
Preliminary W91030B
Publication Release Date: March 2000
- 7 - Revision A1
The diode bridge shown in Figure 7-1 works for both single ended ring signal and balanced ringing.
R1 and R2 are used to set the maximum loading and must be of equal value to achieve balanced
loading at both the tip and ring line. R1, R3 and R4 form a resistor divider to supply a reduced voltage
to the RNGDI input. The attenuation value is determined by the detection of minimal ring voltage and
maximum noise tolerance between tip/ring and ground.
Input Pre-processor
The input signal is processed by an Input Pre-Processor, which is added to the offset voltage to adjust
the input amplitude and to filter out unwanted frequencies. The gain control op-amp is used to bias
the input voltage with the VREF signal voltage. The voltage of VREF pin is V
DD
/2 typically, this pin
had
better connected a 0.1uF capacitor to V
SS
. It is also used to select the input gain by connecting a
feedback resistor between this pin and the INN pin. Figure 7-2 shows the necessary connections with
the tip/ring line inputs. In a single-ended configuration, the gain control op-amp is connected as
shown in Figure 7-3.
W91030B
C1 = C2
Figure 7-2 Differential Input Gain Control Circuit
Figure 7-3 Single-ended Input Gain Control Circuit
Dual Tone Alert Signal Detection
The dual tone alert signal is separated into high and low tones and detected by a high/low tone
detector. The dual tone alert signal detection circuit is always enabled when the W91030B/BS is not
in sleep state. The ALGR is the output of the dual tone detector and when high indicates that the high
tone and low tone alert signals have been detected. The guard time improves detection performance
by rejecting detected signals with insufficient duration and by masking momentary detection dropout.
Figure 7-4 shows the relationship between the ALGR, ALGRC and ALGO pins and Figure 7-5 shows
the guard time waveform of the same pins. The total recognition time is t
REC
= t
DP
+ t
GP
, where t
DP
is
the tone present detect time and t
GP
is the tone present guard time. The tone present guard time is
the RC time constant with the capacitor discharging from V
SS
to V
DD
( the ALGRC pin discharges
from V
SS
to V
DD
through a resistor). The capacitor will discharge rapidly via a discharge switch after
ALGO returns high. The total absent time is t
ABS
= t
DA
+ t
GA
, where t
DA
is the tone absent detect time
and t
GA
is the tone absent guard time. The tone absent guard time is the RC time constant with the
INP
GCFB
INN
+
-
R1
R2
C1
C2
R4
R3
R5
VREF
Tip
Ring
Differential Input Amplifier
R3 = (R4 R5) / (R4 +R5)
R1 = R2
Voltage Gain
Input Impedance
Av = R5 / R1
Zin = 2 R1
2
+ (1 / wC)
2
0.1 uF
C
R1
R2
Input
+
-
INP
INN
GCFB
VREF
Voltage Gain
V
A
= R2 / R1
W91030B
0.1 uF