W91031 Datasheet, PDF (8/29 Pages) Winbond – LOW POWER CMOS INTERGRATED CIRCUIT

Preliminary W91031
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 VDD/2 typically, this pin
must connected a 0.1 µF capacitor to VSS. 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.
C1 _R1
Tip
Ring
C2
R2
Differential Input Amplifier
C1 = C2
R1 = R2
R3 = (R4 R5) / (R4 +R5)
W91031
VREF
0.1 uF
R3
R4
INP
+
INN
-
R5
GCFB
Voltage Gain
Av = R5 / R1
Input Impedance
Zin = 2 R1 ² + (1 / wC) ²
Input ^C
R1
Voltage Gain
AV = R2 / R1
W91031
VREF
0.1 uF
INP
+
INN
-
R2 _GCFB
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 enabled when the FSKE signal is low. It
requires an enable time to enable the dual tone alert signal detector when FSKE goes from high to
low. 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 tREC = tDP + tGP, where tDP is
the tone present detect time and tGP is the tone present guard time. The tone present guard time is
the RC time constant with the capacitor discharging from VSS to VDD (the ALGRC pin discharges
from VSS to VDD through a resistor). The capacitor will discharge rapidly via a discharge switch after
ALGO returns high. The total absent time is tABS = tDA + tGA, where tDA is the tone absent detect time
and tGA is the tone absent guard time. The tone absent guard time is the RC time constant with the
capacitor charging from VDD to VSS (the ALGRC pin charges from VDD to VSS through a resistor). The
capacitor will charge rapidly via a charge switch after ALGO returns low. To obtain unequal present
and absent guard times, a diode can be connected as shown in Figure 7-6, to give the unequal
resistance required during capacitor charging and discharging.
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