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MH88400-1

Line Interface Circuit

Mitel Networks 

Preliminary Information

MH88400

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.

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.

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”.

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.

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

D1

RING

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.

14

13

R1

11

TIP

RLS

TF

MH88400

6

VX

7

VR

C2

TXIN

10

9 RING

VDD

4

RVLC

3

LC

AGND

1

2

+

Audio Output

Audio Input

Ring Voltage & Loop

Current Detect Output

Loop Control Input

C1

+5V

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

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