
AD8065/AD8066
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 effective at lower frequencies,
the current return path distance is less critical.
Rev. E | Page 22 of 28
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 G = 10 pA. Similarly, any contaminants on the board
can create significant leakage (skin oils are a common problem).
To significantly reduce leakage, 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
completely 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 or ceramic, could be necessary in
some instances.
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 imped-
ance 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 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 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.
As shown in Figure 57, put a small value resistor (R
S
) in
series with the output to isolate the load capacitor from the
amp’s output stage. A good value to choose is 20 (see
Figure 10).
Increase the phase margin with higher noise gains or add a
pole with a parallel resistor and capacitor from IN to the
output.
R
S
= 20
V
I
AD8065
C
L
V
O
0
Figure 57. Output Isolation Resistor
R
SH
= 10
11
V
O
R
F
C
F
C
M
R
F
C
M
C
D
C
F
+ C
S
C
S
V
B
I
PHOTO
0
Figure 58. Wideband Photodiode Preamp