OP249
Data Sheet
A
B
4µs
4µs
100
90
100
90
10
10
0%
0%
500mV
1µs
C = 5pF
500mV
1µs
C = 15pF
RESPONSE IS GROSSLY UNDERDAMPED,
AND EXHIBITS RINGING
FAST RISE TIME CHARACTERISTICS, BUT AT EXPENSE
OF SLIGHT PEAKING IN RESPONSE
Figure 49. Effect of Altering Compensation from Circuit in Figure 47—PM7545 CMOS DAC with 1/2 OP249, Unipolar Operation;
Critically Damped Response Is Obtained with C ≈ 33 pF
+15V
0.1µF
Figure 49 illustrates the effect of altering the compensation on
the output response of the circuit in Figure 47. Compensation is
required to address the combined effect of the output capacitance
of the DAC, the input capacitance of the op amp, and any stray
capacitance. Slight adjustments to the compensation capacitor may
be required to optimize settling response for any given application.
8
3
2
1/2
1
7A13 PLUG-IN
7A13 PLUG-IN
OP249
4
0.1µF
*
–15V
The settling time of the combination of the current output DAC
and the op amp can be approximated by
1kΩ
300pF
|V
REF
1kΩ
|
ΔI
=
OUT
+15V
2
2
tS TOTAL =
(
tS DAC
)
+
(
tS AMP
)
1.5kΩ
2N3904
10µF
TTL INPUT
The actual overall settling time is affected by the noise gain of
the amplifier, the applied compensation, and the equivalent
input capacitance at the input of the amplifier.
1N4148
2N2907
1kΩ
1.8kΩ
+15V
220Ω
0.47µF
DISCUSSION ON DRIVING ADCs
0.01µF
0.1µF
*
Settling characteristics of op amps also include the ability of the
amplifier to recover, that is, settle, from a transient current output
load condition. An example of this includes an op amp driving
the input from a SAR-type ADC. Although the comparison
point of the converter is usually diode clamped, the input swing
of plus-and-minus a diode drop still gives rise to a significant
modulation of input current. If the closed-loop output impedance
is low enough and bandwidth of the amplifier is sufficiently
large, the output settles before the converter makes a comparison
decision, which prevents linearity errors or missing codes.
V
REF
*DECOUPLE CLOSE TOGETHER ON GROUND
PLANE WITH SHORT LEAD LENGTHS.
Figure 50. Transient Output Impedance Test Fixture
As seen in Figure 51, the OP249 has an extremely fast recovery
of 247 ns (to 0.01%) for a 1 mA load transient. The performance
makes it an ideal amplifier for data acquisition systems.
247.4ns
100
90
Figure 50 shows a settling measurement circuit for evaluating
recovery from an output current transient. An output disturbing
current generator provides the transient change in output load
current of 1 mA.
10
0%
2mV
2V
100ns
Figure 51. Transient Recovery Time of the OP249 from
a 1 mA Load Transient to 0.01%
Rev. I | Page 16 of 18
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