APPLICATION NOTES
Driving Capacitive Loads
The SGM863x can directly drive 1000pF 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
ISO
and the load capacitor C
L
form a zero to increase stability. The bigger the R
ISO
resistor
value, the more stable V
OUT
will be. Note that this method
results in a loss of gain accuracy because R
ISO
forms a voltage
divider with the R
LOAD
.
Power-Supply Bypassing and Layout
The SGM863x 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
DD
with a
0.1µF ceramic capacitor which should be placed close to the
V
DD
pin. For dual-supply operation, both the V
DD
and the V
SS
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).
R
ISO
SGM8631
V
OUT
C
L
V
IN
V
DD
Figure 1. Indirectly Driving Heavy Capacitive Load
An improvement circuit is shown in Figure 2. It provides DC
accuracy as well as AC stability. R
F
provides the DC accuracy
by connecting the inverting signal with the output. C
F
and R
Iso
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.
10µF
0.1µF
V
DD
10µF
0.1µF
Vn
V
OUT
10µF
Vn
SGM8631
V
OUT
SGM8631
Vp
Vp
C
F
R
F
R
ISO
SGM8631
V
SS
(GND)
0.1µF
V
OUT
C
L
R
L
V
SS
Figure 3. Amplifier with Bypass Capacitors
V
IN
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.
Grounding
A ground plane layer is important for SGM863x 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.
９
SGM8631/2/3/4
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
