AD8651/AD8652

Rev. B | Page 15 of 20

Input 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. The inputs of the

AD8651 are protected with ESD diodes to either power supply.

Excess input voltage will energize internal PN junctions in the

AD8651, allowing current to flow from the input to the

supplies. This results in an input stage with picoamps of input

current that can withstand up to 4000 V ESD events (human

body model) with no degradation.

Excessive power dissipation through the protection devices will

destroy or degrade the performance of any amplifier. Differen-

tial voltages greater than 7 V will result in an input current of

resistance in series with the inputs. For input voltages beyond

of the amplifier sustain differential voltages greater than 7 V or

input voltages beyond the amplifier power supply, limit the

input current to 10 mA by using an appropriately sized input

+

–

30mA

SUPPLY VOLTAGES

30mA

30mA

AD8651

Figure 54. Input Protection Method

Overdrive Recovery

Overdrive recovery is defined as the time it takes for the output

of an amplifier to come off the supply rail after an overload

signal is initiated. This is usually tested by placing the amplifier

in a closed-loop gain of 15 with an input square wave of

200 mV p-p while the amplifier is powered from either 5 V or

3 V. The AD8651 has excellent recovery time from overload

conditions (see Figure 31 and Figure 32). The output recovers

from the positive supply rail within 200 ns at all supply voltages.

Recovery from the negative rail is within 100 ns at 5 V supply.

LAYOUT, GROUNDING, AND BYPASSING

CONSIDERATIONS

Power Supply Bypassing

Power supply pins can act as inputs for noise, so care must be

taken that a noise-free, stable dc voltage is applied. The purpose

of bypass capacitors is to create low impedances from the supply

to ground at all frequencies, thereby shunting or filtering most

of the noise. Bypassing schemes are designed to minimize the

supply impedance at all frequencies with a parallel combination

of capacitors of 0.1 µF and 4.7 µF. Chip capacitors of 0.1 µF

(X7R or NPO) are critical and should be as close as possible to

the amplifier package. The 4.7 µF tantalum capacitor is less

critical for high frequency bypassing, and, in most cases, only

one is needed per board at the supply inputs.

Grounding

A ground plane layer is important for densely packed PC

boards to spread the current-minimizing parasitic inductances.

However, an understanding of where the current flows in a

circuit is critical to implementing effective high speed circuit

design. The length of the current path is directly proportional to

the magnitude of parasitic inductances and, therefore, the high

frequency impedance of the path. High speed currents in an

inductive ground return will create an unwanted voltage noise.

The length of the high frequency bypass capacitor leads is

critical. A parasitic inductance in the bypass grounding will

work against the low impedance created by the bypass capacitor.

Place the ground leads of the bypass capacitors at the same

physical location. Because load currents also flow from the

supplies, the ground for the load impedance should be at the

same physical location as the bypass capacitor grounds. For the

larger value capacitors, intended to be effective at lower fre-

quencies, the current return path distance is less critical.

Leakage Currents

Poor PC board layout, contaminants, and the board insulator

material can create leakage currents that are much larger than

the input bias current of the AD8651/AD8652. Any voltage

differential between the inputs and nearby traces will set up

leakage currents through the PC board insulator, for example,

1 V/100 G = 10 pA. Similarly, any contaminants on the board

can create significant leakage (skin oils are a common problem).

To significantly reduce leakages, put a guard ring (shield)

around the inputs and input leads that are driven to the same

voltage potential as the inputs. This ensures that there is no

voltage potential between the inputs and the surrounding area

to set up any leakage currents. To be effective, the guard ring

must be driven by a relatively low impedance source and should

completely surround the input leads on all sides, above and

below, using a multilayer board.

Another effect that can cause leakage currents is the charge

absorption of the insulator material itself. Minimizing the

amount of material between the input leads and the guard

ring will help to reduce the absorption. Also, low absorption

materials, such as Teflon® or ceramic, may be necessary in

some instances.

Input Capacitance

Along with bypassing and ground, high speed amplifiers can be

sensitive to parasitic capacitance between the inputs and

ground. A few picofarads of capacitance will reduce the input

impedance at high frequencies, which in turn increases the

amplifier’s gain, causing peaking in the frequency response or