OPA687 Burr-Brown, OPA687 Datasheet - Page 8

• Manufacturer Part Number: OPA687
• Description: Wideband / Ultra-Low Noise / Voltage Feedback OPERATIONAL AMPLIFIER With Power Down
• Manufacturer: Burr-Brown

APPLICATIONS INFORMATION
WIDEBAND, NON-INVERTING OPERATION
The OPA687 provides a unique combination of a very 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 to either 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 OPA687, 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 +20 circuit used as
the basis for most of the Typical Performance Curves. Most
of the curves were characterized using signal sources with
50
presenting a 50
shunt resistor at the V
ance of the test generator, while the 50 series resistor at the
V
ment equipment load. Generally, data sheet voltage swing
specifications are at the output pin (V
output power specifications are at the matched 50
The total 100 load at the output, combined with the 790
total feedback network load, presents the OPA687 with an
effective output load of 89
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
an R
Using this guideline will guarantee that the noise added at the
output due to Johnson noise of the resistors will not signifi-
cantly increase the total over that due to the 0.95nV/ Hz input
voltage noise for the op amp itself. This R
good starting point for design. Other values are certainly
acceptable if required by the design.
FIGURE 1. Non-Inverting G = +20 Specifications and Test
O
50 Source
terminal provides a matching resistor for the measure-
G
driving impedance, and with measurement equipment
V
I
set to 39.2
Circuit.
®
50
OPA687
39.2
load impedance. In Figure 1, the 50
0.1 F
and R
R
G
I
terminal matches the source imped-
+5V
–5V
F
OPA687
adjusted to get the desired gain.
+V
–V
for the circuit of Figure 1.
750
S
6.8 F
S
+
R
+
F
6.8 F
O
V
O
in Figure 1), while
G
0.1 F
is suggested as a
50
50 Load
load.
8
WIDEBAND, INVERTING GAIN OPERATION
There can be significant benefits to operating the OPA687 as
an inverting amplifier. This is particularly true when a
matched input impedance is required. Figure 2 shows the
inverting gain circuit used as a starting point for the Typical
Performance Curves showing inverting mode performance.
FIGURE 2. Inverting G = –40 Specifications and Test
Driving this circuit from a 50 source, and constraining the
gain resistor, R
bandwidth and noise advantage. R
as both the input termination resistor and the gain setting
resistor for the circuit. Although the signal gain for the
circuit of Figure 2 is double that for Figure 1, their noise
gains are equal when the 50
This has the interesting effect of doubling the equivalent
GBP for the amplifier. This can be seen in comparing the
G = +12 and G = –20 small-signal frequency response
curves. Both show approximately 500MHz bandwidth with
3dB peaking, but the inverting configuration of Figure 2 is
giving 4.4dB higher signal gain. The noise gains are ap-
proximately equal in this case. If the signal source is
actually the low impedance output of another amplifier, R
should be increased to be greater than the minimum value
allowed at the output of that amplifier and R
get the desired gain. It is critical for stable operation of the
OPA687 that this driving amplifier show a very low output
impedance through frequencies exceeding the expected
closed-loop bandwidth for the OPA687.
WIDEBAND, HIGH SENSITIVITY,
TRANSIMPEDANCE DESIGN
The high Gain Bandwidth Product (GBP) and low input voltage
and current noise for the OPA687 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
50 Source
V
I
0.1 F
50
Circuit.
R
G
G
, to equal 50 , will give both a signal
95.3
–5V
+5V
OPA687
+V
–V
source resistor is included.
S
S
2k
R
G
F
0.1 F
0.1 F
, in this case, is acting
V
O
+
+
6.8 F
6.8 F
50
F
adjusted to
50 Load
G