AD827
VIDEO LINE DRIVER
The AD827 functions very well as a low cost, high speed line
driver for either terminated or unterminated cables. Figure 23
shows the AD827 driving a doubly terminated cable in a
follower configuration.
+V
S
A HIGH SPEED THREE OP AMP INSTRUMENTATION
AMPLIFIER CIRCUIT
The instrumentation amplifier circuit shown in Figure 24 can
provide a range of gains. Table II details performance.
+V
S
0.1µF
TRIM FOR BEST
SETTLING TIME
2 – 8pF
V
IN
1/2
AD827
0.1 µF
R
BT
V
OUT
50Ω
500Ω
R
T
50Ω
0.1 µF
–V
IN
3
8
1/2
2 AD827
–
+
1kΩ
1
+V
S
2kΩ
50Ω
0.1µF
2kΩ
R
G
TRIM FOR
OPTIMUM
BANDWIDTH
7 – 15 pF
2
3
7
6
AD847
+
4
0.1µF
–
V
OUT
2kΩ
R
L
–V
S
2kΩ
3pF
2kΩ
C
C
500Ω
+V
IN
–
1/2
5 AD827
+
4
6
1kΩ
7
0.1µF
–V
S
CIRCUIT GAIN = 2000 + 1
R
G
–V
S
NOTE: PINOUT SHOWN IS FOR MINIDIP PACKAGE
Figure 23. A Video Line Driver
The termination resistor, R
T
, (when equal to the cable’s
characteristic impedance) minimizes reflections from the far end
of the cable. While operating from
±
5 V supplies, the AD827
maintains a typical slew rate of 200 V/µs, which means it can
drive a
±
1 V, 30 MHz signal into a terminated cable.
Table I. Video Line Driver Performance Summary
Figure 24. A High Bandwidth Three Op Amp
Instrumentation Amplifier
Table II. Performance Specifications for the
Three Op Amp Instrumentation Amplifier
Gain
V
IN
*
0 dB or
±
500 mV Step
0 dB or
±
500 mV Step
0 dB or
±
500 mV Step
0 dB or
±
500 mV Step
0 dB or
±
500 mV Step
0 dB or
±
500 mV Step
V
SUPPLY
±
15
±
15
±
15
±
5
±
5
±
5
C
C
20 pF
15 pF
0 pF
20 pF
15 pF
0 pF
–3 dB B
W
23 MHz
21 MHz
13 MHz
18 MHz
16 MHz
11 MHz
Over-
shoot
4%
0%
0%
2%
0%
0%
1
2
10
100
R
G
Open
2k
226
Ω
20
Ω
Small Signal
Bandwidth
@ 1 V p-p Output
16.1 MHz
14.7 MHz
4.9 MHz
660 kHz
*–3
dB bandwidth numbers are for the 0 dBm signal input. Overshoot numbers
are the percent overshoot of the 1 V step input.
A back-termination resistor (R
BT
, also equal to the characteristic
impedance of the cable) may be placed between the AD827
output and the cable input, in order to damp any reflected
signals caused by a mismatch between R
T
and the cable’s
characteristic impedance. This will result in a flatter frequency
response, although this requires that the op amp supply
±
2 V to
the output in order to achieve a
±
1 V swing at resistor R
T
.
REV. C
–7–
AD [ ANALOG DEVICES ]
