AD8655 Analog Devices, AD8655 Datasheet - Page 16

• Manufacturer Part Number: AD8655
• Description: Low Noise, Precision CMOS Amplifier
• Manufacturer: Analog Devices

Specifications of AD8655

• -3db Bandwidth: 28MHz
• Slew Rate: 11V/µs
• Vos: 50µV
• Ib: 10pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 4nV/rtHz
• Vcc-vee: 2.7V to 5.5V
• Isy Per Amplifier: 4.5mA
• Packages: SOIC,SOP

AD8655/AD8656
APPLICATIONS
INPUT OVERVOLTAGE PROTECTION
The internal protective circuitry of the AD8655/AD8656 allows
voltages exceeding the supply to be applied at the input. It is
recommended, however, not to apply voltages that exceed the
supplies by more than 0.3 V at either input of the amplifier. If a
higher input voltage is applied, series resistors should be used to
limit the current flowing into the inputs. The input current
should be limited to less than 5 mA.
The extremely low input bias current allows the use of larger
resistors, which allows the user to apply higher voltages at the
inputs. The use of these resistors adds thermal noise, which
contributes to the overall output voltage noise of the amplifier.
For example, a 10 kΩ resistor has less than 12.6 nV/√Hz of
thermal noise and less than 10 nV of error voltage at room
temperature.
INPUT CAPACITANCE
Along with bypassing and ground, high speed amplifiers can be
sensitive to parasitic capacitance between the inputs and ground.
For circuits with resistive feedback network, the total capacitance,
whether it is the source capacitance, stray capacitance on the
input pin, or the input capacitance of the amplifier, causes a
breakpoint in the noise gain of the circuit. As a result, a
capacitor must be added in parallel with the gain resistor to
obtain stability. The noise gain is a function of frequency and
peaks at the higher frequencies, assuming the feedback capaci-
tor is selected to make the second-order system critically
damped. A few picofarads of capacitance at the input reduce
the input impedance at high frequencies, which increases the
amplifier’s gain, causing peaking in the frequency response or
oscillations. With the AD8655/AD8656, additional input
damping is required for stability with capacitive loads greater
than 200 pF with direct input to output feedback. See the
Driving Capacitive Loads section.
DRIVING CAPACITIVE LOADS
Although the AD8655/AD8656 can drive capacitive loads up to
500 pF without oscillating, a large amount of ringing is present
when operating the part with input frequencies above 100 kHz.
This is especially true when the amplifiers are configured in
positive unity gain (worst case). When such large capacitive
loads are required, the use of external compensation is highly
recommended. This reduces the overshoot and minimizes
ringing, which, in turn, improves the stability of the
AD8655/AD8656 when driving large capacitive loads.
Rev. B | Page 16 of 20
One simple technique for compensation is a snubber that
consists of a simple RC network. With this circuit in place,
output swing is maintained, and the amplifier is stable at all
gains. Figure 57 shows the implementation of a snubber, which
reduces overshoot by more than 30% and eliminates ringing.
Using a snubber does not recover the loss of bandwidth
incurred from a heavy capacitive load.
Figure 58. Driving Heavy Capacitive Loads Using a Snubber Network
Figure 56. Driving Heavy Capacitive Loads Without Compensation
V
A
C
V
A
R
C
C
S
V
L
S
V
S
S
L
= ±2.5V
= 1
= 500pF
= ±2.5V
= 1
= 200Ω
= 500pF
= 500pF
+
–
200mV
Figure 57. Snubber Network
+
–
V
V
V+
V–
CC
EE
TIME (10μs/DIV)
TIME (2 μ s/DIV)
500pF
200Ω
500pF
Data Sheet