
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.
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
Over-
shoot
V
IN
*
V
SUPPLY
C
C
–3 dB B
W
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
±15
±15
±15
±5
±5
±5
20 pF
15 pF
0 pF
20 pF
15 pF
0 pF
23 MHz
21 MHz
13 MHz
18 MHz
16 MHz
11 MHz
4%
0%
0%
2%
0%
0%
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
.
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.
–
+
+
–
+
–
AD847
1
3
2
2
3
6
7
6
5
A1/2
0.1μF
1k
2k
2k
1k
2k
0.1μF
0.1μF
2k
R
L
2 – 8pF
0.1μF
R
G
4
7
2k
8
4
TRIM FOR BEST
SETTLING TIME
1/2
3pF
CIRCUIT GAIN =
+ 1
G
R
2000
TRIM FOR
BOPTIMUM
7 – 15 pF
NOTE: PINOUT SHOWN IS FOR MINIDIP PACKAGE
–V
IN
+V
S
–V
S
+V
IN
+V
S
–V
S
V
OUT
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
Gain
R
G
1
2
10
100
Open
2 k
226
20
16.1 MHz
14.7 MHz
4.9 MHz
660 kHz
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
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
CONTROL
W1
Z1
CH1
OUT
BASE
COM
CH2
OUT
Z2
W2
HF COMP
CH 1
IN
+V
S
CH2
IN
INPUT
COM
OUTPUT
COM
AD539
0.01μF
+
AD827
5
6
2
3+
–
A1/2
1/2
0.1μF
0.1μF
4.7
4.7
+5V
–5V
+5V
0.1μF
8
1
–5V
0.1μF
2pF
2pF
4
7
V
X
50
50
COAX LINE
R
T
OUTPUT
INPUT RANGE:
10MV TO 3V (55dB)
*PINOUT SHOWN IS FOR MINI-DIP PACKAGE
*
*
V
IN
–V
S
8V
2
V AT TERMINATION RESISTOR, R =
C
3
C
4
V AT PIN & OF AD827 =
V
X2
4V
2
IN
V
V
X2
IN
V
Figure 25. A Wide Range Voltage-Controlled
Amplifier Circuit