AD744TA/883B 查看數據表（PDF） - Analog Devices

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AD744TA/883B

Precision, 500 ns Settling BiFET Op Amp

Analog Devices 

AD744

EXTERNAL FREQUENCY COMPENSATION

Even though the AD744 is useable without compensation in

most applications, it may be externally compensated for even

more flexibility. This is accomplished by connecting a capacitor

between Pins 5 and 8. Figure 28, a simplified schematic of the

AD744, shows where this capacitor is connected. This feature is

useful because it allows the AD744 to be used as a unity gain

voltage follower. It also enables the amplifier to drive capacitive

loads up to 2000 pF and greater.

400␮A

+VS

2mA

–IN

NULL /

COMPENSATION

300⍀

300⍀

+IN 5pF

OUTPUT

NULL /

DECOMPENSATION

COMPENSATION

1k⍀

1k⍀ 8k⍀

–VS

Figure 28. AD744 Simplified Schematic

The slew rate and gain bandwidth product of the AD744 are in-

versely proportional to the value of the compensation capacitor,

CCOMP. Therefore, when trying to maximize the speed of the

amplifier, the value of CCOMP should be minimized. CCOMP can

also be used to slow the amplifier to a point where the slew rate

is perfectly symmetrical and well controlled. Figure 29 sum-

marizes the effect of external compensation on slew rate and

bandwidth.

20

100

2

10

0.2

1.0

0.02

0

10

100

CCOMP – pF

0.1

1000

Figure 29. Gain Bandwidth and Slew Rate vs. CCOMP

The following section provides tables to show what CCOMP values

will provide the necessary compensation for given circuit configurations

and capacitive loads. In each case, the recommended CCOMP is a

minimum value. A larger CCOMP can always be used, but slew rate

and bandwidth performance will be degraded.

Figure 30 shows the AD744 configured as a unity gain voltage

follower. In this case, a minimum compensation capacitor of

5 pF is necessary for stable operation. Larger compensation ca-

pacitors can be used for driving larger capacitive loads. Table I

outlines recommended minimum values for CCOMP based on

the desired capacitive load. It also gives the slew rate and band-

width that will be achieved for each case.

+VS

1␮F

0.1␮F

AD744

VIN

CCOMP 5pF

VOUT

1␮F

–VS

0.1␮F

Figure 30. AD744 Connected as a Unity Gain

Voltage Follower

Table I. Recommended Values of CCOMP vs.

Various Capacitive Loads

Gain

1

1

1

Max

CLOAD

(pF)

50

150

2000

CCOMP

(pF)

5

10

25

Slew Rate

(V/␮s)

37

25

12.5

–3 dB

Bandwidth

(MHz)

6.5

4.3

2.0

Figures 31 and 32 show the AD744 as a voltage follower

with gain and as an inverting amplifier. In these cases, external

compensation is not necessary for stable operation. How-

ever, compensation may be applied to drive capacitive loads

above 50 pF. Table II gives recommended CCOMP values, along

with expected slew rates and bandwidths for a variety of load

conditions and gains for the circuits in Figures 31 and 32.

CLEAD*

R1*

R2*

+VS

1␮F

0.1␮F

AD744

VOUT

VIN

OPTIONAL

CCOMP

1␮F

*SEE TABLE II –VS

0.1␮F

Figure 31. AD744 Connected as a Voltage Follower

Operating at Gains of 2 or Greater

–8–

REV. C

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