
AD841
REV. B
–8–
C
P
Figure 25. Circuit for Driving a Large Capacitive Load
USING A HEAT SINK
The AD841 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 4 to 5 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
Engineering #602B.
TERMINATED LINE DRIVER
The AD841 functions very well as a high speed line driver of ei-
ther terminated or unterminated cables. Figure 26 shows the
AD841 driving a doubly terminated cable in a follower configu-
ration. The AD841 maintains a typical slew rate of 300 V/
μ
s,
which means it can drive a
±
10 V, 4.7 MHz signal or a
±
3 V,
15.9 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 AD841 output and the cable in order to damp any stray sig-
nals caused by a mismatch between R
T
and the cable’s charac-
teristic impedance. This will result in a “cleaner” signal, but
since 1/2 the output voltage will be dropped across R
BT
, the op
amp must supply double the output signal required if there is no
back termination. Therefore the full power bandwidth is cut in
half.
If termination is not used, cables appear as capacitive loads. If
this capacitive load is large, it should be decoupled from the
AD841 by a resistor in series with the output (see above:
Driving a Capacitive Load).
Figure 26. Line Driver Configuration
OVERDRIVE RECOVERY
Figure 27 shows the overdrive recovery capability of the AD841.
Typical recovery time is 200 ns from negative overdrive and
700 ns from positive overdrive.
Figure 27. Overdrive Recovery
Figure 28. Overdrive Recovery Test Circuit
E-20A
20-Terminal Leadless
Ceramic Chip Carrier
12-Lead Metal Can Package
(TO-8 Style)
14-Pin Cerdip (Q) Package
14-Pin Plastic (N) Package
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).