Application Hints
OUTPUT DRIVE AND STABILITY
The LME49990 is unity gain stable from both input (both sta-
ble when gain = -1 or gain = 1). It able to drive resistive load
600Ω with output circuit with a typical 27mA. Capacitive loads
up to 100pF will cause little change in the phase characteris-
tics of the amplifiers and are therefore allowable.
Capacitive loads greater than 100pF must be isolated from
the output. The most straight forward way to do this is to put
a resistor in series with the output. This resistor will also pre-
vent excess power dissipation if the output is accidentally
shorted.
The effective load impedance (including feedback resistance)
should be kept above 600Ω for fast settling. Load capacitance
should also be minimized if good settling time is to be opti-
mized. Large feedback resistors will make the circuit more
susceptible to stray capacitance, so in high-speed applica-
tions keep the feedback resistors in the 1kΩ to 2 kΩ range
whenever practical.
300597c7
FIGURE 3. LME4990 Output Compensation Network
SUPPLY BYPASSING
To achieve a low noise and high-speed audio performance,
power supply bypassing is extremely important. Applying
multiple bypass capacitors is highly recommended. From ex-
periment results, a 10μF tantalum, 2.2μF ceramic, and a
0.47μF ceramic work well. All bypass capacitors leads should
be very short. The ground leads of capacitors should also be
separated to reduce the inductance to ground. To obtain the
best result, a large ground plane layout technique is recom-
mended and it was applied in the LME49990 evaluation
board.
OUTPUT COMPENSATION
In most of the audio applications, the device will be operated
in a room temperature and compensation networks are not
necessary. However, the consideration of output network as
shown in Figure 3 may be taken into account for some of the
high performance audio applications such as high speed data
conversion or when operating in a relatively low junction tem-
perature. The compensation network will also provide a small
improvement in settling time for the response time demanding
applications.
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