AD827
VIDEO LINE DRIVER
A HIGH SPEED 3 OP AMP INSTRUMENTATION
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.
AMPLIFIER CIRCUIT
The instrumentation amplifier circuit shown in Figure 24 can
provide a range of gains. The chart of Table II details
performance.
+VS
TRIM FOR BEST
SETTLING TIME
0.1µF
2 – 8pF
–V
IN
3
2
8
+
1
2kΩ
1/2
AD827
–
+VS
0.1µF
1kΩ
2kΩ
2kΩ
7
2
3
TRIM FOR
–
6
RG
OPTIMUM
BANDWIDTH
7 – 15 pF
AD847
VOUT
2kΩ
+
4
0.1µF
3pF
RL
2kΩ
1kΩ
6
–
1/2
7
–V
S
AD827
5
+V
IN
+
4
2000
RG
0.1µF
CIRCUIT GAIN =
+ 1
–V
NOTE: PINOUT SHOWN IS FOR MINIDIP PACKAGE
S
Figure 23. A Video Line Driver
The termination resistor, RT, (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.
Figure 24. A High Bandwidth Three Op Amp
Instrumentation Amplifier
Table II. Performance Specifications for the
Three Op Amp Instrumentation Amplifier
Small Signal
Bandwidth
@ 1 V p-p Output
Table I. Video Line Driver Performance Summary
Over-
Gain
RG
VIN
*
VSUPPLY CC
–3 dB BW shoot
1
2
10
100
Open
2 k
226 Ω
20 Ω
16.1 MHz
14.7 MHz
4.9 MHz
660 kHz
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
0 pF
4%
0%
0%
2%
0%
0%
A TWO-CHIP VOLTAGE-CONTROLLED AMPLIFIER
(VCA) WITH EXPONENTIAL RESPONSE
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
11 MHz
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 (RBT, 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 RT 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 RT.
INPUT RANGE:
10MV TO 3V (55dB)
+5V
0.1µF
AD539
16
15
VX
1
W1
CONTROL
2pF
C3
2
3
2
3
–
8
HF COMP
Z1
CH1
OUT
1/2
V
AD827
+
IN
0.01µF
CH 1
IN
14
13
1
*
+5V 4.7Ω
4
+VS
–VS
BASE
COM 12
0.1µF
5
–5V 4.7Ω
5
6
+
COAX LINE
*
OUTPUT
1/2
0.1µF
7
CH2
IN
INPUT
COM
OUTPUT
COM
6
11
10
9
AD827
–
CH2
OUT
50Ω
4
2pF
C4
RT
50Ω
7
8
0.1µF
Z2
W2
–5V
*PINOUT SHOWN IS FOR MINI-DIP PACKAGE
2
VX VIN
8V2
VOUT AT TERMINATION RESISTOR, RT
=
2
VX VIN
VOUT AT PIN & OF AD827 =
4V2
Figure 25. A Wide Range Voltage-Controlled
Amplifier Circuit