AD818AN(2000) データシートの表示（PDF） - Analog Devices
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AD818AN Datasheet PDF : 12 Pages
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AD818
+VS
OUTPUT
–IN
+IN
–VS
NULL 1
NULL 8
Figure 34. AD818 Simplified Schematic
THEORY OF OPERATION
The AD818 is a low cost, video operational amplifier designed
to excel in high performance, high output current video
applications.
The AD818 (Figure 34) consists of a degenerated NPN differ-
ential pair driving matched PNPs in a folded-cascode gain stage.
The output buffer stage employs emitter followers in a class AB
amplifier which delivers the necessary current to the load, while
maintaining low levels of distortion.
The AD818 will drive terminated cables and capacitive loads of
10 pF or less. As the closed-loop gain is increased, the AD818
will drive heavier cap loads without oscillating.
INPUT CONSIDERATIONS
An input protection resistor (RIN in Figure 28) is required in
circuits where the input to the AD818 will be subjected to tran-
sient of continuous overload voltages exceeding the ± 6 V maxi-
mum differential limit. This resistor provides protection for the
input transistors by limiting their maximum base current.
For high performance circuits, it is recommended that a “bal-
ancing” resistor be used to reduce the offset errors caused by
bias current flowing through the input and feedback resistors.
The balancing resistor equals the parallel combination of RIN
and RF and thus provides a matched impedance at each input
terminal. The offset voltage error will then be reduced by more
than an order of magnitude.
GROUNDING AND BYPASSING
When designing high frequency circuits, some special precau-
tions are in order. Circuits must be built with short interconnect
leads. When wiring components, care should be taken to pro-
vide a low resistance, low inductance path to ground. Sockets
should be avoided, since their increased interlead capacitance
can degrade circuit bandwidth.
Feedback resistors should be of low enough value (≤1 kΩ) to
assure that the time constant formed with the inherent stray
capacitance at the amplifier’s summing junction will not limit
performance. This parasitic capacitance, along with the parallel
resistance of RF/RIN, form a pole in the loop transmission which
may result in peaking. A small capacitance (1–5 pF) may be
used in parallel with the feedback resistor to neutralize this
effect.
Power supply leads should be bypassed to ground as close as
possible to the amplifier pins. Ceramic disc capacitors of
0.1 µF are recommended.
+VS
2
7
AD818 6
8
3
1
4
10k⍀
–VS
VOS ADJUST
Figure 35. Offset Null Configuration
OFFSET NULLING
The input offset voltage of the AD818 is inherently very low.
However, if additional nulling is required, the circuit shown in
Figure 35 can be used. The null range of the AD818 in this
configuration is ± 10 mV.
SINGLE SUPPLY OPERATION
Another exciting feature of the AD818 is its ability to perform
well in a single supply configuration. The AD818 is ideally
suited for applications that require low power dissipation and
high output current.
Referring to Figure 36, careful consideration should be given to
the proper selection of component values. The choices for
this particular circuit are: R1 + R3ʈR2 combine with C1 to
form a low frequency corner of approximately 10 kHz. C4 was
inserted in series with R4 to maintain amplifier stability at high
frequency.
Combining R3 with C2 forms a low pass filter with a corner
frequency of approximately 500 Hz. This is needed to maintain
amplifier PSRR, since the supply is connected to VIN through
the input divider. The values for R2 and C2 were chosen to
demonstrate the AD818’s exceptional output drive capability. In
this configuration, the output is centered around 2.5 V. In order
to eliminate the static dc current associated with this level, C3
was inserted in series with RL.
+VS
R3
100⍀
C2
3.3F
R4
1k⍀
C4
0.001F
C1
0.01F
VIN
R1
3.3k⍀
R2
3.3k⍀
1k⍀
3.3F
SELECT C1, R1, R2
FOR DESIRED LOW
FREQUENCY CORNER.
2
7
AD818
3
4
0.01F
6
VOUT
RL
150⍀
C3
0.1F
Figure 36. Single Supply Amplifier Configuration
–10–
REV. B
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