TS472 datasheet(13/20 Pages) STMICROELECTRONICS

13 / 20 page

TS472

Application Information

13/20

4.3

Lower cut-off frequency

The lower cut-off frequency FCL of the microphone preamplifier depends on the input

capacitors Cin and output capacitors Cout. These input and output capacitors are mandatory in

a application because of DC voltage blocking.

The input capacitors Cin in serial with the input impedance of the TS472 (100kΩ) are equivalent

to a first order high pass filter. Assuming that FCL is the lowest frequency to be amplified (with a

3dB attenuation), the minimum value of Cin is:

The capacitors Cout in serial with the output resistors Rout (or an input impedance of the next

stage) are also equivalent to a first order high pass filter. Assuming that FCL is the lowest

frequency to be amplified (with a 3dB attenuation), the minimum value of Cout is:

Figure 30 and Figure 32 give directly the lower cut-off frequency (with 3dB attenuation) versus

the value of the input or output capacitors

Note:

In case FCL is kept the same for calculation, It must be taken in account that the 1st order high-

pass filter on the input and the 1st order high-pass filter on the output create a 2nd order high-

pass filter in the audio signal path with an attenuation of 6dB on FCL and a rolloff of

40db

⁄ decade.

4.4

Low-noise microphone bias source

The TS472 provides a very low noise voltage and power supply rejection BIAS source designed

for biasing an electret condenser microphone cartridges. The BIAS output is typically set at

2.0 VDC (no load conditions), and can typically source 2mA with respect to drop-out,

determined by the internal resistance 100

Ω (for detailed load regulation curves see Figure 6).

Figure 31. Lower cut-off frequency vs.

input capacitors

Figure 32. Lower cut-off frequency vs.

output capacitors

π F

100

×10

⋅⋅

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

out

π F

ou t

⋅⋅

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

110

100

1000

Typical Zin

ZinMAX

Cin (nF)

ZinMIN

100

1000

100

1000

Rout=10k

Ω

Cout (nF)

Rout=100k

Ω

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