LME49724 Datasheet, PDF (17/30 Pages) National Semiconductor (TI) – High Performance, High Fidelity, Fully-Differential Audio Operational Amplifier

LME49724
www.ti.com
SNAS438A – NOVEMBER 2008 – REVISED APRIL 2013
SUPPLY BYPASSING
The LME49724 should have its supply leads bypassed with low-inductance capacitors such as leadless surface
mount (SMT) capacitors located as close as possible to the supply pins. It is recommended that a 10μF tantalum
or electrolytic capacitor be placed in parallel with a 0.1μF ceramic or film type capacitor on each supply pin.
These capacitors should be star routed with a dedicated ground return plane or large trace for best THD
performance. Placing capacitors too far from the power supply pins, especially with thin connecting traces, can
lead to excessive inductance, resulting in degraded high-frequency bypassing. Poor high-frequency bypassing
can result in circuit instabilities. When using high bandwidth power supplies, the value and number of supply
bypass capacitors should be reduced for optimal power supply performance.
BALANCE CABLE DRIVER
With high peak-to-peak differential output voltage and plenty of low distortion drive current, the LME49724 makes
an excellent balanced cable driver. Combining the single-to-differential configuration with a balanced cable driver
results in a high performance single-ended input to balanced line driver solution.
Although the LME49724 can drive capacitive loads up to 100pF, cable loads exceeding 100pF can cause
instability. For such applications, series resistors are needed on the outputs before the capacitive load.
ANALOG-TO-DIGITAL CONVERTER (ADC) APPLICATION
Figure 53 is a typical fully differential application circuit for driving an analog-to-digital converter (ADC). The
additional components of R₅, R₆, and C₇ are optional components and are for stability and proper ADC sampling.
ADC's commonly use switched capacitor circuitry at the input. When the ADC samples the signal the current
momentarily increases and may disturb the signal integrity at the sample point causing a signal glitch.
Component C₇ is significantly larger than the input capacitance of a typical ADC and acts as a charge reservoir
greatly reducing the effect of the signal sample by the ADC. Resistors R₅ and R₆ decouple the capacitive load,
C₇, for stability. The values shown are general values. Specific values should be optimized for the particular ADC
loading requirements.
The output reference voltage from the ADC can be used to drive the V_OCM pin to set the common-mode DC
voltage on the outputs of the LME49724. A buffer may be needed to drive the LME49724's V_OCM pin if the ADC
cannot drive the 50kΩ input impedance of the V_OCM pin.
In order to minimize circuit distortion when using capacitors in the signal path, the capacitors should be
comprised of either NPO ceramic, polystyrene, polypropylene or mica composition. Other types of capacitors
may provide a reduced distortion performance but for a cost improvement, so capacitor selection is dependent
upon design requirements. The performance/cost tradeoff for a specific application is left up to the user.
Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LME49724
Submit Documentation Feedback
17