LM4671 datasheet(12/18 Pages) NSC

12 / 18 page

Application Information (Continued)

the LM4671 is 0V. Input capacitors create a high-pass filter

with the input resistors, R

i. The –3dB point of the high-pass

filter is found using Equation (3) below.

C =1/(2

πR

i Ci ) (Hz)

(3)

The input capacitors may also be used to remove low audio

frequencies. Small speakers cannot reproduce low bass

frequencies so filtering may be desired . When the LM4671

is using a single-ended source, power supply noise on the

ground is seen as an input signal by the +IN input pin that is

capacitor coupled to ground (See Figures 5 – 7). Setting the

high-pass filter point above the power supply noise frequen-

cies, 217Hz in a GSM phone, for example, will filter out this

noise so it is not amplified and heard on the output. Capaci-

tors with a tolerance of 10% or better are recommended for

impedance matching.

DIFFERENTIAL CIRCUIT CONFIGURATIONS

The LM4671 can be used in many different circuit configu-

rations. The simplest and best performing is the DC coupled,

differential input configuration shown in Figure 2. Equation

(2) above is used to determine the value of the R

i resistors

for a desired gain.

Input capacitors can be used in a differential configuration as

shown in Figure 3. Equation (3) above is used to determine

the value of the C

i capacitors for a desired frequency re-

sponse due to the high-pass filter created by C

i and Ri.

Equation (2) above is used to determine the value of the R

resistors for a desired gain

The LM4671 can be used to amplify more than one audio

source. Figure 4 shows a dual differential input configuration.

The gain for each input can be independently set for maxi-

mum design flexibility using the R

i resistors for each input

and Equation (2). Input capacitors can be used with one or

more sources as well to have different frequency responses

depending on the source or if a DC voltage needs to be

blocked from a source.

SINGLE-ENDED CIRCUIT CONFIGURATIONS

The LM4671 can also be used with single-ended sources but

input capacitors will be needed to block any DC at the input

terminals. Figure 5 shows the typical single-ended applica-

tion configuration. The equations for Gain, Equation (2), and

frequency response, Equation (3), hold for the single-ended

configuration as shown in Figure 5.

When using more than one single-ended source as shown in

Figure 6, the impedance seen from each input terminal

should be equal. To find the correct values for C

i3 and Ri3

connected to the +IN input pin the equivalent impedance of

all the single-ended sources are calculated. The single-

ended sources are in parallel to each other. The equivalent

capacitor and resistor, C

i3 and Ri3, are found by calculating

the parallel combination of all C

ivalues and then all Ri val-

ues. Equations (4) and (5) below are for any number of

single-ended sources.

i3 =Ci1 +Ci2 +Cin ... (µF)

(4)

i3 = 1 / (1/Ri1 + 1/Ri2 + 1/Rin ...) (

Ω)

(5)

The LM4671 may also use a combination of single-ended

and differential sources. A typical application with one single-

ended source and one differential source is shown in Figure

7. Using the principle of superposition, the external compo-

nent values can be determined with the above equations

corresponding to the configuration.

201073I7

FIGURE 2. Differential input configuration

LM4671 Datasheet(PDF) 12 Page - National Semiconductor (TI)

LM4671 Datasheet(HTML) 12 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL