AD8632AR Analog Devices Inc, AD8632AR Datasheet - Page 10

• Manufacturer Part Number: AD8632AR
• Description: IC OPAMP GP R-R 5MHZ DUAL 8SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD8632AR

• Rohs Status: RoHS non-compliant
• Amplifier Type: General Purpose
• Number Of Circuits: 2
• Output Type: Rail-to-Rail
• Slew Rate: 3 V/µs
• Gain Bandwidth Product: 5MHz
• Current - Input Bias: 250nA
• Voltage - Input Offset: 800µV
• Current - Supply: 300µA
• Current - Output / Channel: 10mA
• Voltage - Supply, Single/dual (±): 1.8 V ~ 6 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• -3db Bandwidth: -

AD8631/AD8632
DRIVING CAPACITIVE LOADS
Capacitive Load vs. Gain
Most amplifiers have difficulty driving capacitance due to degra-
dation of phase margin caused by additional phase lag from the
capacitive load. Higher capacitance at the output can increase the
amount of overshoot and ringing in the amplifier’s step response
and could even affect the stability of the device. The value of
capacitive load that an amplifier can drive before oscillation varies
with gain, supply voltage, input signal, temperature, among oth-
ers. Unity gain is the most challenging configuration for driving
capacitive load. However, the AD8631 offers reasonably good
capacitive driving ability. Figure 22 shows the AD8631’s ability to
drive capacitive loads at different gains before instability occurs.
This graph is good for all V
In-the-Loop Compensation Technique for Driving
Capacitive Loads
When driving capacitance in low gain configuration, the in-the-loop
compensation technique is recommended to avoid oscillation as is
illustrated in Figure 23.
Snubber Network Compensation for Driving Capacitive Loads
As load capacitance increases, the overshoot and settling time
will increase and the unity gain bandwidth of the device will
decrease. Figure 24 shows an example of the AD8631 in a non-
inverting configuration driving a 10 kΩ resistor and a 600 pF
capacitor placed in parallel, with a square wave input set to a
frequency of 90 kHz and unity gain.
Figure 23. In-the-Loop Compensation Technique for
Driving Capacitive Loads
100k
V
10k
100
1M
IN
R
1k
10
X
1
=
Figure 22. Capacitive Load vs. Gain
R
R
O
F
R
R
F
2
G
C
F
WHERE R
=
R
[
AD8631
3
G
1 +
UNSTABLE
O
C
SY
4
A
= OPEN-LOOP OUTPUT RESISTANCE
F
1
CL
.
GAIN – V/V
5
[
R
F
STABLE
R
6
+ R
F
R
G
X
7
C
L
R
O
8
C
L
9
V
OUT
10
–10–
By connecting a series R–C from the output of the device to
ground, known as the “snubber” network, this ringing and over-
shoot can be significantly reduced. Figure 25 shows the network
setup, and Figure 26 shows the improvement of the output
response with the “snubber” network added.
Figure 25. Snubber Network Compensation for Capacitive
Loads
The network operates in parallel with the load capacitor, C
and provides compensation for the added phase lag. The actual
values of the network resistor and capacitor have to be empirically
determined. Table II shows some values of snubber network for
large capacitance load.
Figure 24. Driving Capacitive Loads without Compensation
Figure 26. Photo of a Square Wave with the Snubber
Network Compensation
V
IN
90kHz INPUT SIGNAL
A
C = 600pF
90kHz INPUT SIGNAL
A
C = 600pF
V
V
= 1
= 1
AD8631
5V
TIME – 2 s/DIV
TIME – 2 s/DIV
R
C
X
X
C
L
V
OUT
L
REV. 0
,