APA2121 datasheet(21/30 Pages) ANPEC

21 / 30 page

 ANPEC Electronics Corp.

Rev. A.1 - Mar., 2003

APA2120/2121

www.anpec.com.tw

The gain for each audio input of the APA2120/1 is

set by the internal resistors (Ri and Rf) of volume

control amplifier in inverting configuration.

Ri vs Gain(BTL)

100

120

-40

-30

-20

-10

Gain(dB)

Ri(k

Ω)

Application Descriptions (Cont.)

Volume Control Function (Cont.)

For highest accuracy, the voltage shown in the ‘rec-

ommended voltage’ column of the table is used to

select a desired gain. This recommended voltage is

exactly halfway between the two nearest transitions.

The gain levels are 2dB/step from 20dB to -40dB in

BTL mode, and the last step at -80dB as mute mode.

Input Resistance, Ri

BTL mode operation brings the factor of 2 in the gain

equation due to the inverting amplifier mirroring the

voltage swing across the load. For the varying gain

setting, APA2120/1 generates each input resistance

on figure 4. The input resistance will affect the low

frequency performance of audio signal. The minmum

input resistance is 10k

Ω when gain setting is 20dB

and the resistance will ramp up when close loop gain

below 20dB. The input resistance has wide variation

(+/-10%) caused by process variation.

Figure 4: Input resistance vs Gain setting

(3)

BTL Gain = -2 x

In the typical application an input capacitor, Ci, is re-

quired to allow the amplifier to bias the input signal to

the proper DC level for optimum operation. In this

case, Ci and the minimum input impedance Ri (10k

Ω)

form a high-pass filter with the corner frequency de-

termined in the follow equation :

Input Capacitor, Ci

The value of Ci is important to consider as it directly

affects the low frequency performance of the circuit.

Consider the example where Ri is 10k

Ω and the speci-

fication calls for a flat bass response down to 100Hz.

Equation is reconfigured as follow :

Consider to input resistance variation, the Ci is 0.16

µF

so one would likely choose a value in the range

of 0.22

µF to 1.0µF.

FC(highpass)=

πx10kΩxCi

(4)

Ci=

πx10kΩxfC

(5)

SE Gain =

(2)

AV = -

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