Preliminary Information
However, by adding external resistors to the VX
output, the Transmit Gain can be reduced as shown in
Figure 5. To limit the output current drawn, the
minimum recommended resistance to ground is 2kΩ.
MH88400
Overall Transmit Gain =
-0.4 dB + 20log (R4 / (R4 + R3))
The output impedance of the VX output is about 5Ω,
therefore, with two external 1kΩ resistors configured
as per Figure 5, this will result in 6dB of attenuation for
a total gain of -6.4dB. For correct gain, the MH88400
input impedance must match the line impedance.
TIP-RING Drive Circuit
The audio input ground (AGND) referenced signal at
VR, biased at 2.5V, is converted to a balanced output
signal at Tip-Ring. 2-4 wire isolation is achieved
through optical isolation. The LC input is activated,
low, and loop current must be flowing.
Receive Gain
Receive Gain (VR to Tip-Ring) is fixed as indicated in
“AC Electrical Characteristics”. However, by adding a
single external resistor in series with the VR input,
the Receive Gain can be reduced.
Overall Receive Gain =
3.5dB + 20log (47kΩ /(47kΩ + Rext),
The input impedance of the VR input is 47kΩ,
therefore, an external 100kΩ resistor in series with
this input will result in 9.9dB of attenuation for a total
gain of -6.4dB.
For correct gain, the MH88400 input impedance
must match the line impedance.
TIP-RING Receive Circuit
The
differential audio signal at Tip-Ring is converted to
a ground (AGND) referenced signal, biased at 2.5V, at
the VX output. 2-4 Wire isolation is achieved through a
combination of optical isolation and transformerler
isolation. The receive circuit operated with or without
loop current and LC can be either high or low. Signal
reception with no loop current can be used for on-hook
reception of, for example, caller identification signals.
Transmit Gain
Transmit Gain (Tip-Ring to VX) is fixed as indicated in
“AC Electrical Characteristics”.
14
13
R1
TIP
11
C2
D1
TIP
RLS
MH88400
VX
6
Audio Output
7
VR
Audio Input
TF
TXIN
10
RVLC
4
Ring Voltage & Loop
Current Detect Output
3
RING
9
RING
VDD
AGND
LC
Loop Control Input
1
+
C1
2
+5V
NOTES:
Calls are set up and cleared in this mode by the
external controller. It turns on the loop via Loop
Control, dialling by pulsing loop control (or via
DTMF tones) and clearing down by turning Loop
Control off.
1) C1: 10µF, 6V Tantalum
2) C2: 0.1µF, 250V (Typical Value)
3) D1: 180VDC Foldback Diode, e.g. TISP4180, TISP5180
4) VX and VR are biased at 2.5V, therefore, coupling capacitors
may be required depending on application.
5) VX and VR are biased at 2.5V, therefore, coupling capacitors
may be required depending on application.
Figure 3 - Typical Application Circuit
2-17
MITEL [ MITEL NETWORKS CORPORATION ]
