Datasheet

AD8657/AD8659 Data Sheet
Rev. B | 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 R
ON
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)
VOLTAGE (5V/DIV)
V
SY
= ±9V
R
L
= 1MΩ
08804-057
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, 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
. 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
R
L
–V
SY
R
L, EFF
= R
L
|| R2
+V
SY
V
IN
V
OUT
08804-058
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, 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/
AD8659
Figure 71. Noninverting Op Amp Configuration