MT8870D/MT8870D-1
Applications
ISO
2
-CMOS
RECEIVER SYSTEM FOR BRITISH TELECOM
SPEC POR 1151
The circuit shown in Fig. 9 illustrates the use of
MT8870D-1 device in a typical receiver system. BT
Spec defines the input signals less than -34 dBm as
the non-operate level.
This condition can be
attained by choosing a suitable values of R
1
and R
2
to provide 3 dB attenuation, such that -34 dBm input
signal will correspond to -37 dBm at the gain setting
pin GS of MT8870D-1. As shown in the diagram, the
component values of R
3
and C
2
are the guard time
requirements when the total component tolerance is
6%. For better performance, it is recommended to
use the non-symmetric guard time circuit in Fig. 8.
t
GTP
=(R
P
C
1
)In[V
DD
/(V
DD
-V
TSt
)]
t
GTA
=(R
1
C
1
)In(V
DD
/V
TSt
)
R
P
=(R
1
R
2
)/(R
1
+R
2
)
C
1
St/GT
V
DD
R
1
ESt
R
2
Notes:
R
1
=368K
Ω ±
1%
R
2
=2.2M
Ω ±
1%
C
1
=100nF
±
5%
Figure 8 - Non-Symmetric Guard Time Circuit
V
DD
C
1
DTMF
Input
C
2
R
1
MT8870D-1
IN+
IN-
GS
V
Ref
INH
PWDN
OSC 1
OSC 2
V
SS
V
DD
St/GT
ESt
StD
Q4
Q3
Q2
Q1
TOE
R
3
R
2
X
1
NOTES:
R
1
= 102KΩ
±
1%
R
2
= 71.5KΩ
±
1%
R
3
= 390KΩ
±1
%
C
1
,C
2
= 100 nF
±
5%
X
1
= 3.579545 MHz
±
0.1%
V
DD
= 5.0V
±
5%
Figure 9 - Single-Ended Input Configuration for BT or CEPT Spec
4-16
MITEL [ MITEL NETWORKS CORPORATION ]
