Non-Inverting Configuration

Inverting Configuration

IN+

IN-

VCM

IN-

IN+

VCM

LM48557

LM48557

LM48557

SNAS486D – JULY 2009 – REVISED MAY 2013

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Input Capacitor Selection

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 LM48557. The input capacitors create a high-pass filter with the

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

f = 1 / 2

πRINCIN (Hz)

(1)

Where the value of RIN is given in the Electrical Characteristics Table.

High pass filtering the audio signal helps protect the speakers. When the LM48557 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.

COMMON MODE SENSE

The LM48557 features a common mode sense pin (VCM, pin A3) that includes additional common mode

cancelling circuitry that improves the CMRR. When the volume control is set at a high gain step such as 48dB,

any mismatch in the input capacitors would degrade CMRR performance significantly. With the VCM pin

connected to the ground of the input source, it takes the input capacitor mismatches out of the equation and

therefore improves the CMRR. Another advantage with this feature is that only one input capacitor is needed in

the single-ended configuration as opposed to two well matched capacitors. See next section for details of

different configurations of the LM48857.

SINGLE-ENDED INPUT CONFIGURATION

Ground-Referenced Audio Source

The LM48557 input stage is compatible with ground-referenced input sources, such as CODECs with an

integrated headphone amplifier. Connect either input, IN+ or IN- to the CODEC output, and connect the unused

input and VCM to the CODEC output ground (Figure 22). An input coupling capacitor in series with the source

and device input is recommended to block the CODEC output offset voltage, minimizing click and pop and zipper

noise during volume transitions.

Figure 22. Single-Ended Input Configuration with a Ground-Referenced Source

NON-GROUND REFERENCED AUDIO SOURCE

Stereo-to-Mono Conversion

The LM48557 can convert a single-ended stereo signal to a mono BTL signal (Figure 23). Connect the left and

right CODEC outputs in parallel through two equal value resistors to either IN+ or IN-, and connect the unused

input and VCM to the CODEC ground. Select the value of the resistors based on the desired frequency response

created by the combination of the input resistor and the input coupling capacitor.

14

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