M-88L70 CLARE [Clare, Inc.], M-88L70 Datasheet - Page 3

• Manufacturer Part Number: M-88L70
• Description: 3V DTMF Receiver
• Manufacturer: CLARE [Clare, Inc.]

ered a valid pause. This capability, together with the
ability to select the steering time constants externally,
allows the designer to tailor performance to meet a wide
variety of system requirements.
Guard Time Adjustment
Where independent selection of receive and pause are
not required, the simple steering circuit of Figure 3 is
applicable. Component values are chosen according to
the formula:
The value of t
the minimum signal duration to be recognized by the
receiver. A value for C of 0.1 F is recommended for
most applications, leaving R to be selected by the
designer. For example, a suitable value of R for a t
40 ms would be 300 K ohm. A typical circuit using this
steering configuration is shown in Figure 4. The timing
requirements for most telecommunication applications
are satisfied with this circuit. Different steering arrange-
ments may be used to select independently the guard
times for tone-present (t
may be necessary to meet system specifications that
place both accept and reject limits on both tone duration
and interdigit pause.
Guard time adjustment also allows the designer to tailor
system parameters such as talkoff and noise immunity.
Increasing t
reduces the probability that tones simulated by speech
will maintain signal condition long enough to be regis-
tered. On the other hand, a relatively short t
long t
ronments where fast acquisition time and immunity to
dropouts would be required. Design information for
guard time adjustment is shown in Figure 5.
Input Configuration
The input arrangement of the M-88L70 provides a dif-
ferential input operational amplifier as well as a bias
source (V
made for connection of a feedback resistor to the op-
amp output (GS) for gain adjustment.
In a single-ended configuration, the input pins are con-
nected as shown in Figure 4 with the op-amp connect-
ed for unity gain and V
Figure 7 shows the differential configuration, which per-
mits gain adjustment with the feedback resistor R5.
Rev. 1
DO
would be appropriate for extremely noisy envi-
REF
REC
) to bias the inputs at mid-rail. Provision is
DP
is a parameter of the device and t
improves talkoff performance, since it
t
t
REC
GTP
@ 0.67 RC
= t
GTP
REF
DP
) and tone-absent (t
biasing the input at 1/2V
+ t
GTP
REC
GTA
). This
with a
www.clare.com
REC
REC
DD
of
is
.
Figure 3 Basic Steering Circuit
Table 2 Tone Decoding
L = logic low, H = logic high, Z = high impedance, X = don’t care
F
ANY
697
697
697
770
770
770
852
852
852
941
941
941
697
770
852
941
697
770
852
941
LOW
F
1209
1336
1477
1209
1336
1477
1209
1336
1477
1336
1209
1477
1633
1633
1633
1633
1633
1633
1633
1633
ANY
HIGH
(ref.)
Key
ANY
D
D
1
2
3
4
5
6
7
8
9
0
*
#
A
B
C
A
B
C
OE INH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
D
L
X
X
X
X
X
X
X
X
X
X
X
X
X
L
L
L
L
H
H
H
H
ESt
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
L
L
L
L
Undetected, the output
code will remain the
same as the previous
detected code.
Q4
Z
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
Q3
Z
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
M-88L70
Q2
Z
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
Q1
Z
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
3