SGM8543 Datasheet, PDF (7/12 Pages) SG Micro Corp – Operational Amplifier with Shutdown

SGM8543
1.1MHz, 48μA, Rail-to-Rail I/O, CMOS
Operational Amplifier with Shutdown
APPLICATION NOTES
Driving Capacitive Loads
The SGM8543 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 R_ISO and the load capacitor C_L form a zero to
increase stability. The bigger the RISO resistor value, the more
stable V_OUT will be. Note that this method results in a loss of gain
accuracy because RISO forms a voltage divider with the RLOAD.
DISABLE
SGM8543
VIN
RISO
V_OUT
CL
Power-Supply Bypassing and Layout
The SGM8543 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 +VS with a 0.1µF ceramic
capacitor which should be placed close to the +V_S pin. For
dual-supply operation, both the +VS and the -VS 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
DISABLE
Vn
SGM8543
Vp
VOUT
+VS
10µF
0.1µF
DISABLE
Vn
SGM8543
VOUT
Vp
10µF
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.
-VS (GND)
0.1µF
-VS
Figure 3. Amplifier with Bypass Capacitors
CF
DISABLE
R_F
RISO
SGM8543
V_OUT
VIN
CL
RL
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
www.sg-micro.com