Tone Detector
FX105A
1.6.2.7 Calculate RWC3A (C3A = C3B) Use nearest preferred values.
RWC3A » TWORD/ [-3In (1 - VTH/ VINMIN ) ]
where VTH is the word filter
sensitivity, see Section 1.7.1
A signal amplitude of 200mV and a resistor value RW of 465kW with a 220nF capacitor for C3A and
C3B will yield a TWORD time of 41msec. This in turn yields a response time of 9msec + 41msec =
50msec.
1.6.2.8 Calculate the Maximum De-response Time
T
OFF » -3 RWC3A In (VTH/ VIN
)
where VTH is the word filter
sensitivity, see Section 1.7.1
MAX
For improved de-response time, a diode (1N914 or similar) can be placed between pins 5 and 6, as
shown in Figure 2. The formula and figure below show the approximate time the FX105A will take to
turn off after an in-band signal has been removed. The effect of this diode is to greatly reduce the
turn-off time with signal input amplitudes greater than 300mVrms. Figure 4 is for VDD = 5V; for lower
VDD then KDT increases.
T
OFF » KDTRWC3A
So for a maximum signal amplitude of 400mV, a resistor value RW of 465kW with a 220nF capacitor
for C3A and C3B and a diode between pins 5 and 6, a de-response time of » 363msec is obtained.
Figure 4 KDT Factor for TOFF vs. Signal Input Amplitude
ã 1999 Consumer Microcircuits Limited
9
D/105A/5
CMLMICRO [ CML MICROCIRCUITS ]
