AD812
Power Supply Bypassing
The input and output signal return paths must also be kept from
overlapping. Since ground connections are not of perfectly zero
impedance, current in one ground return path can produce a
voltage drop in another ground return path if they are allowed
to overlap.
Adequate power supply bypassing can be very important when
optimizing the performance of high speed circuits. Inductance
in the supply leads can (for example) contribute to resonant
circuits that produce peaking in the amplifier’s response. In
addition, if large current transients must be delivered to a load,
then large (greater than 1 µF) bypass capacitors are required to
produce the best settling time and lowest distortion. Although
0.1 µF capacitors may be adequate in some applications, more
elaborate bypassing is required in other cases.
Electric field coupling external to (and across) the package can
be reduced by arranging for a narrow strip of ground plane to be
run between the pins (parallel to the pin rows). Doing this on
both sides of the board can reduce the high frequency crosstalk
by about 5 dB or 6 dB.
When multiple bypass capacitors are connected in parallel, it is
important to be sure that the capacitors themselves do not form
resonant circuits. A small (say 5 Ω) resistor may be required in
series with one of the capacitors to minimize this possibility.
Driving Capacitive Loads
When used with the appropriate output series resistor, any load
capacitance can be driven without peaking or oscillation. In
most cases, less than 50 Ω is all that is needed to achieve an
extremely flat frequency response. As illustrated in Figure 44,
the AD812 can be very attractive for driving largely capacitive
loads. In this case, the AD812’s high output short circuit
current allows for a 150 V/µs slew rate when driving a 510 pF
capacitor.
As discussed below, power supply bypassing can have a signifi-
cant impact on crosstalk performance.
Achieving Low Crosstalk
Measured crosstalk from the output of amplifier 2 to the input
of amplifier 1 of the AD812 is shown in Figure 40. The crosstalk
from the output of amplifier 1 to the input of amplifier 2 is a few
dB better than this due to the additional distance between criti-
cal signal nodes.
R
F
+V
0.1F
1.0F
S
A carefully laid-out PC board should be able to achieve the level
of crosstalk shown in the figure. The most significant contribu-
tors to difficulty in achieving low crosstalk are inadequate power
supply bypassing, overlapped input and/or output signal paths,
and capacitive coupling between critical nodes.
R
G
8
R
S
V
AD812
O
The bypass capacitors must be connected to the ground plane at
a point close to and between the ground reference points for the
two loads. (The bypass of the negative power supply is particu-
larly important in this regard.) There are two amplifiers in the
package, and low impedance signal return paths must be pro-
vided for each load. (Using a parallel combination of 1 µF,
0.1 µF, and 0.01 µF bypass capacitors will help to achieve opti-
mal crosstalk.)
V
C
R
IN
L
4
L
1.0F
0.1F
R
T
–V
S
Figure 41. Circuit for Driving a Capacitive Load
–10
–20
V
= ؎5V
S
G = +2
R
= 150⍀
R
R
C
= 750⍀
L
F
L
L
–30
–40
= 1k⍀
= 10pF
12
9
6
–50
R
= 0
S
–60
R
= 30⍀
S
3
0
–70
R
= 50⍀
S
–80
–3
–6
–90
–100
–110
1
10
100
1000
100k
1M
10M
100M
FREQUENCY – MHz
FREQUENCY – Hz
Figure 42. Response to a Small Load Capacitor at ±5 V
Figure 40. Crosstalk vs. Frequency
REV. B
–13–
ADI [ ADI ]
