AD8657/AD8659

Data Sheet

Rev. C | Page 20 of 24

OUTPUT STAGE

The AD8657/AD8659 feature a complementary output stage

consisting of the M16 and M17 transistors. These transistors are

configured in Class AB topology and are biased by the voltage

source, VB2. This topology allows the output voltage to go

within millivolts of the supply rails, achieving a rail-to-rail output

swing. The output voltage is limited by the output impedance of the

transistors, which are low RON MOS devices. The output voltage

swing is a function of the load current and can be estimated using

the output voltage to the supply rail vs. load current diagrams (see

Figure 21, Figure 22, Figure 24, and Figure 25).

RAIL TO RAIL

The AD8657/AD8659 feature rail-to-rail input and output with

a supply voltage from 2.7 V to 18 V. Figure 69 shows the input

and output waveforms of the AD8657/AD8659 configured as a

unity-gain buffer with a supply voltage of ±9 V and a resistive

load of 1 MΩ. With an input voltage of ±9 V, the AD8657/AD8659

allow the output to swing very close to both rails. Additionally,

they do not exhibit phase reversal.

TIME (200µs/DIV)

VSY = ±9V

RL = 1MΩ

INPUT

OUTPUT

Figure 69. Rail-to-Rail Input and Output

RESISTIVE LOAD

The feedback resistor alters the load resistance that an amplifier

sees. It is, therefore, important to be aware of the value of feed-

back resistors chosen for use with the AD8657/AD8659. The

amplifiers are capable of driving resistive loads down to 100 kΩ.

The following two examples, inverting and noninverting

configurations, show how the feedback resistor changes the

actual load resistance seen at the output of the amplifier.

Inverting Op Amp Configuration

Figure 70 shows the AD8657/AD8659 in an inverting configu-

ration with a resistive load, RL, at the output. The actual load

seen by the amplifier is the parallel combination of the feedback

resistor, R2, and load, RL. For example, the combination of a feed-

back resistor of 1 kΩ and a load of 1 MΩ results in an equivalent

load resistance of 999 Ω at the output. Because the AD8657/

AD8659 are incapable of driving such a heavy load, performance

degrades greatly.

To avoid loading the output, use a larger feedback resistor, but

consider the resistor thermal noise effect on the overall circuit.

AD8657/

AD8659

R1

R2

RL

–VSY

RL, EFF = RL || R2

+VSY

VIN

VOUT

Figure 70. Inverting Op Amp Configuration

Noninverting Op Amp Configuration

Figure 71 shows the AD8657/AD8659 in a noninverting configu-

ration with a resistive load, RL, at the output. The actual load seen

by the amplifier is the parallel combination of R1 + R2 and RL.

R1

R2

RL

–VSY

RL, EFF = RL || (R1 + R2)

+VSY

VIN

VOUT

AD8657/

AD8659

Figure 71. Noninverting Op Amp Configuration