Advance Information
Ringing Voltage Detection
The sensitivity of the ringing voltage detection
circuitry can be adjusted by applying an external
resistor (R7, Figure 5) between the RS and LOOP
pins. With a short circuit, the threshold sensitivity is
~10Vrms, therefore R7 = 30kΩ x (Desired threshold
voltage - 10Vrms).
Example: 300kΩ gives ~20Vrms and 600kΩ gives
~30Vrms.
An AC ringing voltage across Tip and Ring will cause
RV to output TTL pulses at the ringing frequency,
with an envelope determined by the ringing cadence.
MH88437-P
2-4 Wire Conversion
The device converts the balanced 2-Wire input,
presented by the line at Tip and Ring, to a ground
referenced signal at VX, biased at 2.0V. This
simplifies the interface to a modem chip set.
Conversely, the device converts the differential signal
input at VR+ and VR- to a balanced 2-Wire signal at
Tip and Ring. The device can also be used in a
single ended mode at the receive input, by leaving
VR+ open circuit and connecting the input signal to
VR- only. Both inputs are biased at 2.0V.
During full duplex transmission, the signal at Tip and
Ring consists of both the signal from the device to
the line and the signal from the line to the device.
The signal input at VR+ and VR- being sent to the
line, must not appear at the output VX. In order to
prevent this, the device has an internal cancellation
circuit, the measure of this attenuation is Transhybrid
Loss (THL).
The MH88437 has the ability to transmit analog
signals from Tip and Ring through to VX when on-
hook. This can be used when receiving caller line
identification information.
Parallel Phone and Dummy Ringer
An external parallel phone or dummy ringer circuit
can be connected across Tip and Ring as shown in
Figure 5. A Dummy Ringer is an AC load which
represents a telephone’s mechanical ringer.
In normal circumstances when a telephone is On-
Hook and connected to the PSTN, its AC (Ringer)
load is permanently presented to the network. This
condition is used by many PTT’s to test line
continuity, by placing a small AC current onto the line
and measuring the voltage across tip (A) and ring
(B).
Today’s telecom equipment may not have an AC load
present across tip and ring (e.g. modems), therefore
any testing carried out by the PTT will see an open
circuit across tip and ring. In this instance the PTT
assumes that the line continuity has been damaged.
To overcome this problem many PTT’s specify that a
"Dummy Ringer" is presented to the network at all
times. Ideally its impedance should be low in the
audio band and high at the ringing frequencies (e.g.
25Hz). Note that the requirement for the "Dummy
Ringer" is country specific.
Parallel phone detection is used mostly in set-top
box applications. This is when a modem call will
need to be disconnected from the central office by
the equipment when the parallel phone is in the off-
hook state. This is to allow the subscriber to make
emergency calls.
To detect this state, additional circuitry will be
required. Refer to Application Note MSAN-154.
Transmit Gain
The Transmit Gain of the MH88437 is the gain from
the differential signal across Tip and Ring to the
ground referenced signal at VX. The internal
Transmit Gain of the device is fixed as shown in the
AC Electrical Characteristics table. For the correct
gain, the Input Impedance of the MH88437, must
match the specified line impedance.
By adding an external potential divider to VX, it is
possible to reduce the overall gain in the application.
The output impedance of VX is approximately 10Ω
and the minimum resistance from VX to ground
should be 2kΩ.
Example: If R3 = R4 = 2kΩ, in Figure 5, the overall
gain would reduce by 6.0dB.
Receive Gain
The Receive Gain of the MH88437 is the gain from
the differential signal at VR+ and VR- to the
differential signal across Tip and Ring. The internal
Receive Gain of the device is fixed as shown in the
AC Electrical Characteristics table. For the correct
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