AD744 Analog Devices, AD744 Datasheet - Page 8

• Manufacturer Part Number: AD744
• Description: Precision/ 500 ns Settling BiFET Op Amp
• Manufacturer: 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.
The slew rate and gain bandwidth product of the AD744 are in-
versely proportional to the value of the compensation capacitor,
C
amplifier, the value of C
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.
DECOMPENSATION
COMP
COMPENSATION
0.02
0.2
. Therefore, when trying to maximize the speed of the
20
2
0
NULL /
NULL /
–IN
400 A
300
1k
10
COMP
300
1k
C
COMP
should be minimized. C
8k
+IN
– pF
2mA
5pF
100
COMPENSATION
+V
–V
OUTPUT
S
S
COMP
1000
100
10
1.0
0.1
can
The following section provides tables to show what C
will provide the necessary compensation for given circuit configurations
and capacitive loads. In each case, the recommended C
minimum value. A larger C
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
the desired capacitive load. It also gives the slew rate and band-
width that will be achieved for each case.
Gain
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
with expected slew rates and bandwidths for a variety of load
conditions and gains for the circuits in Figures 31 and 32.
1
1
1
SEE TABLE II
V
Table I. Recommended Values of C
Various Capacitive Loads
C
50
150
2000
R1
IN
Max
(pF)
V
LOAD
IN
AD744
–V
C
AD744
–V
S
(pF)
C
COMP
+V
5
10
25
S
LEAD
COMP
+V
S
1 F
S
1 F
C
can always be used, but slew rate
C
COMP
1 F
COMP
R2
1 F
OPTIONAL
Slew Rate
0.1 F
(V/ s)
0.1 F
37
25
12.5
5pF
0.1 F
0.1 F
COMP
COMP
COMP
V
values, along
OUT
Bandwidth
V
COMP
vs.
OUT
(MHz)
based on
–3 dB
COMP
6.5
4.3
2.0
values
is a