SGM8624_15 Datasheet, PDF (9/19 Pages) SG Micro Corp – Rail-to-Rail I/O CMOS Operational Amplifiers

SGM8621/2/3/4
APPLICATION NOTES
Driving Capacitive Loads
The SGM8621/2/3/4 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 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.
SGM8621
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
SGM8621
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.
250μA, 3MHz, Rail-to-Rail I/O
CMOS Operational Amplifiers
Power-Supply Bypassing and Layout
The SGM8621/2/3/4 operate 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_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.
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).
+V_S
10µF
+V_S
10µF
0.1µF
0.1µF
Vn
SGM8621
Vp
Vn
V_{OUT Vp}
SGM8621
V_OUT
10µF
-V_S (GND)
0.1µF
-V_S
Figure 3. Amplifier with Bypass Capacitors
Grounding
A ground plane layer is important for SGM8621/2/3/4 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.
SG Micro Corp
9
www.sg-micro.com