SGM8541_17 Datasheet, PDF (8/18 Pages) SG Micro Corp – CMOS Operational Amplifiers

SGM8541/2/4
APPLICATION NOTES
Driving Capacitive Loads
The SGM8541/2/4 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 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.
SGM8541
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. 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.
C_F
SGM8541
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.
1.1MHz, 46μA, Rail-to-Rail I/O
CMOS Operational Amplifiers
Power-Supply Bypassing and Layout
The SGM8541/2/4 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_n
SGM8541
V_p
V_n
V_OUT
V_p
+V_S
10μF
0.1μF
SGM8541
V_OUT
10μF
-V_S(GND)
0.1μF
-V_S
Figure 3. Amplifier with Bypass Capacitors
SG Micro Corp
www.sg-micro.com
MARCH 2017
8