Dual SPM Detector
Page 12 of 16
MX641 Preliminary Information
5. Applications
5.1.1 Input Configurations
Differential Input
Common Mode Input
INPUT AMP
-
INPUT AMP
-
Tip (a)
Ring (b)
+
+
MX641
MX641
VBIAS
VBIAS
Figure 8: Input Configurations
5.1.2 Protection Against High Voltages
Telephone systems may have high d.c. and a.c. voltages present on the line. If the MX641 is part of a host
equipment that has its own signal input protection circuitry, there will be no need for further protection as long
as the voltage on any pin is limited to within V +0.3V and V -0.3V.
DD
SS
If the host system does not have input protection, or there are signals present outside the device's specified
limits, the MX641 will require protection diodes at its signal inputs (+ and -). The breakdown voltage of
capacitors and the peak inverse voltage of the diodes must be sufficient to withstand the sum of the d.c.
voltages plus all expected signal peaks.
5.1.3 Aliasing
Due to the sampling nature of switched-capacitor filters used in the MX641, high frequency noise or unwanted
signals can alias into the passband, disrupting detection. External components must be chosen carefully to
avoid alias effects.
Possible Alias Frequencies:
12kHz Mode = 52kHz
16kHz Mode = 69kHz
If other filtering in the system has not attenuated these alias frequencies, capacitors should be employed
across resistors R3, R4, R7 and R8 to provide anti-alias filtering.
The low-pass cutoff frequency should be chosen to be approximately 20kHz to 25kHz for a 12kHz system, or
25kHz to 30kHz for a 16kHz system.
1
i.e. C =
2 ×π × fΟ × R3
When anti-alias capacitors are used, an allowance must be made for reduced gain at the SPM frequency
(12kHz or 16kHz).
ã2001 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 20480115.004
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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