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Data Sheet AS2540
austriamicrosystems
Revision 3.1
Page 5 of 18
The DC characteristic is determined by the voltage at LI
pin and a 30
resistor between LI and LS pin. It can be
calculated by the following equation: VLS = VLI + ILine *
30
. The t.m. voltage at the LI pin is 4.5V. The calculation
leads to the following DC resistances: 330
to 75 at line
currents from 15mA to 100mA.
2/4 wire conversion
AS2540 has a built-in dual Wheatstone bridge with one
common ground. This provides a maximum of
independence of AC impedance and side tone from each
other. One can adapt side tone without changing the AC
impedance.
A1=3.4/3.5
A2=-0.4
AC impedance
The ac impedance of AS2540 is set to t.m. 1000
. With
the external capacitor at CI pin it can be programmed
complex. With a parallel impedance at the LS pin it can be
programmed to 600
.
Side Tone
A good sidetone cancellation can be achieved by using the
following equation:
ZBAL/ZLINE = 10
Transmit path
The gain of the M1/M2 —> LS transmit path is set to 38dB
for ac-impedance 1000
. The input is differential with an
impedance of 25k
. The soft clip circuit limits the output
voltage at LS to 2Vp. The attack time is 30s/6dB and the
decay time is 20ms/6dB.
The gain of the TIT —> LS transmit path is set to 8dB for
ac-impedance 1000
. The input is single ended with an
impedance of 25k
. The soft clip circuit is not active for
this input. The dynamic range of this path is 2.5Vp at LS.
Receive path
The gain of the LS —> ROH receive path is set to 2dB for
ac-impedance 1000
. The receive input is the differential
signal of RI and STB. During DTMF dialling a MF comfort
tone is applied to ROH which is -30dB relative to the line
level. The soft clip circuit limits the output voltage at ROH
to 1Vp. It prevents harsh distortion and acoustic shock.
The gain of the LS —> ROT receive path is set to 0dB for
ac-impedance 1000
. The receive input is the differential
signal of RI and STB. The soft clip circuit is not active for
this output.
Modescan
The DTMF-level on the line and the function of the line loss
compensation can be selected with the connection of the
MODE-pin to the keyboard matrix. The MODE-pin is
scanned once after each OFF-Hook transition.
R1
1
PIN
C1
2
3
R2
4
5
6
7
8
9
*
0
#
PIN
C2
PIN
C3
PIN
R1
PIN
R2
PIN
MODE
c1
C2
C3
C2
C1
R2
R1
LLC
Dtmf Level
45mA-75mA
-6dBm
XX
45mA-75mA
-8dBm
X
45mA-75mA
-10dBm
X
20mA-50mA
-6dBm
X
20mA-50mA
-8dBm
XX
20mA-50mA
-10dBm
XX
No LLC
-6dBm
XX
No LLC
-8dBm
XX
X
No LLC
-10dBm
X indicates connection to pin mode
ams
AG
Technical
content
still
valid