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AD8655/AD8656

Data Sheet

Rev. E | Page 16 of 20

APPLICATIONS INFORMATION

INPUT OVERVOLTAGE PROTECTION

The internal protective circuitry of the AD8655/AD8656 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 0.3 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. The input current

should be limited to less than 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.

For example, a 10 kΩ resistor has less than 12.6 nV/√Hz of

thermal noise and less than 10 nV of error voltage at room

temperature.

INPUT CAPACITANCE

Along with bypassing and ground, high speed amplifiers can be

sensitive to parasitic capacitance between the inputs and ground.

For circuits with resistive feedback network, the total capacitance,

whether it is the source capacitance, stray capacitance on the

input pin, or the input capacitance of the amplifier, causes a

breakpoint in the noise gain of the circuit. As a result, a

capacitor must be added in parallel with the gain resistor to

obtain stability. The noise gain is a function of frequency and

peaks at the higher frequencies, assuming the feedback capaci-

tor is selected to make the second-order system critically damped.

A few picofarads of capacitance at the input reduce the input

impedance at high frequencies, which increases the amplifier’s

gain, causing peaking in the frequency response or oscillations.

With the AD8655/AD8656, additional input damping is required

for stability with capacitive loads greater than 200 pF with

direct input to output feedback. See the Driving Capacitive

Loads section.

DRIVING CAPACITIVE LOADS

Although the AD8655/AD8656 can drive capacitive loads up to

500 pF without oscillating, a large amount of ringing is present

when operating the part with input frequencies above 100 kHz.

This is especially true when the amplifiers are configured in

positive unity gain (worst case). When such large capacitive

loads are required, the use of external compensation is highly

recommended. This reduces the overshoot and minimizes

ringing, which, in turn, improves the stability of the AD8655/

AD8656 when driving large capacitive loads.

One simple technique for compensation is a snubber that

consists of a simple RC network. With this circuit in place,

output swing is maintained, and the amplifier is stable at all

gains. Figure 55 shows the implementation of a snubber, which

reduces overshoot by more than 30% and eliminates ringing.

Using a snubber does not recover the loss of bandwidth

incurred from a heavy capacitive load.

Figure 54. Driving Heavy Capacitive Loads Without Compensation

Figure 55. Snubber Network

Figure 56. Driving Heavy Capacitive Loads Using a Snubber Network

TIME (2

µs/DIV)

VS = ±2.5V

AV = 1

CL = 500pF

+IN

200Ω

500pF

–IN

VCC

VEE

200mV

–

VS = ±2.5V

AV = 1

RS = 200Ω

CS = 500pF

CL = 500pF

TIME (10

µs/DIV)

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