Datasheet AD744 (Analog Devices) - 9

AD744 EXTERNAL FREQUENCY COMPENSATION
The following section provides tables to show what CCOMP values Even though the AD744 is useable without compensation in will provide the necessary compensation for given circuit configurations most applications, it may be externally compensated for even and capacitive loads. In each case, the recommended CCOMP is a more flexibility. This is accomplished by connecting a capacitor minimum value. A larger CCOMP can always be used, but slew rate between Pins 5 and 8. Figure 28, a simplified schematic of the and bandwidth performance will be degraded. AD744, shows where this capacitor is connected. This feature is useful because it allows the AD744 to be used as a unity gain Figure 30 shows the AD744 configured as a unity gain voltage voltage follower. It also enables the amplifier to drive capacitive follower. In this case, a minimum compensation capacitor of loads up to 2000 pF and greater. 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
+V
COMP based on
S
the desired capacitive load. It also gives the slew rate and band-
400
␮
A 2mA
width that will be achieved for each case.
300
⍀
300
⍀
–IN +IN +VS 5pF OUTPUT NULL / 1
␮
F 0.1
␮
F COMPENSATION NULL / AD744 V DECOMPENSATION OUT COMPENSATION VIN C 1k
⍀
1k
⍀
8k
⍀
COMP 5pF –VS 1
␮
F 0.1
␮
F
Figure 28. AD744 Simplified Schematic
–VS
The slew rate and gain bandwidth product of the AD744 are in- versely proportional to the value of the compensation capacitor, Figure 30. AD744 Connected as a Unity Gain C Voltage Follower COMP. 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
Table I. Recommended Values of CCOMP vs.
is perfectly symmetrical and well controlled. Figure 29 sum-
Various Capacitive Loads
marizes the effect of external compensation on slew rate and
Max –3 dB
bandwidth.
CLOAD CCOMP Slew Rate Bandwidth Gain (pF) (pF) (V/
␮
s) (MHz) 20 100
1 50 5 37 6.5 1 150 10 25 4.3 1 2000 25 12.5 2.0
MHz s – 2 10
␮ Figures 31 and 32 show the AD744 as a voltage follower
V/ –
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
SLEW RATE
above 50 pF. Table II gives recommended C
0.2 1.0
COMP values, along
GAIN BANDWIDTH
with expected slew rates and bandwidths for a variety of load conditions and gains for the circuits in Figures 31 and 32.
CLEAD* 0.02 0.1 0 10 100 1000 R1* R2* CCOMP – pF +VS
Figure 29. Gain Bandwidth and Slew Rate vs. CCOMP
1
␮
F 0.1
␮
F AD744 VOUT V OPTIONAL IN CCOMP 1
␮
F 0.1
␮
F *SEE TABLE II –VS
Figure 31. AD744 Connected as a Voltage Follower Operating at Gains of 2 or Greater –8– REV. C