Tone Detector
FX105A
1.6.2.7 Calculate R
W
C
3A
(C
3A
= C
3B
)
Use nearest preferred values.
R
W
C
3A
»
T
WORD
/ [-3In (1 - V
TH
/ V
IN
MIN
) ]
where V
TH
is the word filter
sensitivity, see Section 1.7.1
A signal amplitude of 200mV and a resistor value R
W
of 465k
Ω
with a 220nF capacitor for C
3A
and
C
3B
will yield a T
WORD
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
R
W
C
3A
In (V
TH
/ V
IN
MAX
)
where V
TH
is the word filter
sensitivity, see Section 1.7.1
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 300mV
rms
. Figure 4 is for V
DD
= 5V; for lower
V
DD
then K
DT
increases.
T
OFF
»
K
DT
R
W
C
3A
So for a maximum signal amplitude of 400mV, a resistor value R
W
of 465k
Ω
with a 220nF capacitor
for C
3A
and C
3B
and a diode between pins 5 and 6, a de-response time of
≈
363msec is obtained.
Figure 4 K
DT
Factor for T
OFF
vs. Signal Input Amplitude
©
1999 Consumer Microcircuits Limited
9
D/105A/5
CMLMICRO [ CML MICROCIRCUITS ]
