AD711
OP AMP SETTLING TIME—
A MATHEMATICAL MODEL
The design of the AD711 gives careful attention to optimizing
individual circuit components; in addition, a careful tradeoff was
made: the gain bandwidth product (4 MHz) and slew rate
(20 V/µs) were chosen to be high enough to provide very fast
settling time but not too high to cause a significant reduction in
phase margin (and therefore stability). Thus designed, the
AD711 settles to
±
0.01%, with a 10 V output step, in under
1
µs,
while retaining the ability to drive a 100 pF load capaci-
tance when operating as a unity gain follower.
If an op amp is modeled as an ideal integrator with a unity gain
crossover frequency of
ω
ο
/2π, Equation 1 will accurately de-
scribe the small signal behavior of the circuit of Figure 26a, con-
sisting of an op amp connected as an I-to-V converter at the
output of a bipolar or CMOS DAC. This equation would com-
pletely describe the output of the system if not for the op amp’s
finite slew rate and other nonlinear effects.
Equation 1.
V
O
–
R
=
I
IN
R(C
f
=
C
X
)
2
G
N
s
+
+
RC
f
s
+
1
ω
ο
ω
ο
When R
O
and I
O
are replaced with their Thevenin V
IN
and R
IN
equivalents, the general purpose inverting amplifier of Figure
26b is created. Note that when using this general model, capaci-
tance C
X
is EITHER the input capacitance of the op amp if a
simple inverting op amp is being simulated OR it is the com-
bined capacitance of the DAC output and the op amp input if
the DAC buffer is being modeled.
Figure 26b. Simplified Model of the AD711
Used as an Inverter
where
2
=op amp’s unity gain frequency
π
G
N
= “noise” gain of circuit
1
+
R
O
This equation may then be solved for C
f
:
Equation 2.
C
f
=
2
−
G
N
2
RC
X
ω
ο
+
(1
−
G
N
)
+
Rω
ο
Rω
ο
ω
ο
R
In either case, the capacitance C
X
causes the system to go from
a one-pole to a two-pole response; this additional pole increases
settling time by introducing peaking or ringing in the op amp
output. Since the value of C
X
can be estimated with reasonable
accuracy, Equation 2 can be used to choose a small capacitor,
C
F
, to cancel the input pole and optimize amplifier response.
Figure 27 is a graphical solution of Equation 2 for the AD711
with R = 4 kΩ.
In these equations, capacitor C
X
is the total capacitor appearing
the inverting terminal of the op amp. When modeling a DAC
buffer application, the Norton equivalent circuit of Figure 26a
can be used directly; capacitance C
X
is the total capacitance of
the output of the DAC plus the input capacitance of the op amp
(since the two are in parallel).
Figure 27. Value of Capacitor C
F
vs. Value of C
X
The photos of Figures 28a and 28b show the dynamic response
of the AD711 in the settling test circuit of Figure 29.
The input of the settling time fixture is driven by a flat-top pulse
generator. The error signal output from the false summing node
of A1 is clamped, amplified by A2 and then clamped again. The
error signal is thus clamped twice: once to prevent overloading
amplifier A2 and then a second time to avoid overloading the
oscilloscope preamp. The Tektronix oscilloscope preamp type
7A26 was carefully chosen because it does not overload with
these input levels. Amplifier A2 needs to be a very high speed
FET-input op amp; it provides a gain of 10, amplifying the error
signal output of A1.
Figure 26a. Simplified Model of the AD711 Used as a
Current-Out DAC Buffer
–8–
REV. A
AD [ ANALOG DEVICES ]
