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參數(shù)資料
| 型號(hào)： | OPA685N |
| 英文描述： | Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable TM |
| 中文描述： | 超寬帶，電流反饋運(yùn)算放大器商標(biāo)使用禁用 |
| 文件頁數(shù)： | 28/28頁 |
| 文件大?。?/td> | 283K |
| 代理商： | OPA685N |
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OPA685
c) Careful selection and placement of external compo-
nents will preserve the high frequency performance of
the OPA685.
Resistors should be a very low reactance type.
Surface-mount resistors work best and allow a tighter overall
layout. Metal-film and carbon composition, axially-leaded
resistors can also provide good high frequency performance.
Again, keep their leads and PC board trace length as short as
possible. Never use wirewound-type resistors in a high
frequency application. Since the output pin and inverting
input pin are the most sensitive to parasitic capacitance,
always position the feedback and series output resistor, if
any, as close as possible to the output pin. Other network
components, such as non-inverting input termination resis-
tors, should also be placed close to the package. Where
double-side component mounting is allowed, place the feed-
back resistor directly under the package on the other side of
the board between the output and inverting input pins. The
frequency response is primarily determined by the feedback
resistor value, as described previously. Increasing its value
will reduce the bandwidth, while decreasing it will give a
more peaked frequency response. The 402
feedback resis-
tor (used in the typical performance specifications at a gain
of +8 on
±
5V supplies) is a good starting point for design.
Note that a 523
feedback resistor, rather than a direct short,
is required for the unity gain follower application. A current-
feedback op amp requires a feedback resistor—even in the
unity gain follower configuration—to control stability.
d) Connections to other wideband devices on the board
may be made with short direct traces or through on-
board transmission lines.
For short connections, consider
the trace and the input to the next device as a lumped
capacitive load. Relatively wide traces (50mils to 100mils)
should be used, preferably with ground and power planes
opened up around them. Estimate the total capacitive load
and set R
S
from the plot of “Recommended R
S
vs Capacitive
Load”. Low parasitic capacitive loads (< 5pF) may not need
an R
S
since the OPA685 is nominally compensated to
operate with a 2pF parasitic load. If a long trace is required,
and the 6dB signal loss intrinsic to a doubly-terminated
transmission line is acceptable, implement a matched im-
pedance transmission line using microstrip or stripline tech-
niques (consult an ECL design handbook for microstrip and
stripline layout techniques). A 50
environment is usually
not necessary on board. In fact, a higher impedance environ-
ment will improve distortion as shown in the distortion
versus load plots. With a characteristic board trace imped-
ance defined (based on board material and trace dimen-
sions), a matching series resistor into the trace from the
output of the OPA685 is used. A terminating shunt resistor
at the input of the destination device is used as well.
Remember also that the terminating impedance will be the
parallel combination of the shunt resistor and the input
impedance of the destination device; this total effective
impedance should be set to match the trace impedance. The
high output voltage and current capability of the OPA685
allows multiple destination devices to be handled as separate
transmission lines, each with their own series and shunt
terminations. If the 6dB attenuation of a doubly-terminated
transmission line is unacceptable, a long trace can be series-
terminated at the source end only. Treat the trace as a
capacitive load in this case and set the series resistor value
as shown in the plot of “R
S
vs Capacitive Load”. This will
not preserve signal integrity as well as a doubly-terminated
line. If the input impedance of the destination device is low,
there will be some signal attenuation due to the voltage
divider formed by the series output into the terminating
impedance.
e) Socketing a high-speed part like the OPA685 is not
recommended.
The additional lead length and pin-to-pin
capacitance introduced by the socket can create an ex-
tremely troublesome parasitic network which can make it
almost impossible to achieve a smooth, stable frequency
response. Best results are obtained by soldering the OPA685
onto the board.
INPUT AND ESD PROTECTION
The OPA685 is built using a very high-speed, complemen-
tary bipolar process. The internal junction breakdown volt-
ages are relatively low for these very small geometry de-
vices. These breakdowns are reflected in the Absolute Maxi-
mum Ratings table where an absolute maximum 13V supply
is reported. All device pins have limited ESD protection
using internal diodes to the power supplies as shown in
Figure 20.
External
Pin
+V
CC
–V
CC
Internal
Circuitry
FIGURE 20. Internal ESD Protection.
These diodes also provide moderate protection to input
overdrive voltages above the supplies. The protection diodes
can typically support 30mA continuous current. Where higher
currents are possible (e.g., in systems with
±
15V supply
parts driving into the OPA685), current-limiting series resis-
tors should be added into the two inputs. Keep these resistor
values as low as possible since high values degrade both
noise performance and frequency response.
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| OPA686 | 制造商:BB 制造商全稱:BB 功能描述:Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER |
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