OP249 Datasheet, PDF (13/17 Pages) Analog Devices – Dual, Precision JFET High Speed Operational Amplifier

OP249
A
B
C = 5pF
RESPONSE IS GROSSLY UNDERDAMPED,
AND EXHIBITS RINGING
C = 15pF
FAST RISE TIME CHARACTERISTICS, BUT AT EXPENSE
OF SLIGHT PEAKING IN RESPONSE
Figure 48. Effect of Altering Compensation from Circuit in Figure 47a—PM7545 CMOS DAC with 1/2 OP249, Unipolar
Operation. Critically Damped Response Will Be Obtained with C à´  33 pF
Figure 48 illustrates the effect of altering the compensation on
the output response of the circuit in Figure 48a. Compensation
is required to address the combined effect of the DAC’s output
capacitance, the op amp’s input capacitance and any stray ca-
pacitance. Slight adjustments to the compensation capacitor
may be required to optimize settling response for any given
application .
The settling time of the combination of the current output DAC
and the op amp can be approximated by:
t_S TOTAL = (t_S DAC )² + (t_S AMP )²
The actual overall settling time is affected by the noise gain of
the amplifier, the applied compensation, and the equivalent
input capacitance at the amplifier’s input.
DISCUSSION ON DRIVING A/D CONVERTERS
Settling characteristics of operational amplifiers also include an
amplifier’s ability to recover, i.e., settle, from a transient current
output load condition. An example of this includes an op amp
driving the input from a SAR type A/D converter. 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 out-
put impedance is low enough and bandwidth of the amplifier is
sufficiently large, the output will settle before the converter
makes a comparison decision which will prevent linearity errors
or missing codes.
Figure 49 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. As seen in Figure 50, the OP249 has extremely
fast recovery of 274 ns (to 0.01%), for a 1 mA load transient.
The performance makes it an ideal amplifier for data acquisition
systems.
+15V _0.1␮F
1/2
OP249
0.1␮F
7A13 PLUG-IN
*
–15V
7A13 PLUG-IN
300pF
+15V
1k⍀
⌬I_OUT =
| V_REF |
1k⍀
TTL INPUT
+15V
1.5k⍀
1N4148
1.8k⍀
2N2907
220⍀
2N3904
1k⍀
10␮F
0.1␮F
0.01␮F 0.47␮F
*
*NOTE: DECOUPLE CLOSE TOGETHER
ON GROUND PLANEWITH
SHORT LEAD LENGTHS
V_REF
Figure 49. Transient Output Impedance Test Fixture
The combination of high speed and excellent dc performance of
the OP249 makes it an ideal amplifier for 12-bit data acquisition
systems. Examining the circuit in Figure 51, one amplifier in the
OP249 provides a stable –5 V reference voltage for the V_REF
input of the ADC912. The other amplifier in the OP249 per-
forms high speed buffering of the A/D’s input.
Examining the worst case transient voltage error (Figure 52) at
the Analog In node of the A/D converter: the OP249 recovers in
less than 100 ns. The fast recovery is due to both the OP249’s
wide bandwidth and low dc output impedance.
REV. C
–13–