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.
A HIGH SPEED 3 OP AMP INSTRUMENTATION
AMPLIFIER CIRCUIT
The instrumentation amplifier circuit shown in Figure 24 can
provide a range of gains. The chart of Table II details
performance.
+V
S
0.1µF
TRIM FOR BEST
SETTLING TIME
2 – 8pF
–V
IN
3
8
1/2
AD827
2
–
+
1kΩ
2kΩ
R
G
TRIM FOR
OPTIMUM
BANDWIDTH
7 – 15 pF
1
+V
S
2kΩ
0.1µF
7
6
AD847
+
4 0.1µF
–
V
OUT
2kΩ
R
L
2
3
2kΩ
3pF
2kΩ
+V
IN
1/2
5 AD827
+
4
6 –
1kΩ
7
0.1µF
–V
S
CIRCUIT GAIN =
2000
+1
R
G
Figure 23. A Video Line Driver
–V
S
NOTE: PINOUT SHOWN IS FOR MINIDIP PACKAGE
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
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%
Gain
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
A TWO-CHIP VOLTAGE-CONTROLLED AMPLIFIER
(VCA) WITH EXPONENTIAL RESPONSE
NOTE
*–3 dB bandwidth numbers are for the 0 dBm signal input. Overshoot numbers
are the percent overshoot of the 1 Volt 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
.
Voltage-controlled amplifiers are often used as building blocks
in automatic gain control systems. Figure 25 shows a two-chip
VCA built using the AD827 and the AD539, a dual, current-
output multiplier. As configured, the circuit has its two
INPUT RANGE:
10MV TO 3V (55dB)
+5V
AD539
V
X
V
IN
+5V
–5V
1
2
CONTROL
W1
16
15
2pF
2
–
1/2
0.1µF
8
HF COMP
Z1
0.01µF 3 CH 1
CH1 14
IN
OUT
4.7Ω
4
13
+V
S
BASE
0.1µF
4.7Ω
COM 12
5 –V
S
0.1µF
CH2
6
CH2 11
IN
OUT
10
7 INPUT
Z2
COM
8 OUTPUT
9
W2
COM
C
3
3
AD827
1
+
*
5 +
1/2
* 7
6
AD827
COAX LINE
50Ω
R
T
OUTPUT
50Ω
2pF
–
4
0.1µF
C
4
–5V
*PINOUT SHOWN IS FOR MINI-DIP PACKAGE
V
OUT
AT TERMINATION RESISTOR, R
T
=
V
OUT
AT PIN & OF AD827 =
V
X 2
V
IN
4V
2
V
X 2
V
IN
8V
2
Figure 25. A Wide Range Voltage-Controlled
Amplifier Circuit
AD [ ANALOG DEVICES ]
