OPA686 Burr-Brown, OPA686 Datasheet - Page 11

• Manufacturer Part Number: OPA686
• Description: Wideband / Low Noise / Voltage Feedback OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

the primary through to the secondary as a 200
impedance and likewise, the 200
through to the transformer primary as a 50
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
the R
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 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.
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
PRODUCT
OPA686U
OPA686N
G
resistor, divides it by 5. This gives a net gain of 0.7
5-Lead SOT23-5
8-Pin SO-8
PACKAGE
DEM-OPA68xU
DEM-OPA6xxN
NUMBER
BOARD
PART
R
G
resistor is reflected
input match-
LITERATURE
REQUEST
NUMBER
MKT-351
MKT-348
source
11
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
Dividing this expression by the noise gain (NG = 1+R
will give the equivalent input-referred spot noise voltage at
the non-inverting input as shown in Equation 2.
Equation 2
FIGURE 7. Op Amp Noise Analysis Model.
E
E
E
O
RS
N
R
S
E
E
4kTR
4kT
R
NI
NI
G
S
I
BN
I
I
BN
BN
R
E
NI
S
R
S
R
kTR
G
kTR
OPA686
S
OPA686
NG
S
I
BI
R
F
I
4kT = 1.6E –20J
I
BI
NG
BI
R
R
at 290°K
4kTR
F
F
F
kTR
kTR
NG
F
F
/R
NG
E
F
O
G
)
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