LM4910 datasheet(12/22 Pages) NSC

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Application Information (Continued)

From Equation 2, the minimum A

V is 0.98; use AV = 1. Since

the desired input impedance is 20k

Ω, and with A

V equal to 1,

a ratio of 1:1 results from Equation 1 for R

f to Ri. The values

are chosen with R

i = 20k

Ω and R

f = 20k

Ω.

The last step in this design example is setting the amplifier’s

−3dB frequency bandwidth. To achieve the desired ±0.25dB

pass band magnitude variation limit, the low frequency re-

sponse must extend to at least one-fifth the lower bandwidth

limit and the high frequency response must extend to at least

five times the upper bandwidth limit. The gain variation for

both response limits is 0.17dB, well within the ±0.25dB

desired limit. The results are an

L = 100Hz/5 = 20Hz

(3)

and an

H = 20kHzx5= 100kHz

(4)

As mentioned in the Selecting Proper External Compo-

nents section, R

i and Ci create a highpass filter that sets the

amplifier’s lower bandpass frequency limit. Find the coupling

capacitor’s value using Equation (3).

≥ 1/(2πR

ifL)

(5)

The result is

1/(2

π*20kΩ*20Hz) = 0.397µF

Use a 0.39µF capacitor, the closest standard value.

The high frequency pole is determined by the product of the

desired frequency pole, f

H, and the differential gain, AV. With

an A

V = 1 and fH = 100kHz, the resulting GBWP = 100kHz

which is much smaller than the LM4910 GBWP of 11MHz.

This figure displays that if a designer has a need to design

an amplifier with higher differential gain, the LM4910 can still

be used without running into bandwidth limitations.

MINIMIZING OUTPUT NOISE / REDUCING OUTPUT POWER

Output noise delivered to the load can be minimized with the

use of an external resistor, R

SERIES, placed in series with

each load as shown in Figure 3.R

SERIES forms a voltage

divider with the impedance of the headphone driver R

L.As a

result, output noise is attenuated by the factor R

L /(RL +

SERIES). Figure 4 illustrates the relationship between output

noise and R

SERIES for different loads. RSERIES also de-

creases output power delivered to the load by the factor R

/(R

L +RSERIES)

2. However, this may not pose a problem

since most headphone applications require less than 10mW

of output power. Figure 5 illustrates output power (@1%

THD+N) vs R

SERIES for different loads.

Figure 4 shows an optional resistor connected between the

amplifier output that drives the headphone jack sleeve and

ground. This resistor provides a ground path that supressed

power supply hum. This hum may occur in applications such

as notebook computers in a shutdown condition and con-

nected to an external powered speaker. The resistor’s 100

Ω

value is a suggested starting point. Its final value must be

determined based on the tradeoff between the amount of

noise suppression that may be needed and minimizing the

additional current drawn by the resistor (25mA for a 100

Ω

resistor and a 5V supply).

20030568

FIGURE 3.

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