MC34216A Datasheet, PDF (11/20 Pages) Motorola, Inc – PROGRAMMABLE TELEPHONE LINE INTERFACE CIRCUIT WITH LOUDSPEAKER AMPLIFIER

MC34216A
AC CHARACTERISTICS
Impedance
In Figure 5, the block diagram of the MC34216A
performing the ac impedance is depicted. As can be seen, it
is partly in common with the dc mask block diagram. The part
generating the dc mask is replaced by a dc voltage source
because for ac, this part has no effect.
Line +
Figure 5. AC Stage of the MC34216A
T2
Transmit
When on Pin LAI a current is injected, via the loop
depicted in Figure 5, a signal is created on the line. In this
way, the microphone signals and DTMF signals from the
internal generator are transmitted. It can be derived that the
signal voltage on the line (Vline) depends on the signal
current injected in LAI (ILAI) according to:
+ ₎ Vline
–ILAI
x
Z0
Zin
x
Zline
Zline
Z0
LAI
Z21
SAO
+ _VO1
MC34216A
+
LAO
+
Gnd
VO2
SAI
R1
R5
VCC
T3
+
C7
With this relation, a simplified replacement circuit can be
made for the transmit amplifier (see Figure 7). Here the
product of ILAI and Z0 is replaced by one voltage source.
Figure 7. Replacement Diagram for the
Transmit Amplifier
Zin
+
+
–ILAI*Z0
Vline _Zline
Line –
When calculating the ac loop, it can be derived that the set
+ + ǒ ) Ǔ [ ^{impedance Zin equals:}
Zin
Vline
Iline
R1 1
Z0
Z21
R1
x
Z0
Z21
As can be noticed, the formula for the ac impedance Zin
equals the formula for the dc slope in Regions 1 and 3.
However, because for the dc slopes the resistive part of Z0
and Z21 are used, the actual values for Zin and the dc slopes
do not have to be equal.
A complex impedance can be made by making either Z0
or Z21 complex. When Z0 is made complex to fit the set
impedance, the transmit characteristics will be complex as
well. The complex impedance is therefore preferably made
via the Z21 network. Because Z21 is in the denominator of
the Zin formula, Z21 will not be a direct copy of the required
impedance, but a derivative of it. Figure 6 shows the derived
network to be used for Z21.
Figure 6. Derived Network for Z21 in Case
of Complex Set Impedance
Rv
SAO
Ra
Rv
LAI
Rw
Rb
Cb
Cw
–
The microphone signal current is derived from the
microphone signal according to the schematic in Figure 8.
Figure 8. Microphone Amplifier Input Stage
VCC
Cmic _{R Mic}
Ru ^Cu
Iu
TXI
MIC
Handset
Microphone
MC34216A
Line AGC
RTXI _Ku
ILAI
Microphone Threshold
The input stage in Figure 8 consists of a current amplifier
with transfer Ku and impedance RTXI = 1.0 kΩ plus an
attenuator which reduces the signal current at high line
currents, the so–called line length regulated gain or line AGC.
This attenuator can be switched on/off via the microcontroller.
The input current Iu within the telephony speech band is
derived from the microphone signal according to:
+ _{) )} [ Iu
RMic
Vu
Ru
RTXI
Vu
Ru
+ ₎ Rv
R1 x Z0
2(Ra Rb – R1)
Gnd
+ ₎ Rw
R1 x Z0 (Ra – R1)
4Rb (Ra Rb – R1)
+ Cw
4Rb² x Cb
R1 x Z0
With: Vu = signal of the microphone only loaded with RMic.
The overall gain from microphone to line (ATX) now follows
as:
+ + ₎ ATX
Vline
Vu
Ku
Ru
x
Z0
Zin
x
Zline
Zline
Practically, the gain can only be varied with Z0, Ru and
RMic.
MOTOROLA ANALOG IC DEVICE DATA
11