp-p for each test tone) shows

that the two-tone intermodulation intercept has improved to

40dBm versus the 35dBm shown in the Typical Perfor-

mance Curves, giving a 72dBc SFDR for the two 4dBm test

tones at the load .

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

Two PC boards are available to assist in the initial evaluation

of circuit performance using the OPA686 in its two package

styles. Both of these are available free as an unpopulated PC

board delivered with descriptive documentation. The sum-

mary information for these boards is shown in the table

below.

BOARD

LITERATURE

PART

REQUEST

PRODUCT

PACKAGE

NUMBER

NUMBER

OPA686U

8-Pin SO-8

DEM-OPA68xU

MKT-351

OPA686N

5-Lead SOT23-5

DEM-OPA6xxN

MKT-348

Contact the Burr-Brown applications support line to request

any of these boards.

MACROMODELS AND APPLICATIONS SUPPORT

Computer simulation of circuit performance using SPICE is

often useful when analyzing the performance of analog

circuits and systems. This is particularly true for video and

RF amplifier circuits where parasitic capacitance and induc-

tance can have a major effect on circuit performance. A

SPICE model for the OPA686 is available through either the

Burr-Brown Internet web page (http://www.burr-brown.com)

or as one model on a disk from the Burr-Brown Applications

department (1-800-548-6132). The Applications department

is also available for design assistance at this number. These

models do a good job of predicting small-signal AC and

transient performance under a wide variety of operating

conditions. They do not do as well in predicting the har-

monic distortion characteristics. These models do not at-

tempt to distinguish between the package types in their

small-signal AC performance.

OPERATING SUGGESTIONS

SETTING RESISTOR VALUES TO MINIMIZE NOISE

The OPA686 provides a very low input noise voltage while

requiring a low 12mA quiescent current. To take full advan-

tage of this low input noise, careful attention to the other

possible noise contributors is required. Figure 7 shows the

op amp noise analysis model with all the noise terms

included. In this model, all the 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 squared contributing terms to the output

noise voltage. This computation adds all the contributing

noise powers at the output by superposition, then takes the

square root to get back to a spot noise voltage. Equation 1

shows the general form for this output noise voltage using

the terms shown in Figure 7.

Equation 1

will give the equivalent input-referred spot noise voltage at

the non-inverting input as shown in Equation 2.

Equation 2

4kT

R

G

R

G

R

F

R

S

OPA686

I

BI

E

O

I

BN

4kT = 1.6E –20J

at 290°K

E

RS

E

NI

4kTR

S

√

4kTR

F

√

E

E

NI

S

E

E

NI

I

NG









2

+ 4

kTR

F

NG

FIGURE 7. Op Amp Noise Analysis Model.