SGM724 sg-micro, SGM724 Datasheet - Page 9

• Manufacturer Part Number: SGM724
• Description: 970?a, 10mhz, Rail-to-rail I/o Cmos Operational Amplifier
• Manufacturer: sg-micro

APPLICATION NOTES
Driving Capacitive Loads
The SGM72x can directly drive 4700pF in unity-gain without
oscillation. The unity-gain follower (buffer) is the most sensitive
configuration to capacitive loading. Direct capacitive loading
reduces the phase margin of amplifiers and this results in
ringing or even oscillation. Applications that require greater
capacitive drive capability should use an isolation resistor
between the output and the capacitive load like the circuit in
Figure 1. The isolation resistor R
form a zero to increase stability. The bigger the R
value, the more stable V
results in a loss of gain accuracy because R
divider with the R
An improvement circuit is shown in Figure 2. It provides DC
accuracy as well as AC stability. R
by connecting the inverting signal with the output. C
serve to counteract the loss of phase margin by feeding the
high frequency component of the output signal back to the
amplifier’s inverting input, thereby preserving phase margin in
the overall feedback loop.
Figure 2. Indirectly Driving Heavy Capacitive Load with DC
Accuracy
For no-buffer configuration, there are two others ways to
increase the phase margin: (a) by increasing the amplifier’s
gain or (b) by placing a capacitor in parallel with the feedback
resistor to counteract the parasitic capacitance associated with
inverting node.
Figure 1. Indirectly Driving Heavy Capacitive Load
V
V
IN
IN
SGM721
SGM721
C
F
LOAD
.
OUT
R
will be. Note that this method
R
ISO
C
ISO
ISO
L
F
provides the DC accuracy
and the load capacitor C
R
F
C
L
ISO
R
V
L
OUT
forms a voltage
V
OUT
ISO
F
and R
resistor
Iso
L
８
Power-Supply Bypassing and Layout
The SGM72x family operates from either a single +2.5V to
+5.5V supply or dual ±1.25V to ±2.75V supplies. For
single-supply operation, bypass the power supply V
0.1µF ceramic capacitor which should be placed close to the
V
supplies should be bypassed to ground with separate 0.1µF
ceramic capacitors. 2.2µF tantalum capacitor can be added for
better performance.
Good PC board layout techniques optimize performance by
decreasing the amount of stray capacitance at the op amp’s
inputs and output. To decrease stray capacitance, minimize
trace lengths and widths by placing external components as
close to the device as possible. Use surface-mount
components whenever possible.
For the operational amplifier, soldering the part to the board
directly is strongly recommended. Try to keep the high
frequency big current loop area small to minimize the EMI
(electromagnetic interfacing).
Grounding
A ground plane layer is important for SGM72x circuit design.
The length of the current path speed currents in an inductive
ground return will create an unwanted voltage noise. Broad
ground plane areas will reduce the parasitic inductance.
Input-to-Output Coupling
To minimize capacitive coupling, the input and output signal
traces should not be parallel. This helps reduce unwanted
positive feedback.
Vn
Vp
DD
pin. For dual-supply operation, both the V
Figure 3. Amplifier with Bypass Capacitors
V
SGM721
V
DD
SS
(GND)
0.1µF
10µF
V
OUT
SGM721/2/3/4
Vn
Vp
V
SGM721
DD
V
SS
DD
0.1µF
0.1µF
and the V
10µF
10µF
DD
with a
V
OUT
SS