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參數(shù)資料
| 型號(hào)： | AD842* |
| 英文描述： | Wideband. High Output Current. Fast Settling Op Amp |
| 中文描述： | 寬帶。高輸出電流?？焖俜€(wěn)定運(yùn)算放大器 |
| 文件頁(yè)數(shù)： | 8/10頁(yè) |
| 文件大?。?/td> | 475K |
AD842
–
8
–
REV. E
GROUNDING AND BYPASSING
In designing practical circuits with the AD842, the user must
remember that whenever high frequencies are involved, some
Figure 24. AD842 Settling Demonstrating No Settling
Tails
special precautions are in order. Circuits must be built with
short interconnect leads. Large ground planes should be used
whenever possible to provide a low resistance, low inductance
circuit path, as well as minimizing the effects of high frequency
coupling. Sockets should be avoided because the increased
interlead capacitance can degrade bandwidth.
Feedback resistors should be of low enough value to assure that
the time constant formed with the circuit capacitances will not
limit the amplifier performance. Resistor values of less than
5 k
are recommended. If a larger resistor must be used, a small
(<10 pF) feedback capacitor connected in parallel with the feed-
back resistor, R
F
, may be used to compensate for these stray
capacitances and optimize the dynamic performance of the
amplifier in the particular application.
Power supply leads should be bypassed to ground as close as
possible to the amplifier pins. A 2.2
μ
F capacitor in parallel with
a 0.1
μ
F ceramic disk capacitor is recommended.
CAPACITIVE LOAD DRIVING ABILITY
Like all wideband amplifiers, the AD842 is sensitive to capaci-
tive loading. The AD842 is designed to drive capacitive loads of
up to 20 pF without degradation of its rated performance. Ca-
pacitive loads of greater than 20 pF will decrease the dynamic
performance of the part although instability should not occur
unless the load exceeds 100 pF.
USING A HEAT SINK
The AD842 draws less quiescent power than most precision
high speed amplifiers and is specified for operation without a
heat sink. However, when driving low impedance loads, the cur-
rent to the load can be 10 times the quiescent current. This will
create a noticeable temperature rise. Improved performance can
be achieved by using a small heat sink such as the Aavid Engi-
neering #602B.
TERMINATED LINE DRIVER
The AD842 is optimized for high speed line driver applications.
Figure 25 shows the AD842 driving a doubly terminated cable
in a gain-of-2 follower configuration. The AD842 maintains a
typical slew rate of 375 V/
μ
s, which means it can drive a
±
10 V,
6.0 MHz signal or a
±
3 V, 19.9 MHz signal.
The termination resistor, R
T
, (when equal to the characteristic
impedance of the cable) minimizes reflections from the far end
of the cable. A back-termination resistor (R
BT
, also equal to the
characteristic impedance of the cable) may be placed between
the AD842 output and the cable in order to damp any stray
signals caused by a mismatch between R
T
and the cable’s char-
acteristic impedance. This will result in a “cleaner” signal. With
this circuit, the voltage on the line equals V
IN
because one half
of V
OUT
is dropped across R
BT
.
The AD842 has
±
100 mA minimum output current and, there-
fore, can drive
±
5 V into a 50
cable.
The feedback resistors, R1 and R2, must be chosen carefully.
Large value resistors are desirable in order to limit the amount
of current drawn from the amplifier output. But large resistors
can cause amplifier instability because the parallel resistance
R1 R2 combines with the input capacitance (typically 2–5 pF) to
create an additional pole. Also, the voltage noise of the AD842
is equivalent to a 5 k
resistor, so large resistors can signifi-
cantly increase the system noise. Resistor values of 1 k
or 2 k
are recommended.
If termination is not used, cables appear as capacitive loads and
can be decoupled from the AD842 by a resistor in series with
the output.
AD842
–
TERMINATION
RESISTOR FOR
INPUT SIGNAL
V
IN
+V
S
0.1 F
2.2 F
–
V
S
0.1 F
2.2 F
R2
R1
R
ST
50 OR 75
CABLE
R
T
R
T
= R
ST
= CABLE CHARACTERISTIC
IMPEDANCE
+
Figure 25. Line Driver Configuration
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