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Preliminary Information MH89770
AC Electrical Characteristics† - MH88422-3
† 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 ot herwise st ate d.
Characteristics Sym Min Typ‡Max Units Test Conditions
1 2-Wire In put Impedance
(370Ω + 6 20 Ω // 310nF)
Zin 700 Ω @ 1kHz
2 Retu rn Loss at 2-Wire
(370Ω + 6 20 Ω // 310nF)
RL 18 20 dB 200-2400Hz
3 Longit udi nal to Metallic B alance 50 60 dB 300-3400H z
4 Idle Channel Noise
at 2-Wire
at VX
Nc
-80
-80
-70
-68
dBmp
dBmp
5 Transmit Gain (2-Wire to Vx) -0.4 dB Input 0.5V @ 1kHz
Off-Hook
6 Freque nc y Gain
(relative to gain @ 1kHz)
-0.6
-0.6
dB
dB
300Hz
3400Hz
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.
TIP-RING Receive Circuit
The differential audio signal at Tip-Ring is converted to
a ground (AGND) referenced si gnal, bia sed at 2. 5V, at
the VX output. 2-4 Wire isolat ion is achieved through
the use of o ptocouplers. The receive circuit operated
when LC
is low and loop current is flowing
Transmit Ga in
Transmit Gain (Tip-Ring to VX) is fixed as indicat ed 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 recomm ended resista nce to ground is 2kΩ.
Overall Transmit Gain =
-0.4 dB + 20log (R4 / (R4 + R3))
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 cor rec t gai n, t he M H8 842 2
input impedance must match the line impedance.
Receive G ain
Receive Gain (VR to Tip-Ring) is fixed as indicated in
“AC Electrical Cha ra c te ristics”. Howeve r, 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. 5d B .
For correct gain, the MH88422 input impedance
must match the line impedance.