2024年8月26日发(作者：贝知)

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.

+V

S

400␮A

300⍀

–IN

NULL/

COMPENSATION

NULL/

DECOMPENSATION

1k⍀1k⍀8k⍀

–V

S

300⍀

+IN

5pF

OUTPUT

2mA

The following section provides tables to show what C

COMP

values

will provide the necessary compensation for given circuit configurations

and capacitive loads. In each case, the recommended C

COMP

is a

minimum value. A larger C

COMP

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 C

COMP

based on

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

width that will be achieved for each case.

+V

S

1␮F0.1␮F

COMPENSATION

V

IN

AD744

C

COMP

5pF

V

OUT

Figure 744 Simplified Schematic

1␮F

–V

S

0.1␮F

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

versely proportional to the value of the compensation capacitor,

C

COMP

. Therefore, when trying to maximize the speed of the

amplifier, the value of C

COMP

should be minimized. C

COMP

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.

20100

Figure 744 Connected as a Unity Gain

Voltage Follower

Table I. Recommended Values of C

COMP

vs.

Various Capacitive Loads

Gain

1

1

1

Max

C

LOAD

(pF)

50

150

2000

C

COMP

(pF)

5

10

25

Slew Rate

(V/␮s)

37

25

12.5

–3 dB

Bandwidth

(MHz)

6.5

4.3

2.0

G

A

I

N

B

A

N

D

W

I

D

T

H

–

M

H

z

2

10

S

L

E

W

R

A

T

E

–

V

/

␮

s

0.21.0

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 C

COMP

values, along

with expected slew rates and bandwidths for a variety of load

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

C

LEAD

*

R1*

+V

S

1␮F

0.1␮F

R2*

0.02

0

10

C

COMP

– pF

100

0.1

1000

Figure Bandwidth and Slew Rate vs. C

COMP

AD744

V

IN

V

OUT

OPTIONAL

C

COMP

1␮F

0.1␮F

*SEE TABLE II

–V

S

Figure 31. AD744 Connected as a Voltage Follower

Operating at Gains of 2 or Greater

–8–

REV.

D

AD744

Table II. Recommended Values of C

COMP

vs. Various Load Conditions for the Circuits of

Figures 31 and 32.

R1

(⍀)

4.99 k

4.99 k

4.99 k

4.99 k

499 Ω

499 Ω

499 Ω

R2

(⍀)

4.99 k

4.99 k

4.99 k

4.99 k

4.99 k

4.99 k

4.99 k

Gain

Follower

2

2

2

2

11

11

11

Gain

Inverter

1

1

1

1

10

10

10

Max

C

LOAD

(pF)

50

150

1000

>2000

270

390

1000

C

COMP

(pF)

0

5

20

25

0

2

5

C

LEAD

(pF)

7

7

–

–

–

–

–

Slew

Rate

(V/␮s)

75

37

14

12.5

2

75

50

37

2

–3 dB

Bandwidth

(MHz)

2.5

1

2.3

1

1.2

1.0

1.2

0.85

0.60

NOTES

1

Bandwidth with C

LEAD

adjusted for minimum settling time.

2

Into large capacitive loads the AD744’s 25 mA output current limit sets the slew rate of the amplifier, in V/µs, equal to 0.025

amps divided by the value of C

LOAD

in µF. Slew rate is specified into rated max C

LOAD

except for cases marked

2

, which are

specified with a 50 pF. load.

C

LEAD

*

R2*

+V

S

R1*

V

IN

1␮F

0.1␮F

Due to manufacturing variations in the value of the internal

C

COMP

, it is recommended that the amplifier’s response be

optimized for the desired gain by using a 2 to 10 pF trimmer

capacitor rather than using a fixed value.

R1*

R2*

+V

S

1␮F

0.1␮F

AD744

OPTIONAL

C

COMP

*SEE TABLE II

1␮F

–V

S

0.1␮F

V

OUT

AD744

V

OUT

NOT CONNECTED

2 – 10pF

*SEE TABLE III

0.1␮F

Figure 32. AD744 Connected as an Inverting Amplifier

Operating at Gains of 1 or Greater

Using Decompensation to Extend the Gain Bandwidth

Product

V

IN

1␮F

–V

S

When the AD744 is used in applications where the closed-loop

gain is greater than 10, gain bandwidth product may be enhanced

by connecting a small capacitor between Pins 1 and 5 (Figure

33). At low frequencies, this capacitor cancels the effects of the

chip’s internal compensation capacitor, C

COMP

, effectively dec-

ompensating the amplifier.

Figure 33. Using the Decompensation Connection to

Extend Gain Bandwidth

Table III. Performance Summary for the Circuit of Figure 33

R1

(⍀)

1 k

100

100

R2

(⍀)

Gain

Follower

Gain

Inverter

10

100

1000

–3 dB

Bandwidth

2.5 MHz

760 kHz

225 kHz

Gain/BW

Product

25 MHz

76 MHz

225 MHz

10 k11

10 k101

100 k1001

REV.

D

–9–

2024年8月26日发(作者：贝知)

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.

+V

S

400␮A

300⍀

–IN

NULL/

COMPENSATION

NULL/

DECOMPENSATION

1k⍀1k⍀8k⍀

–V

S

300⍀

+IN

5pF

OUTPUT

2mA

The following section provides tables to show what C

COMP

values

will provide the necessary compensation for given circuit configurations

and capacitive loads. In each case, the recommended C

COMP

is a

minimum value. A larger C

COMP

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 C

COMP

based on

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

width that will be achieved for each case.

+V

S

1␮F0.1␮F

COMPENSATION

V

IN

AD744

C

COMP

5pF

V

OUT

Figure 744 Simplified Schematic

1␮F

–V

S

0.1␮F

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

versely proportional to the value of the compensation capacitor,

C

COMP

. Therefore, when trying to maximize the speed of the

amplifier, the value of C

COMP

should be minimized. C

COMP

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.

20100

Figure 744 Connected as a Unity Gain

Voltage Follower

Table I. Recommended Values of C

COMP

vs.

Various Capacitive Loads

Gain

1

1

1

Max

C

LOAD

(pF)

50

150

2000

C

COMP

(pF)

5

10

25

Slew Rate

(V/␮s)

37

25

12.5

–3 dB

Bandwidth

(MHz)

6.5

4.3

2.0

G

A

I

N

B

A

N

D

W

I

D

T

H

–

M

H

z

2

10

S

L

E

W

R

A

T

E

–

V

/

␮

s

0.21.0

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 C

COMP

values, along

with expected slew rates and bandwidths for a variety of load

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

C

LEAD

*

R1*

+V

S

1␮F

0.1␮F

R2*

0.02

0

10

C

COMP

– pF

100

0.1

1000

Figure Bandwidth and Slew Rate vs. C

COMP

AD744

V

IN

V

OUT

OPTIONAL

C

COMP

1␮F

0.1␮F

*SEE TABLE II

–V

S

Figure 31. AD744 Connected as a Voltage Follower

Operating at Gains of 2 or Greater

–8–

REV.

D

AD744

Table II. Recommended Values of C

COMP

vs. Various Load Conditions for the Circuits of

Figures 31 and 32.

R1

(⍀)

4.99 k

4.99 k

4.99 k

4.99 k

499 Ω

499 Ω

499 Ω

R2

(⍀)

4.99 k

4.99 k

4.99 k

4.99 k

4.99 k

4.99 k

4.99 k

Gain

Follower

2

2

2

2

11

11

11

Gain

Inverter

1

1

1

1

10

10

10

Max

C

LOAD

(pF)

50

150

1000

>2000

270

390

1000

C

COMP

(pF)

0

5

20

25

0

2

5

C

LEAD

(pF)

7

7

–

–

–

–

–

Slew

Rate

(V/␮s)

75

37

14

12.5

2

75

50

37

2

–3 dB

Bandwidth

(MHz)

2.5

1

2.3

1

1.2

1.0

1.2

0.85

0.60

NOTES

1

Bandwidth with C

LEAD

adjusted for minimum settling time.

2

Into large capacitive loads the AD744’s 25 mA output current limit sets the slew rate of the amplifier, in V/µs, equal to 0.025

amps divided by the value of C

LOAD

in µF. Slew rate is specified into rated max C

LOAD

except for cases marked

2

, which are

specified with a 50 pF. load.

C

LEAD

*

R2*

+V

S

R1*

V

IN

1␮F

0.1␮F

Due to manufacturing variations in the value of the internal

C

COMP

, it is recommended that the amplifier’s response be

optimized for the desired gain by using a 2 to 10 pF trimmer

capacitor rather than using a fixed value.

R1*

R2*

+V

S

1␮F

0.1␮F

AD744

OPTIONAL

C

COMP

*SEE TABLE II

1␮F

–V

S

0.1␮F

V

OUT

AD744

V

OUT

NOT CONNECTED

2 – 10pF

*SEE TABLE III

0.1␮F

Figure 32. AD744 Connected as an Inverting Amplifier

Operating at Gains of 1 or Greater

Using Decompensation to Extend the Gain Bandwidth

Product

V

IN

1␮F

–V

S

When the AD744 is used in applications where the closed-loop

gain is greater than 10, gain bandwidth product may be enhanced

by connecting a small capacitor between Pins 1 and 5 (Figure

33). At low frequencies, this capacitor cancels the effects of the

chip’s internal compensation capacitor, C

COMP

, effectively dec-

ompensating the amplifier.

Figure 33. Using the Decompensation Connection to

Extend Gain Bandwidth

Table III. Performance Summary for the Circuit of Figure 33

R1

(⍀)

1 k

100

100

R2

(⍀)

Gain

Follower

Gain

Inverter

10

100

1000

–3 dB

Bandwidth

2.5 MHz

760 kHz

225 kHz

Gain/BW

Product

25 MHz

76 MHz

225 MHz

10 k11

10 k101

100 k1001

REV.

D

–9–