Datasheet

AD812
REV. B
–14–
1
10 1000100
FREQUENCY – MHz
6
9
3
0
–3
CLOSED-LOOP GAIN – dB
12
–6
–9
V
S
= 615V
G = +2
R
F
= 750V
R
L
= 1kV
C
L
= 510pF, R
S
= 15V
C
L
= 150pF, R
S
= 30V
Figure 43. Response to Large Load Capacitor, V
S
= ± 15 V
10
100
0%
100ns
5V
5V
V
IN
V
OUT
90
Figure 44. Pulse Response of Circuit of Figure 41 with
C
L
= 510 pF, R
L
= 1 k Ω , R
F
= R
G
= 715 Ω , R
S
= 15 Ω
Overload Recovery
There are three important overload conditions to consider.
They are due to input common mode voltage overdrive, input
current overdrive, and output voltage overdrive. When the
amplifier is configured for low closed-loop gains, and its input
common-mode voltage range is exceeded, the recovery time will
be very fast, typically under 10 ns. When configured for a higher
gain, and overloaded at the output, the recovery time will also
be short. For example, in a gain of +10, with 6 dB of input
overdrive, the recovery time of the AD812 is about 10 ns.
10
90
100
0%
2V
1V
50ns
V
IN
V
OUT
Figure 45. 6 dB Overload Recovery; G = 10, R
L
= 500 Ω ,
V
S
= ± 5 V
In the case of high gains with very high levels of input overdrive,
a longer recovery time may occur. For example, if the input
common-mode voltage range is exceeded in a gain of +10, the
recovery time will be on the order of 100 ns. This is primarily
due to current overloading of the input stage.
As noted in the warning under “Maximum Power Dissipation,”
a high level of input overdrive in a high noninverting gain circuit
can result in a large current flow in the input stage. For differ-
ential input voltages of less than about 1.25 V, this will be inter-
nally limited to less than 20 mA (decreasing with supply voltage).
For input overdrives which result in higher differential input
voltages, power dissipation in the input stage must be consid-
ered. It is recommended that external diode clamps be used in
cases where the differential input voltage is expected to exceed
1.25 V.
High Performance Video Line Driver
At a gain of +2, the AD812 makes an excellent driver for a back-
terminated 75 Ω video line. Low differential gain and phase
errors and wide 0.1 dB bandwidth can be realized over a wide
range of power supply voltage. Outstanding gain and group
delay matching are also attainable over the full operating supply
voltage range.
AD812
8
4
R
G
R
F
V
IN
75V
V
OUT
75V
+V
S
0.1mF
0.1mF
–V
S
75V
CABLE
75V
CABLE
75V
Figure 46. Gain of +2 Video Line Driver (R
F
= R
G
from
Table I)