TS4956 datasheet(41/51 Pages) STMICROELECTRONICS

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TS4956

Application information

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4.4

Low frequency response

4.4.1

Input capacitor Cin

The input coupling capacitor blocks the DC part of the input signal at the amplifier input. In

the low-frequency region, Cin starts to have an effect. Cin with Zin forms a first-order, high-

pass filter with -3 dB cut-off frequency.

Zin is the input impedance of the corresponding input.

Note:

For all inputs, the impedance value remains constant for all gain settings. This means that

the lower cut-off frequency doesn’t change with the gain setting. Note also that 30 k

Ω is a

typical value and there is tolerance around this value. Using Figure 129 you can easily

establish the Cin value required for a -3dB cut-off frequency.

Figure 129. 3dB lower cut off frequency vs. input capacitance

4.4.2

Output capacitor Cout

In the single-ended configuration an external output coupling capacitor, Cout, is needed.

This coupling capacitor Cout, together with the output load RL, forms a first-order high-pass

filter with -3 dB cut off frequency.

See

Figure 130 to establish the Cout value for a -3dB cut-off frequency required.

These two first-order filters form a second-order high-pass filter. The -3 dB cut-off frequency

of these two filters should be the same, so the following formula should be respected:

πZ

inCin

------------------------ (Hz)

100

Typical Input

Impedance

Maximum Input

Impedance

Minimum Input

Impedance

All gain setting

Tamb=25

°C

Input Capacitor Cin (

µF)

0.1

πR

LCout

-------------------------- Hz

()

πZ

inCin

------------------------

πR

L Cout

--------------------------

≅

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