OP249

–13–

REV. C

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

2

+(t

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.

B

A

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

33 pF

1k

1/2

OP249

+15V

–15V

0.1 F

0.1 F

+15V

1.5k

1N4148

0.1 F

220

1.8k

1k

2N3904

1k

10 F

0.47 F

0.01 F

*

*

*NOTE: DECOUPLE CLOSE TOGETHER

ON GROUND PLANEWITH

SHORT LEAD LENGTHS

TTL INPUT

+15V

2N2907

7A13 PLUG-IN

7A13 PLUG-IN

300pF

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

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.