AD847
VID EO LINE D RIVER
Figure 24 shows the AD847 driving 100 pF and 1000 pF loads.
T he AD847 functions very well as a low cost, high speed line
driver for either terminated or unterminated cables. Figure 23
shows the AD847 driving a doubly terminated cable in a fol-
lower configuration.
T he termination resistor, RT , (when equal to the cable’s charac-
teristic impedance) minimizes reflections from the far end of the
cable. While operating from ±5 V supplies, the AD847 main-
tains a typical slew rate of 200 V/µs, which means it can drive a
±1 V, 30 MHz signal into a terminated cable.
+V
S
75Ω
COAX
R
IN
100Ω
0.1 µF
Figure 24. AD847 Driving Capacitive Loads
75Ω
COAX
V
IN
V
AD847
OUT
FLASH AD C INP UT BUFFER
75Ω
R
BT
75Ω
R
75Ω
T
T he 35 MHz unity gain bandwidth of the AD847 makes it an
excellent choice for buffering the input of high speed flash A/D
converters, such as the AD9048.
0.1 µF
500Ω
–V
S
Figure 25 shows the AD847 as a unity inverter for the input to
the AD9048.
500Ω
C
C
0.1
SEE TABLE I
–5.2V
AD589
2k
10kΩ
Figure 23. Video Line Driver
Table I. Video Line D river P erform ance Chart
27
100
2N3906
5
0.1
AD741
1k
O ver-
–3 dB BW shoot
1k
VIN
*
VSUP P LY CC
1.5kΩ
0.1µF
0 dB or ±500 mV Step ±15
0 dB or ±500 mV Step ±15
0 dB or ±500 mV Step ±15
0 dB or ±500 mV Step ±5
0 dB or ±500 mV Step ±5
0 dB or ±500 mV Step ±5
20 pF 23 MHz
15 pF 21 MHz
0 pF 13 MHz
20 pF 18 MHz
15 pF 16 MHz
4%
0%
0%
2%
0%
0%
ANALOG
INPUT
(0V TO +2V)
R
B
R
1.5kΩ
50Ω
T
43Ω
D1
(MSB)
V
AD847
IN
AD9048
CONVERT
TTL
CONVERT
SIGNAL
0 pF
11 MHz
D8
(LSB)
V
V
*–3 dB bandwidth numbers are for the 0 dBm signal input. Overshoot numbers
are the percent overshoot of the 1 volt step input.
EE
CC
0.1µF
0.1µF
A back-termination resistor (RBT, also equal to the characteristic
impedance of the cable) may be placed between the AD847 out-
put and the cable input, in order to damp any reflected signals
caused by a mismatch between RT and the cable’s characteristic
impedance. T his will result in a flatter frequency response, al-
though this requires that the op amp supply ±2 V to the output
in order to achieve a ±1 V swing at resistor RT .
+5.0V
–5.2V
Figure 25. Flash ADC Input Buffer
–10–
REV. F
ROCHESTER [ Rochester Electronics ]
