AD8602DRM Analog Devices, AD8602DRM Datasheet - Page 14

• Manufacturer Part Number: AD8602DRM
• Description: Precision CMOS Single Supply Rail-to-Rail Input/Output Wideband Operational Amplifiers
• Manufacturer: Analog Devices

AD8601/AD8602
Input Overvoltage Protection
As with any semiconductor device, if a condition could exist for
the input voltage to exceed the power supply, the device’s input
overvoltage characteristic must be considered. Excess input voltage
will energize internal PN junctions in the AD860x, allowing
current to flow from the input to the supplies.
This input current will not damage the amplifier provided it is
limited to 5 mA or less. This can be ensured by placing a resistor
in series with the input. For example, if the input voltage could
exceed the supply by 5 V, the series resistor should be at least
(5 V/5 mA) = 1 kΩ. With the input voltage within the supply
rails, a minimal amount of current is drawn into the inputs
which, in turn, causes a negligible voltage drop across the series
resistor. Thus, adding the series resistor will not adversely affect
circuit performance.
Overdrive Recovery
Overdrive recovery is defined as the time it takes the output of an
amplifier to come off the supply rail when recovering from an over-
load signal. This is tested by placing the amplifier in a closed-loop
gain of 10 with an input square wave of 2 V peak-to-peak while the
amplifier is powered from either 5 V or 3 V.
The AD860x has excellent recovery time from overload conditions.
The output recovers from the positive supply rail within 200 ns at all
supply voltages. Recovery from the negative rail is within 500 ns
at 5 V supply, decreasing to within 350 ns when the device is
powered from 2.7 V.
Power-On Time
Power-on time is important in portable applications, where the
supply voltage to the amplifier may be toggled to shut down the
device to improve battery life. Fast power-up behavior ensures
the output of the amplifier will quickly settle to its final voltage,
thus improving the power-up speed of the entire system. Once
the supply voltage reaches a minimum of 2.5 V, the AD860x
will settle to a valid output within 1 µs. This turn-on response
time is faster than many other precision amplifiers, which can
take tens or hundreds of microseconds for their output to settle.
Using the AD8602 in High Source Impedance Applications
The CMOS rail-to-rail input structure of the AD860x allows
these amplifiers to have very low input bias currents, typically
0.2 pA. This allows the AD860x to be used in any application
that has a high source impedance or must use large value resistances
around the amplifier. For example, the photodiode amplifier circuit
shown in Figure 3 requires a low input bias current op amp to
reduce output voltage error. The AD8601 minimizes offset errors
due to its low input bias current and low offset voltage.
The current through the photodiode is proportional to the incident
light power on its surface. The 4.7 MΩ resistor converts this
current into a voltage, with the output of the AD8601 increas-
ing at 4.7 V/µA. The feedback capacitor reduces excess noise at
higher frequencies by limiting the bandwidth of the circuit to:
Using a 10 pF feedback capacitor limits the bandwidth to approxi-
mately 3.3 kHz.
BW
=
2 4 7
π . MΩ
(
1
)
C
F
(1)
High- and Low-Side Precision Current Monitoring
Because of its low input bias current and low offset voltage, the
AD860x can be used for precision current monitoring. The true
rail-to-rail input feature of the AD860x allows the amplifier to
monitor current on either high-side or low-side. Using both
amplifiers in an AD8602 provides a simple method for monitoring
both current supply and return paths for load or fault detection.
Figure 4 and 54 demonstrate both circuits.
Voltage drop is created across the 0.1 Ω resistor that is propor-
tional to the load current. This voltage appears at the inverting
input of the amplifier due to the feedback correction around the
op amp. This creates a current through R1 which, in turn, pulls
current through R2. For the low side monitor, the monitor
output voltage is given by:
For the high-side monitor, the monitor output voltage is:
MONITOR
MONITOR
Monitor Output
OUTPUT
OUTPUT
3V
D1
2N3905
100
2N3904
R1
100
Q1
3V
R1
Q1
R
(OPTIONAL)
R2
2.49k
0.1
SENSE
4.7m
R2
2.49k
10pF
R
0.1
SENSE
=
AD8601
R
2
×
3V



AD8602
R
1/2 AD8602
SENSE
1/2
R
3V
I
1
L
V
4.7V/ A
RETURN TO
GROUND
OUT
0.1 F

 ×

I
V+
L
(2)