AD8615/AD8616/AD8618
APPLICATIONS INFORMATION
This reduces the overshoot and minimizes ringing, which in
turn improves the frequency response of the AD8615/AD8616/
AD8618. One simple technique for compensation is the snubber,
which consists of a simple RC network. With this circuit in place,
output swing is maintained and the amplifier is stable at all gains.
INPUT OVERVOLTAGE PROTECTION
The AD8615/AD8616/AD8618 have internal protective circuitry
that 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 1.5 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.
Figure 38 shows the implementation of the snubber, which
reduces overshoot by more than 30% and eliminates ringing
that can cause instability. Using the snubber does not recover
the loss of bandwidth incurred from a heavy capacitive load.
The input current should be limited to <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.
V
A
C
= ±2.5V
= 1
= 500pF
S
V
L
For example, a 10 kΩ resistor has less than 13 nV/√Hz of
thermal noise and less than 10 nV of error voltage at room
temperature.
OUTPUT PHASE REVERSAL
The AD8615/AD8616/AD8618 are immune to phase inversion,
a phenomenon that occurs when the voltage applied at the input of
the amplifier exceeds the maximum input common mode.
Phase reversal can cause permanent damage to the amplifier
and can create lock ups in systems with feedback loops.
TIME (2µs/DIV)
Figure 37. Driving Heavy Capacitive Loads Without Compensation
V
V
A
R
= ±2.5V
S
= 6V p-p
= 1
IN
V
L
= 10kΩ
V
EE
+
–
V–
V+
200Ω
V
OUT
V
500pF
IN
500pF
–
V
CC
200mV
Figure 38. Snubber Network
V
= ±2.5V
= 1
= 200Ω
= 500pF
= 500pF
S
A
R
C
C
V
S
S
L
TIME (2ms/DIV)
Figure 36. No Phase Reversal
DRIVING CAPACITIVE LOADS
Although the AD8615/AD8616/AD8618 are capable of driving
capacitive loads of up to 500 pF without oscillating, a large amount
of overshoot is present when operating at frequencies above
100 kHz. This is especially true when the amplifier is configured
in positive unity gain (worst case). When such large capacitive
loads are required, the use of external compensation is highly
recommended.
TIME (10µs/DIV)
Figure 39. Driving Heavy Capacitive Loads Using the Snubber Network
Rev. E | Page 11 of 20
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