LME49724 Datasheet, PDF (14/30 Pages) National Semiconductor (TI) – High Performance, High Fidelity, Fully-Differential Audio Operational Amplifier

LME49724
SNAS438A – NOVEMBER 2008 – REVISED APRIL 2013
APPLICATION INFORMATION
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GENERAL OPERATION
The LME49724 is a fully differential amplifier with an integrated common-mode reference input (V_OCM). Fully
differential amplification provides increased noise immunity, high dynamic range, and reduced harmonic
distortion products.
Differential amplifiers typically have high CMRR providing improved immunity from noise. When input, output,
and supply line trace pairs are routed together, noise pick up is common and easily rejected by the LME49724.
CMRR performance is directly proportional to the tolerance and matching of the gain configuring resistors. With
0.1% tolerance resistors the worst case CMRR performance will be about 60dB (20LOG(0.001)).
A differential output has a higher dynamic range than a single-ended output because of the doubling of output
voltage. The dynamic range is increased by 6dB as a result of the outputs being equal in magnitude but opposite
in phase. As an example, a single-ended output with a 1V_PP signal will be two 1V_PP signals with a differential
output. The increase is 20LOG(2) = 6dB. Differential amplifiers are ideal for low voltage applications because of
the increase in signal amplitude relative to a single-ended amplifier and the resulting improvement in SNR.
Differential amplifiers can also have reduced even order harmonics, all conditions equal, when compared to a
single-ended amplifier. The differential output causes even harmonics to cancel between the two inverted outputs
leaving only the odd harmonics. In practice even harmonics do not cancel completely, however there still is a
reduction in total harmonic distortion.
OUTPUT COMMON-MODE VOLTAGE (V_OCM pin)
The output common-mode voltage is the DC voltage on each output. The output common-mode voltage is set by
the V_OCM pin. The V_OCM pin can be driven by a low impedance source. If no voltage is applied to the V_OCM pin,
the DC common-mode output voltage will be set by the internal resistor divider to the midpoint of the voltages on
the V_CC and V_EE pins. The input impedance of the V_OCM pin is 50kΩ. The V_OCM pin can be driven up to V_CC -
1.5V and V_EE + 1.5V. The V_OCM pin should be bypassed to ground with a 0.1μF to 1μF capacitor. The V_OCM pin
should be connected to ground when the desired output common-mode voltage is ground reference. The value
of the external capacitor has an effect on the PSRR performance of the LME49724. With the V_OCM pin only
bypassed with a low value capacitor, the PSRR performance of the LME49724 will be reduced, especially at low
audio frequencies. For best PSRR performance, the V_OCM pin should be connected to stable, clean reference.
Increasing the value of the bypass capacitor on the V_OCM pin will also improve PSRR performance.
ENABLE FUNCTION
The LME49724 can be placed into standby mode to reduce system current consumption by driving the ENABLE
pin below V_EE + 1.75V. The LME49724 is active when the voltage on the ENABLE pin is above V_EE + 2.35V. The
ENABLE pin should not be left floating. For best performance under all conditions, drive the ENABLE pin to the
V_EE pin voltage to enter standby mode and to ground for active operation when operating from split supplies.
When operating from a single supply, drive the ENABLE pin to ground for standby mode and to V_CC for active
mode.
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