Datasheet

AD8657
Rev. A | Page 18 of 24
V+
V–
+IN x
R1
D1 D2
M1 M2
M7 M6
M3 M4
M5
VB1
M8
M10
M9
M16
M17
M11
VB2
OUT x
M12
M14
M13
M15
I1
R2
–IN x
08804-056
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)
VOLTAGE (5V/DIV)
V
SY
= ±9V
R
L
= 1M
08804-057
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, R
L
, at the output. The actual load seen by the
amplifier is the parallel combination of the feedback resistor,
R2, and load, R
L
. 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
R1
R2
R
L
–V
SY
R
L, EFF
= R
L
|| R2
+V
SY
V
IN
V
OUT
08804-058
Figure 65. Inverting Op Amp
Noninverting Configuration
Figure 66 shows the AD8657 in a noninverting configuration
with a resistive load, R
L
, at the output. The actual load seen by
the amplifier is the parallel combination of R1 + R2 and R
L
.
R1
R2
R
L
–V
SY
R
L, EFF
= R
L
|| (R1 + R2)
+V
SY
V
IN
V
OUT
08804-059
AD8657
1/2
Figure 66. Noninverting Op Amp