AD8601/AD8602/AD8604

Rev. G | Page 16 of 24

INPUT OVERVOLTAGE PROTECTION

As with any semiconductor device, if a condition could exist

that could cause the input voltage to exceed the power supply,

the device’s input overvoltage characteristic must be considered.

Excess input voltage energizes the internal PN junctions in the

AD860x, allowing current to flow from the input to the supplies.

This input current does 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. Therefore, adding the series resistor does 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 overload signal. This is tested by placing the amplifier in a

closed-loop gain of 10 with an input square wave of 2 V p-p

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 a 5 V supply, decreasing to within 350 ns when the

device is powered from 2.7 V.

POWER-ON TIME

The 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

that the output of the amplifier quickly settles to its final voltage,

improving the power-up speed of the entire system. When the

supply voltage reaches a minimum of 2.5 V, the AD860x settles 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 outputs 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 56 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 increasing at 4.7 V/μA.

The feedback capacitor reduces excess noise at higher frequencies

by limiting the bandwidth of the circuit to

()

F

C

π

BW

Ω

M

7

.

4

2

1

=

(1)

Using a 10 pF feedback capacitor limits the bandwidth to

approximately 3.3 kHz.

AD8601

10pF

(OPTIONAL)

4.7V/µA

4.7M

Ω

D1

Figure 56. Amplifier Photodiode Circuit

HIGH SIDE 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 the high side or the 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 57 and Figure 58 demonstrate both circuits.

1/2 AD8602

RETURN TO

GROUND

0.1

Ω

R1

100

Ω

R2

249k

Ω

Q1

2N3904

MONITOR

OUTPUT

3V

3V

Figure 57. Low-Side Current Monitor

3V

V+

3V

MONITOR

OUTPUT

R1

100

Ω

R2

2.49k

Ω

0.1

Ω

Q1

2N3905

1/2 AD8602

Figure 58. High-Side Current Monitor