DSB75 Development Support Board Rev. B1 Hardware Description
Confidential / Released
DSB75_hd_v12
Page 40 of 96
2008-08-26
3.6.3
Microphone Circuit (Feeding Bridges)
This section describes the microphone signal path through feeding bridges. The location of
the audio connectors can be seen in Figure 14. Figure 17 shows the principal circuit of
microphone feeding bridges.
The bridge for the audio interface 1 is optimized for feeding the Votronic headset. The
balanced microphone is supplied with filtered 5V.
The bridge for the unbalanced interface 2 is supplied directly from VMIC_I (2.5V). The GSM
module requires a bias voltage of 1V…1.6V between MICP2_I and AGND_I. This is normally
achieved by the voltage drop of the electret microphone in the case of headset operation.
Please note for speakerphone operation, that no voltage drop occurs using the “Siemens
Car Kit Portable” or “Siemens Car Kit portable HKP-500”. In this case an additional
2.2 kOhm resistor has to be connected between MICP2_I and AGND_I. It is recommended
to connect this resistor between X102, pin 34 (AGND) and X102, pin 40 (MICP2).
X502
100nF
47k
47k
10µF
4.7k
4.7k
100nF
600R/200MHz
600R/200MHz
1n
10n
10n
2.2k
2.2k
5V0_I
VMIC_I
AGND_I
MICP1_I
MICN1_I
2.2nF
2.2k
5.6k
X700, X701
(Lumberg)
AGND_I
VMIC_I
MICP2_I
MICN2_I
AGND
MICP
2.2nF
2.2k
600R/200MHz
600R/200MHz
MICP
MICN
EMC-Filter
Figure 17: Circuit of microphone feeding bridges
