IN+

IN-

CIN

CIN

RINEXT

RINEXT

RIN

RIN

RF

RF

LM48310

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SNAS430D – NOVEMBER 2007 – REVISED MAY 2013

AUDIO AMPLIFIER INPUT CAPACITOR SELECTION

Input capacitors may be required for some applications, or when the audio source is single-ended. Input

capacitors block the DC component of the audio signal, eliminating any conflict between the DC component of

the audio source and the bias voltage of the LM48310. The input capacitors create a high-pass filter with the

input resistors RIN. The -3dB point of the high pass filter is found using Equation 1 below.

f = 1 / 2

πRINCIN

Where

•

RIN is the value of the input resistor given in the Electrical Characteristics table

(1)

The input capacitors can also be used to remove low frequency content from the audio signal. Small speakers

cannot reproduce, and may even be damaged by low frequencies. High pass filtering the audio signal helps

protect the speakers. When the LM48310 is using a single-ended source, power supply noise on the ground is

seen as an input signal. Setting the high-pass filter point above the power supply noise frequencies, 217Hz in a

GSM phone, for example, filters out the noise such that it is not amplified and heard on the output. Capacitors

with a tolerance of 10% or better are recommended for impedance matching and improved CMRR and PSRR.

AUDIO AMPLIFIER GAIN

The gain of the LM48310 is internally set to 12dB. The gain can be reduced by adding additional input resistance

Figure 31. In this configuration, the gain of the device is given by:

AV = 2 x [RF / (RINEXT + RIN)]

Where

•

RF is 40kΩ

•

RIN is 20kΩ

•

RINEXT is the value of the additional external resistor

(2)

Figure 31. Reduced Gain Configuration

SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION

The LM48310 is compatible with single-ended sources. When configured for single-ended inputs, input

capacitors must be used to block and DC component at the input of the device. Figure 32 shows the typical

single-ended applications circuit.

Copyright © 2007–2013, Texas Instruments Incorporated

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