AD8065 AD8066 Analog Devices, AD8065 AD8066 Datasheet - Page 19

• Manufacturer Part Number: AD8065 AD8066
• Description: High Performance, 145 MHz Fast FET Op Amps
• Manufacturer: Analog Devices

The length of the high frequency bypass capacitor leads is most
critical. A parasitic inductance in the bypass grounding will
work against the low impedance created by the bypass capacitor.
Place the ground leads of the bypass capacitors at the same
physical location. Because load currents flow from the supplies
as well, the ground for the load impedance should be at the same
physical location as the bypass capacitor grounds. For the larger
value capacitors, which are intended to be effective at lower
frequencies, the current return path distance is less critical.
Leakage Currents
Poor PC board layout, contaminants, and the board insulator
material can create leakage currents that are much larger than
the input bias current of the AD8065/AD8066. Any voltage
differential between the inputs and nearby runs will set up
leakage currents through the PC board insulator, for example,
1 V/100 GW = 10 pA. Similarly any contaminants on the board
can create significant leakage (skin oils are a common problem).
To significantly reduce leakages, put a guard ring (shield) around
the inputs and input leads that are driven to the same voltage
potential as the inputs. This way there is no voltage potential
between the inputs and surrounding area to set up any leakage
currents. For the guard ring to be completely effective it must
be driven by a relatively low impedance source and should com-
pletely surround the input leads on all sides, above and below
using a multilayer board.
Another effect that can cause leakage currents is the charge
absorption of the insulator material itself. Minimizing the amount
of material between the input leads and the guard ring will help
to reduce the absorption. Also, low absorption materials, such as
Teflon
REV. B
®
or ceramic, may be necessary in some instances.
–19–
Input Capacitance
Along with bypassing and ground, high speed amplifiers can be
sensitive to parasitic capacitance between the inputs and ground.
A few pF of capacitance will reduce the input impedance at high
frequencies, in turn increasing the amplifier’s gain, causing
peaking of the frequency response or even oscillations, if
severe enough. It is recommended that the external passive
components that are connected to the input pins be placed as
close as possible to the inputs to avoid parasitic capacitance. The
ground and power planes must be kept at a small distance away
from the input pins on all layers of the board.
Output Capacitance
To a lesser extent, parasitic capacitances on the output can cause
peaking and ringing of the frequency response. There are two
methods to effectively minimize their effect.
1) As shown in Figure 7, put a small value resistor (R
2) Increase the phase margin with higher noise gains or adding
with the output to isolate the load capacitor from the amp’s
output stage. A good value to choose is 20 W (see TPC 7).
a pole with a parallel resistor and capacitor from –IN to
the output.
V
I
Figure 7. Output Isolation Resistor
AD8065
AD8065/AD8066
R
S
= 20
C
L
S
) in series
V
O