AD8601/AD8602

–14–

REV. 0

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 k

Ω. 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 M

Ω 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:

BW

C

F

=

()

1

24 7

π . MΩ

(1)

Using a 10 pF feedback capacitor limits the bandwidth to approxi-

mately 3.3 kHz.

4.7m

10pF

(OPTIONAL)

D1

4.7V/ A

AD8601

Figure 3. Amplifier Photdiode Circuit

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.

3V

RETURN TO

GROUND

1/2 AD8602

3V

R2

2.49k

MONITOR

OUTPUT

R1

100

0.1

Q1

2N3905

Figure 4. A Low-Side Current Monitor

3V

0.1 F

0.1

V+

Q1

2N3904

MONITOR

OUTPUT

3V

R2

2.49k

R1

100

1/2

AD8602

Figure 5. A High-Side Current Monitor

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:

Monitor Output

R

R

R

I

SENSE

L

=×









×

2

1

(2)

For the high-side monitor, the monitor output voltage is: