AD8061 Analog Devices, AD8061 Datasheet - Page 16

• Manufacturer Part Number: AD8061
• Description: Low Cost, 300 MHz Rail-to-Rail Amplifier (Single)
• Manufacturer: Analog Devices

Specifications of AD8061

• -3db Bandwidth: 320MHz
• Slew Rate: 650V/µs
• Vos: 1mV
• Ib: 350nA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 8.5nV/rtHz
• Vcc-vee: 2.7V to 8V
• Isy Per Amplifier: 9.5mA
• Packages: SOIC,SOT

AD8061/AD8062/AD8063
CAPACITIVE LOAD DRIVE
The AD8061/AD8062/AD8063 family is optimized for
bandwidth and speed, not for driving capacitive loads. Output
capacitance creates a pole in the amplifier’s feedback path,
leading to excessive peaking and potential oscillation. If dealing
with load capacitance is a requirement of the application, the
two strategies to consider are as follows:
Use a small resistor in series with the amplifier’s output and the
Reduce the bandwidth of the amplifier’s feedback loop by
Figure 50 shows a unity-gain follower using the series resistor
strategy. The resistor isolates the output from the capacitance
and, more importantly, creates a zero in the feedback path that
compensates for the pole created by the output capacitance.
Voltage feedback amplifiers like those in the AD8061/AD8062/
AD8063 family are able to drive more capacitive load without
excessive peaking when used in higher gain configurations
because the increased noise gain reduces the bandwidth of the
overall feedback loop. Figure 51 plots the capacitance that
produces 30% overshoot vs. noise gain for a typical amplifier.
load capacitance.
increasing the overall noise gain.
10k
100
1k
10
V
1
IN
Figure 50. Series Resistor Isolating Capacitive Load
Figure 51. Capacitive Load vs. Closed-Loop Gain
R
S
= 4.7
AD8061
2
R
CLOSED-LOOP GAIN
S
= 0
3
R
SERIES
C
LOAD
4
V
O
5
Rev. G | Page 16 of 20
DISABLE OPERATION
The internal circuit for the AD8063 disable function is shown
in Figure 52. When the DISABLE node is pulled below 2 V
from the positive supply, the supply current decreases from
typically 6.5 mA to under 400 μA, and the AD8063 output
enters a high impedance state. If the DISABLE node is not
connected and allowed to float, the AD8063 stays biased at
full power.
Figure 34 shows the AD8063 supply current vs. DISABLE
voltage.
is driven with a 10 MHz sine wave, and
from 0 V to 5 V, illustrating the part’s turn-on and turn-off
time.
the AD8063 shut off.
BOARD LAYOUT CONSIDERATIONS
Maintaining the high speed performance of the AD8061/AD8062/
AD8063 family requires the use of high speed board layout
techniques and low parasitic components.
The PCB should have a ground plane covering unused portions
of the component side of the board to provide a low impedance
path. Remove the ground plane near the package to reduce
parasitic capacitance.
Proper bypassing is critical. Use a ceramic 0.1 μF chip capacitor
to bypass both supplies. Locate the chip capacitor within 3 mm
of each power pin. Additionally, connect in parallel a 4.7 μF to
10 μF tantalum electrolytic capacitor to provide charge for fast,
large signal changes at the output.
Minimizing parasitic capacitance at the amplifier’s inverting
input pin is very important. Locate the feedback resistor close to
the inverting input pin. The value of the feedback resistor may
come into play—for instance, 1 kΩ interacting with 1 pF of
parasitic capacitance creates a pole at 159 MHz. Use stripline
design techniques for signal traces longer than 25 mm. Design
them with either 50 Ω or 75 Ω characteristic impedance and
proper termination at each end.
DISABLE
Figure 33
Figure 35
Figure 52. Disable Circuit of the AD8063
shows the input/output isolation response with
plots the output seen when the AD8063 input
VCC
VEE
DISABLE is toggled
2V
TO AMPLIFIER
BIAS