AD841 Data Sheet
Rev. C | Page 12 of 16
TERMINATED LINE DRIVER
The AD841 functions very well as a high speed line driver of
either terminated or unterminated cables. Figure 33 shows the
AD841 driving a doubly terminated cable in a follower config-
uration. The AD841 maintains a typical slew rate of 300 V/µs,
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 to damp any stray signals
caused by a mismatch between R
T
and the cable’s characteristic
impedance. This results in a cleaner signal, but because half the
output voltage is dropped across R
BT
, the op amp must supply
double the output signal required if there is no back termina-
tion. 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 Figure 32).
Figure 33. Line Driver Configuration
OVERDRIVE RECOVERY
Figure 34 shows the overdrive recovery capability of the AD841.
Typical recovery time is 200 ns from negative overdrive and
700 ns from positive overdrive.
Figure 34. Overdrive Recovery
Figure 35. Overdrive Recovery Test Circuit
11
6
4
5
10
–
+
AD841
+V
S
–V
S
2.2µF
0.1µF
2.2µF
0.1µF
TERMINATION
RESISTOR
FOR INPUT
SIGNAL
R
B
100Ω
V
IN
R
BT
(OPTIONAL)
R
T
V
OUT
50Ω OR
75Ω CABLE
R
T
= R
BT
= CABLE CHARACTERISTIC IMPEDANCE
11340-033
100
90
10
0%
10V
200ns
1V
OVERDRIVEN
OUTPUT: 10V/DIV
INPUT SQUARE
WAVE: 1V/DIV
11340-034
11
6
4
5
10
–
+
AD841
+V
S
–V
S
2.2µF
0.1µF
2.2µF
0.1µF
OUTPUT
1kΩ
50Ω
HP3314A
PULSE GENERATOR
OR EQUIVALENT
1µs ± 1V SQUARE
WAVE INPUT
11340-035
