AD8066AR-REEL Analog Devices Inc, AD8066AR-REEL Datasheet - Page 23

• Manufacturer Part Number: AD8066AR-REEL
• Description: IC OPAMP VF R-R DUAL LN LP 8SOIC
• Manufacturer: Analog Devices Inc
• Series: FastFET™r

Specifications of AD8066AR-REEL

• Rohs Status: RoHS non-compliant
• Design Resources: Precision, Bipolar Configuration for the AD5426/32/43 8-Bit to12-Bit DACs (CN0036) Precision, Bipolar, Configuration for AD5450/1/2/3 8-14bit Multiplying DACs (CN0053)
• Amplifier Type: Voltage Feedback
• Number Of Circuits: 2
• Output Type: Rail-to-Rail
• Slew Rate: 180 V/µs
• -3db Bandwidth: 145MHz
• Current - Input Bias: 3pA
• Voltage - Input Offset: 400µV
• Current - Supply: 6.6mA
• Current - Output / Channel: 30mA
• Voltage - Supply, Single/dual (±): 5 V ~ 24 V, ±2.5 V ~ 12 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Gain Bandwidth Product: -

LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS
POWER SUPPLY BYPASSING
Power supply pins are actually inputs and care must be taken so
that a noise-free stable dc voltage is applied. The purpose of bypass
capacitors is to create low impedances from the supply to ground at
all frequencies, thereby shunting or filtering most of the noise.
Decoupling schemes are designed to minimize the bypassing
impedance at all frequencies with a parallel combination of
capacitors. 0.1 μF (X7R or NPO) chip capacitors are critical
and should be as close as possible to the amplifier package.
The 4.7 μF tantalum capacitor is less critical for high frequency
bypassing, and, in most cases, only one is needed per board at
the supply inputs.
GROUNDING
A ground plane layer is important in densely packed PC boards
to spread the current minimizing parasitic inductances. However,
an understanding of where the current flows in a circuit is critical
to implementing effective high speed circuit design. The length
of the current path is directly proportional to the magnitude of
parasitic inductances and, therefore, the high frequency impedance
of the path. High speed currents in an inductive ground return
create unwanted voltage noise.
The length of the high frequency bypass capacitor leads is most
critical. A parasitic inductance in the bypass grounding works
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.
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 sets up leakage
currents through the PC board insulator, for example, 1 V/100 GΩ
= 10 pA. Similarly, any contaminants on the board can create
significant leakage (skin oils are a common problem). To reduce
leakage significantly, 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
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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
helps 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 reduces 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 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:
•
•
V
As shown in Figure 57, put a small value resistor (R
series with the output to isolate the load capacitor from the
amp’s output stage. A good value to choose is 20 Ω (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.
I
Figure 57. Output Isolation Resistor
AD8065
R
S
= 20Ω
AD8065/AD8066
C
L
S
V
) in
O