Preliminary Information
AC Electrical Characteristics
†
- MH88422-3
Characteristics
1
2
3
4
2-Wire Input Impedance
(370Ω + 620Ω // 310nF)
Return Loss at 2-Wire
(370Ω + 620Ω // 310nF)
Longitudinal to Metallic Balance
Idle Channel Noise
at 2-Wire
at VX
5
6
Transmit Gain (2-Wire to Vx)
Frequency Gain
(relative to gain @ 1kHz)
Nc
-80
-80
-0.4
-0.6
-0.6
-70
-68
dBmp
dBmp
dB
dB
dB
Sym
Zin
RL
18
50
Min
Typ
‡
700
20
60
Max
Units
Ω
dB
dB
MH89770
Test Conditions
@ 1kHz
200-2400Hz
300-3400Hz
Input 0.5V @ 1kHz
Off-Hook
300Hz
3400Hz
† AC Electrical Characteristics are over Recommended Operating Conditions unless otherwise stated.
‡ Typical figures are at 25°C and are for design aid only.
Note 1: All of the above characteristics use a test circuit as per Figure 3.
Note 2: All of the above test conditions use a test source impedance which matches the device’s impedance.
Note 3: dBm is referenced to 600Ω unless otherwise stated.
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 the use of optocouplers when the LC input is
activated, low, and loop current flowing.
The output impedance of the VX output is about 10Ω,
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 MH88422
input impedance must match the line impedance.
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 10dB of attenuation for a total
gain of -6.5dB.
For correct gain, the MH88422 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
the use of optocouplers. The receive circuit operated
when LC is low and loop current is flowing
Transmit Gain
Transmit Gain (Tip-Ring to VX) is fixed as indicated in
“AC Electrical Characteristics”.
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.
Overall Transmit Gain =
-0.4 dB + 20log (R4 / (R4 + R3))
2-27
MITEL [ MITEL NETWORKS CORPORATION ]
