OPA686 Burr-Brown, OPA686 Datasheet - Page 8

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

FIGURE 1. Non-Inverting, G = +10 Specification and Test
APPLICATIONS INFORMATION
WIDEBAND, NON-INVERTING OPERATION
The OPA686 provides a unique combination of features—
low input voltage noise along with a very low distortion
output stage—to give one of the highest dynamic range op
amps available. Its very high Gain Bandwidth Product (GBP)
can be used either to deliver high signal bandwidths at high
gains, or to deliver very low distortion signals at moderate
frequencies and lower gains. To achieve the full perfor-
mance of the OPA686, careful attention to PC board layout
and component selection is required as discussed in the
remaining sections of this data sheet.
Figure 1 shows the non-inverting gain of +10 circuit used as
the basis of the Electrical Specifications and most of the
Typical Performance Curves. Most of the curves were char-
acterized using signal sources with 50 driving impedance,
and with measurement equipment presenting a 50
impedance. In Figure 1, the 50
terminal matches the source impedance of the test generator,
while the 50
matching resistor for the measurement equipment load.
Generally, data sheet voltage swing specifications are at the
output pin (V
specifications are at the matched 50
load at the output, combined with the 503
network load, presents the OPA686 with an effective output
load of 83
Voltage feedback op amps, unlike current feedback designs,
can use a wide range of resistor values to set their gain. The
circuit of Figure 1, and the specifications at other gains, use
the constraint that R
adjusted to get the desired gain. Using this guideline will
guarantee that the noise added at the output due to Johnson
noise of the resistors will not significantly increase the total
noise over that due to the 1.3nV/ Hz input voltage noise for
the op amp itself.
50 Source
V
I
Circuit.
for the circuit of Figure 1.
®
O
50
series resistor at the V
OPA686
in Figure 1), while output power (dBm)
50
0.1µF
R
G
G
should always be set to 50
+5V
–5V
OPA686
+V
–V
453
S
6.8µF
S
+
R
+
F
6.8µF
shunt resistor at the V
O
load. The total 100
V
terminal provides a
O
0.1µF
50
total feedback
50 Load
and R
load
F
I
8
WIDEBAND, INVERTING GAIN OPERATION
Operating the OPA686 as an inverting amplifier has several
benefits and is particularly appropriate when a matched
input impedance is required. Figure 2 shows the inverting
gain circuit used as the basis of the inverting mode Typical
Performance Curves.
Driving this circuit from a 50 source, and constraining the
gain resistor (R
bandwidth and noise advantage. R
termination resistor and the gain setting resistor for the
circuit. Although the signal gain (V
Figure 2 is double that for Figure 1, the noise gains are in
fact equal when the 50
has the interesting effect of doubling the equivalent GBP of
the amplifier. This can be seen in comparing the G = +10
and G = –20 small-signal frequency response curves. Both
show approximately 250MHz bandwidth, but the inverting
configuration of Figure 2 gives 6dB higher signal gain. If
the signal source is actually the low impedance output of
another amplifier, R
load resistance value allowed for that amplifier and R
should be adjusted to achieve the desired gain. For stable
operation of the OPA686, it is critical that this driving
amplifier show a very low output impedance at frequencies
beyond the expected closed-loop bandwidth for the OPA686.
WIDEBAND, HIGH SENSITIVITY, TRANSIMPEDANCE
DESIGN
The high Gain Bandwidth Product (GBP) and the low input
voltage and current noise for the OPA686 make it an ideal
wideband transimpedance amplifier for low to moderate
transimpedance gains. Very high transimpedance gains
(> 100k ) will benefit from the low input noise current of
a FET-input op amp such as the OPA655. Unity gain
stability in the op amp is not required for application as a
FIGURE 2. Inverting, G = –20 Characterization Circuit.
50 Source
V
I
0.1µF
50
R
G
G
) to equal 50 , will give both a signal
G
91
should be increased to the minimum
source resistor is included. This
+5V
–5V
OPA686
+V
–V
S
S
1k
R
G
F
O
0.1µF
0.1µF
acts as both the input
/V
I
) for the circuit of
V
O
+
+
6.8µF
6.8µF
50
50 Load
F