AD8657ACPZ-R7 datasheet(18/24 Pages) AD

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AD8657

Rev. A | Page 18 of 24

V+

V–

+IN x

VB1

M10

M16

M17

M11

VB2

OUT x

M12

M14

M13

M15

–IN x

Figure 63. Simplified Schematic

RAIL TO RAIL

The AD8657 features rail-to-rail input and output with a supply

voltage from 2.7 V to 18 V. Figure 64 shows the input and output

waveforms of the AD8657 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 allows the output to swing

very close to both rails. Additionally, it does not exhibit phase

reversal.

TIME (200µs/DIV)

VSY = ±9V

RL = 1MΩ

INPUT

OUTPUT

Figure 64. 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. The AD8657 is

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 Configuration

Figure 65 shows AD8657 in an inverting configuration 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. Having a feedback resistor of 1 kΩ and a load

of 1 MΩ results in an equivalent load resistance of 999 Ω at the

output. In this condition, the AD8657 is incapable of driving

such a heavy load; therefore, its 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

1/2

–VSY

RL, EFF = RL || R2

+VSY

VIN

VOUT

Figure 65. Inverting Op Amp

Noninverting Configuration

Figure 66 shows the AD8657 in a noninverting configuration

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

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

–VSY

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

+VSY

VIN

VOUT

AD8657

1/2

Figure 66. Noninverting Op Amp

AD8657ACPZ-R7 Datasheet(PDF) 18 Page - Analog Devices

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