APPLICATION NOTES
Driving Capacitive Loads
The SGM805x family is optimized for bandwidth and speed,
not for driving capacitive loads. Output capacitance will
create a pole in the amplifier’s feedback path, leading to
excessive peaking and potential oscillation. If dealing with
load capacitance is a requirement of the application, the two
strategies to consider are (1) using a small resistor in series
with the amplifier’s output and the load capacitance and (2)
reducing the bandwidth of the amplifier’s feedback loop by
increasing the overall noise gain.
Figure 1 shows a unity gain follower using the series resistor
strategy. The resistor isolates the output from the capacitance
and, more importantly, creates a zero in the feedback path
that compensates for the pole created by the output
capacitance.
V
DD
V
DD
10µF
10µF
0.1µF
0.1µF
Vn
Vn
SGM8051
V
OUT
SGM8051
V
OUT
10µF
Vp
Vp
V
SS
(GND)
0.1µF
V
SS
Figure 2. Amplifier with Bypass Capacitors
R
ISO
SGM8051
V
IN
V
OUT
C
LOAD
Grounding
A ground plane layer is important for high speed 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.
Figure 1
. Series Resistor Isolating Capacitive Load
Power-Supply Bypassing and Layout
The SGM805x family operates from either a single +2.7V to
+5.5V supply or dual ±1.35V to ±2.75V supplies. For
single-supply operation, bypass the power supply V
DD
with a
0.1µF ceramic capacitor which should be placed close to the
V
DD
pin. For dual-supply operation, both the V
DD
and the V
SS
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 high speed 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).
Input-to-Output Coupling
To minimize capacitive coupling, the input and output signal
traces should not be parallel. This helps reduce unwanted
positive feedback.
９
SGM8051/2/3/4/5
SGMICRO [ Shengbang Microelectronics Co, Ltd ]
