AD8065 AD8066 Analog Devices, AD8065 AD8066 Datasheet - Page 18

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

AD8065/AD8066
Input and Output Overload Behavior
The AD8065/AD8066 have internal circuitry to guard against
phase reversal due to overdriving the input stage. A simplified
schematic of the input stage, including the input-protection
diodes and antiphase reversal circuitry, is shown in Figure 6.
The circuit is arranged such that when the input common-mode
voltage exceeds a certain threshold, the input JFET pair’s bias
current will turn OFF, and the bias current of an auxiliary NPN
pair will turn ON, taking over control of the amplifier. When
the input common-mode voltage returns to a viable operating value,
the FET stage turns back ON, the NPN stage turns OFF, and
normal operation resumes.
The NPN pair can sustain operation with the input voltage up
to the positive supply, so this is a pseudo rail-to-rail input stage.
For operation beyond the FET stage’s common-mode limit, the
amplifier’s V
160 mV, standard deviation of 820 mV), and I
the NPN pair’s base current up to 45 mA (see TPC 29).
Switchback, or recovery time, is about 100 ns, as shown in TPC 24.
The output transistors of the rail-to-rail output stage have circuitry
to limit the extent of their saturation when the output is overdriven.
This helps output recovery time. Output recovery from a 0.5 V
output overdrive on a ± 5 V supply is shown in TPC 21.
OS
will change to the NPN pair’s offset (mean of
VTHRESHOLD
b
V N
will increase to
Figure 6. Simplified Input Stage
S
–18–
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 a majority
of the noise.
Decoupling schemes are designed to minimize the bypassing imped-
ance at all frequencies with a parallel combination of capacitors.
0.1 mF (X7R or NPO) chip capacitors are critical and should be
as close as possible to the amplifier package. The 4.7 mF tantalum
capacitor is less critical for high frequency bypassing, and in most
cases, only one per board is needed 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 imped-
ance of the path. High speed currents in an inductive ground
return will create an unwanted voltage noise.
V CC
V EE
S
TO REST OF AMP
V P
VBIAS
REV. B