ISO2-CMOS MT8870D/MT8870D-1
Power-down and Inhibit Mode
A logic high applied to pin 6 (PWDN) will power down
the device to minimize the power consumption in a
standby mode. It stops the oscillator and the
functions of the filters.
MT8870D/
MT8870D-1
C
R
1
IN+
IN-
1
+
-
Inhibit mode is enabled by a logic high input to the
pin 5 (INH). It inhibits the detection of tones
representing characters A, B, C, and D. The output
code will remain the same as the previous detected
code (see Table 1).
C
R
4
2
R
GS
5
R
3
R
2
Differential Input Configuration
V
Ref
The input arrangement of the MT8870D/MT8870D-1
provides a differential-input operational amplifier as
Differential Input Amplifier
C =C =10 nF
1
2
R =R =R =100 kΩ
All resistors are 1% tolerance.
All capacitors are 5% tolerance.
1
2
4
5
well as a bias source (V ) which is used to bias the
Ref
R =60kΩ, R =37.5 kΩ
3
inputs at mid-rail. Provision is made for connection of
a feedback resistor to the op-amp output (GS) for
adjustment of gain. In a single-ended configuration,
the input pins are connected as shown in Figure 10
R R
2
5
R =
3
R +R
2
5
R
R
5
VOLTAGE GAIN (A diff)=
v
1
with the op-amp connected for unity gain and V
Ref
1
INPUT IMPEDANCE
biasing the input at / V . Figure 6 shows the
2
DD
differential configuration, which permits the
2
1
ωc
2
(Z
) = 2
R
+
adjustment of gain with the feedback resistor R .
INDIFF
1
5
Crystal Oscillator
Figure 6 - Differential Input Configuration
The internal clock circuit is completed with the
addition of an external 3.579545 MHz crystal and is
normally connected as shown in Figure 10 (Single-
Ended Input Configuration). However, it is possible to
configure several MT8870D/MT8870D-1 devices
employing only a single oscillator crystal. The
oscillator output of the first device in the chain is
coupled through a 30 pF capacitor to the oscillator
input (OSC1) of the next device. Subsequent devices
are connected in a similar fashion. Refer to Figure 7
for details. The problems associated with unbalanced
loading are not a concern with the arrangement
shown, i.e., precision balancing capacitors are not
required.
To OSC1 of next
C
MT8870D/MT8870D-1
X-tal
OSC2
OSC1
OSC1
OSC2
C
C=30 pF
X-tal=3.579545 MHz
Figure 7 - Oscillator Connection
Parameter
Unit
Resonator
R1
L1
Ohms
mH
pF
10.752
.432
C1
C0
Qm
∆f
4.984
37.915
896.37
0.2%
pF
-
%
Table 2. Recommended Resonator Specifications
Note: Qm=quality factor of RLC model, i.e., 1/2ΠƒR1C1.
4-15
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
