AD8657 Analog Devices, AD8657 Datasheet - Page 18

• Manufacturer Part Number: AD8657
• Description: Precision, Micropower 18 V CMOS RRIO Op Amp
• Manufacturer: Analog Devices

Specifications of AD8657

• Vcc-vee: 2.7V to 18V
• Isy Per Amplifier: 18µA
• Packages: SOP,CSP
• -3db Bandwidth: 200kHz
• Slew Rate: 0.07V/µs
• Vos: 350µV
• Ib: 5pA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 50nV/rtHz

AD8657
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.
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.
INPUT
OUTPUT
Figure 64. Rail-to-Rail Input and Output
+IN x
–IN x
V+
V–
TIME (200µs/DIV)
R1
R2
D1
D2
V
R
SY
L
M1
= 1MΩ
M7
= ±9V
M2
Figure 63. Simplified Schematic
VB1
Rev. A | Page 18 of 24
M5
M6
M3
M4
Inverting Configuration
Figure 65 shows AD8657 in an inverting configuration with
a resistive load, R
amplifier is the parallel combination of the feedback resistor,
R2, and load, R
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.
Noninverting Configuration
Figure 66 shows the AD8657 in a noninverting configuration
with a resistive load, R
the amplifier is the parallel combination of R1 + R2 and R
I1
M12
M10
M14
M8
V
IN
L
. Having a feedback resistor of 1 kΩ and a load
L
, at the output. The actual load seen by the
Figure 66. Noninverting Op Amp
R1
VB2
R1
Figure 65. Inverting Op Amp
V
L
IN
, at the output. The actual load seen by
R
M11
M9
L, EFF
M13
M15
R
L, EFF
AD8657
AD8657
= R
+V
–V
1/2
R2
+V
–V
L
1/2
SY
R2
= R
SY
|| (R1 + R2)
SY
SY
M16
M17
L
|| R2
OUT x
V
OUT
R
V
L
R
OUT
L
L
.