SGM8532_15 Datasheet, PDF (7/15 Pages) SG Micro Corp – CMOS Operational Amplifiers

SGM8531/2/4
APPLICATION NOTES
Driving Capacitive Loads
The SGM8531/2/4 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.
SGM8531
V_IN
R_ISO
V_OUT
C_L
Figure 1. Indirectly Driving Heavy Capacitive Load
An improved 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. CF and RIso
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.
500kHz, 18μA, Rail-to-Rail I/O
CMOS Operational Amplifiers
Power-Supply Bypassing and Layout
The SGM8531/2/4 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.
+V_S
10µF
0.1µF
+V_S
10µF
0.1µF
Vn
SGM8531
Vp
Vn
V_OUT
Vp
V_OUT
SGM8531
10µF
-V_S(GND)
0.1µF
-V_S
Figure 3. Amplifier with Bypass Capacitors
C_F
SGM8531
V_IN
R_ISO
C_L
R_F
V_OUT
R_L
Figure 2. Indirectly Driving Heavy Capacitive Load with DC
Accuracy
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.
SG Micro Corp
7
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