SGM8522_15 Datasheet, PDF (7/15 Pages) SG Micro Corp – CMOS Operational Amplifiers

SGM8521/2/4
APPLICATION NOTES
Driving Capacitive Loads
The SGM8521/2/4 can directly drive 250pF 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 driving capability should use an isolation resistor
between the output and the capacitive load like the circuit in
Figure 1. The isolation resistor RISO and the load capacitor CL
form a zero to increase stability. The bigger the R_ISO resistor
value, the more stable VOUT will be. Note that this method
results in a loss of gain accuracy because R_ISO forms a voltage
divider with the RLOAD.
SGM8521
V_IN
RISO
V_OUT
CL
Figure 1. Indirectly Driving Heavy Capacitive Load
An improved circuit is shown in Figure 2. It provides DC
accuracy as well as AC stability. RF 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.
CF
SGM8521
VIN
R_ISO
C_L
RF
VOUT
R_L
150kHz, 5.5μA, Rail-to-Rail I/O
CMOS Operational Amplifiers
For non-buffer configuration, there are two other 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.
Power-Supply Bypassing and Layout
The SGM8521/2/4 family operates from either a single +2.1V to
+5.5V supply or dual ±1.05V to ±2.75V supplies. For
single-supply operation, bypass the power supply +V_S with a
0.1µF ceramic capacitor which should be placed close to the
+V_S pin. For dual-supply operation, both the +V_S and the -V_S
supplies should be bypassed to ground with separate 0.1µF
ceramic capacitors. 2.2µF tantalum capacitor can be added for
better performance.
+VS
10μF
0.1μF
+VS
10μF
0.1μF
Vn
SGM8521
Vp
Vn
VOUT
Vp
VOUT
SGM8521
10μF
-VS (GND)
0.1μF
-V_S
Figure 3. Amplifier with Bypass Capacitors
Figure 2. Indirectly Driving Heavy Capacitive Load with DC
Accuracy
SG Micro Corp
7
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