MT8870D/MT8870D-1
Pin Description
Pin #
18
17
20
19
Name
St/GT
Description
Data Sheet
Steering Input/Guard time (Output) Bidirectional.
A voltage greater than V
TSt
detected at
St causes the device to register the detected tone pair and update the output latch. A
voltage less than V
TSt
frees the device to accept a new tone pair. The GT output acts to
reset the external steering time-constant; its state is a function of ESt and the voltage on St.
Positive power supply (Input).
+5 V typical.
No Connection.
18
20
7,
16
V
DD
NC
Functional Description
The MT8870D/MT8870D-1 monolithic DTMF receiver offers small size, low power consumption and high
performance. Its architecture consists of a bandsplit filter section, which separates the high and low group tones,
followed by a digital counting section which verifies the frequency and duration of the received tones before passing
the corresponding code to the output bus.
Filter Section
Separation of the low-group and high group tones is achieved by applying the DTMF signal to the inputs of two
sixth-order switched capacitor bandpass filters, the bandwidths of which correspond to the low and high group
frequencies. The filter section also incorporates notches at 350 and 440 Hz for exceptional dial tone rejection (see
Figure 3). Each filter output is followed by a single order switched capacitor filter section which smooths the signals
prior to limiting. Limiting is performed by high-gain comparators which are provided with hysteresis to prevent
detection of unwanted low-level signals. The outputs of the comparators provide full rail logic swings at the
frequencies of the incoming DTMF signals.
0
PRECISE
DIAL TONES
X=350 Hz
Y=440 Hz
DTMF TONES
A=697 Hz
B=770 Hz
C=852 Hz
D=941 Hz
E=1209 Hz
F=1336 Hz
G=1477 Hz
H=1633 Hz
10
20
ATTENUATION
(dB)
30
40
50
1kHz
A B C D
E
FREQUENCY (Hz)
X
Y
F
G
H
Figure 3 - Filter Response
3
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
