AD9630
2
1
FREQUENCY RESPONSE – dB
0
–1
–2
–3
–4
–5
–6
–7
–8
<0.1MHz
50pF
10pF
25pF
the device output. To avoid this occurrence, the power supply
leads should be tightly twisted (if appropriate). Ferrite beads
mounted between the tantalum and ceramic capacitors will
serve the same purpose.
All unused pins (except the optional power supply pins) should
be connected to ground to reduce pin-to-pin capacitive coupling
and prevent external RF interference. If the source and drive
electronics require “remote” operation (> 1 inch from the
AD9630), the PC board line impedances should be matched
with the buffer input and output resistances. Basic microstrip
techniques should be observed. R
IN
and R
S
should be connected
as close to the AD9630 as possible.
100MHz
C
L
200MHz
300MHz
Figure 2. Frequency Response vs. C
L
with Recommended R
S
With only minimal pulse overshoot and ringing, the AD9630
can drive terminated cables directly without the use of an output
termination resistor (R
S
). Termination resistors (R
S
and R
IN
)
can be either standard carbon composition or microwave type.
For matching characteristic impedances, precision microwave
resistors of 1% or better tolerance are preferred.
The AD9630 should be soldered directly to the PC board with
as little vertical clearance as possible. The use of zero insertion
sockets is strongly discouraged because of the high effective pin
inductances. Use of this type socket will result in peaking and
possibly induce oscillation.
+V
S
4.7 F
In pulse mode applications, with R
S
equal to approximately
12
Ω,
capacitive loads of up to 50 pF can be driven with mini-
mal settling time degradation.
The output stage has short circuit protection to ground. The
output driver will shut down if more than approximately
130 mA of instantaneous sink or source current is reached. This
level of current ensures that output clipping will not result when
driving heavy capacitive loads during high slew conditions,
although average load currents above 70 mA may reduce device
reliability.
LAYOUT CONSIDERATIONS
0.1 F
0.1 F
Due to the high frequency operation of the AD9630 attention to
board layout is necessary to achieve optimum dynamic perfor-
mance. A two ounce copper ground plane on the top side of the
board is recommended; it should cover as much of the board as
possible with appropriate openings for supply decoupling ca-
pacitors as well as for load and source termination resistors, (see
Figure 3).
Optimum settling time and ac performance results will be
achieved with surface mount 0.1
µF
supply decoupling ceramic
chip capacitors mounted within 50 mils of the corresponding
device pins with the other side soldered directly to the ground
plane. For best high resolution (<0.02%) settling times, the op-
tional power supply pins should be decoupled as shown above.
If the optional power supply pins are not used, they should be
left open.
If surface mount capacitors cannot be used, radial lead ceramic
capacitors with leads less than 30 mils long are recommended.
Low frequency power supply decoupling is necessary and can be
accomplished with 4.7
µF
tantalum capacitors mounted within
0.5 inches of the supply pins. Due to the series inductance of
these capacitors interacting with the 0.1
µF
capacitors and
power supply leads, high frequency oscillations might appear on
1
2 *
V
IN
R
IN
R
S
**
8
AD9630
6 *
5
V
OUT
0.1 F
0.1 F
4.7 F
–V
S
*SEE PINOUTS
**SEE FIGURE 1
Figure 3. AD9630 Application Circuit
–4–
REV. B
AD [ ANALOG DEVICES ]
