2-21

Features

FAX and Modem interface (V29/V22 bis)

Variants match German, UK & 600

network

impedances

Provides reinforced barrier to international PTT
requirements

Transformerless 2-4 Wire conversion

Loop start operation

Pulse and DTMF operation

Accommodates external monitor phone

Line state detection outputs:

-loop current/ringing outputs

Single +5V operation, low idle power (5mW)

Full Duplex

Suitable for power down applications

Applications

Interface to Central Office for:

DAA

Modem

FAX

Answering Machine

Terminal Equipment

Description

The Mitel MH88422 Line Interface Circuit provides a
complete audio and signalling link between audio
equipment and central office. The functions provided
by the MH88422 include 2-4 Wire conversion, loop
seizure, ring voltage and loop current detection. The
device is fabricated as thick film hybrid which
incorporates various technologies for optimum circuit
design, high voltage isolation and very high
reliability.

Variants are provided to meet German (-1) and UK
(-3) line impedance (see section on Line
Impedances), as well as a 600

variants (-2).

ISSUE

April 1995

Ordering Information

MH88422-1

14 Pin DIL Package

MH88422-2

14 Pin DIL Package

MH88422-3

14 Pin DIL Package

Surface Mount Options are available by adding the

Suffix "S"

Power

Supply

Opto-

Isolation

Logic Input

Buffer

Opto-

Isolation

Opto-

Isolation

Opto-

Isolation

Audio

Buffer

Audio
Buffer

Ring & Loop

Buffer

Transhybrid

Loss

Cancellation

Circuit

Active

Termination

Audio Input

Buffer

Input

Buffer

Isolation Barrier

VDD

AGND

RVLC

TIP

RING

TXIN

RLS

USER CONNECTIONS

NETWORK CONNECTIONS

Figure 1 - Functional Block Diagram

MH88422

Line Interface Circuit

Preliminary Information

2-22

MH88422

Preliminary Information

Figure 2 - Pin Connections

Pin Description

Pin #

Name

Description

VDD

Positive Power Supply Voltage. +5V.

AGND

Analog Ground. 4-Wire Ground. Normally connected to System Ground.

Loop Control (Input). A logic low activates internal circuitry which provides a dc
termination across Tip and Ring. Used for seizing the line and dial pulsing.

RVLC

Ring Voltage and Current Detect (Output). A logic low indicates that loop current is
detected. The loop current can be due to the external monitor phone or the MH88422 in
the off-hook mode. The RVLC output pulses when the external monitor phone is dial
pulsing or when the MH88422 is dial pulsing via the LC input. In addition, when the
MH88422 is on the on-hook mode, a pulsing output indicates ringing voltage across the
Tip and Ring leads. The pulsing output frequency is twice the ringing frequency.

No Connection should be made to this pin.

Transmit (Output). 4-Wire ground (AGND) referenced audio output, biased at 2.5V.
Outputs in off-hook mode only.

Receive (Input). 4-Wire ground (AGND) referenced audio input, biased at 2.5V. LC
must be activated low and loop current must be flowing.

No Connection should be made to this pin.

RING

Ring Lead. Connects to the "Ring" lead of the central office through an optional relay
contact. The central office "Tip" and "Ring" leads may be interchanged.

TXIN

Transmit (Input). Connects to the "Ring" lead of the central office through a coupling
capacitor.

Tip Feed. Connects to the "Tip" lead of the central office through an internal resistor and
an optional external resistor.

No Connection should be made to this pin.

RLS

Ring Loop Sense (Input). Sense node for ringing voltage detector and the loop current
detector.

TIP

Tip Lead. Connects to the "Tip" lead of the central office through an optional relay
contact. The central office "Tip" and "Ring" leads may be interchanged.

RLS
NC
TF
TXIN

TIP

AGND

RING

RVLC

VDD

3
4

6
7

Preliminary Information

MH88422

2-23

Functional Description

The MH88422 Line Interface Circuit is a COIC
(Central Office Interface Circuit) used to interface
FAX's Modems or user defined equipment to Central
Office 2-Wire Analog Trunks.

Opto-Isolation

The isolation barrier is designed to meet regulatory
requirements for a reinforced barrier of 3kVac. It
provides full isolation of mains voltages up to 250V
RMS and all telecom voltages.

In order that this barrier is not bypassed a creepage/
clearance distance of 6.4mm minimum must be
maintained between wiring, pcb tracking, etc.,
connected to external circuitry on either side of the
barrier. To make this requirement simpler, pins on the
MH88422 connected to opposite sides of the barrier
are on opposite sides of the package.

External Protection Circuit

To meet regulatory high voltage requirement, an
external protection circuit is required. The protection
circuit shown in Figure 3 (Clamp Diode D1) is
recommended.

DC Loop Termination

The DC loop termination circuitry provides the loop
with an active DC load termination when a logic low
is applied to the LC (Loop Control) input. The
termination is similar to a resistance of
approximately 300

(loop current dependent).

Internal optically isolated circuitry is used to switch
the termination in and out the loop. This is used for
both seizing the line as well as generating dial
pulses. The MH88422 will not seize the line when
powered down.

Supervision Features

The supervision circuitry is capable of detecting
ringing voltage and loop current as well as the status
of an optional external monitor phone. The RVLC
(Ring Voltage Loop Current Detect) output provides
a logic low when loop current flows due to the
external monitor phone or the MH88422 being in the
off-hook mode.

The RVLC outputs pulses when the external monitor
phone is dial pulsing or when the MH88422 is dial
pulsing via the LC mode.

In addition, when the MH88422 is on-hook, a pulsing
output indicates that ringing voltage is across the tip
and ring leads; the pulsing output frequency is twice
the ringing frequency. See Figure 3.

Ringing frequency may require external validation.
An RC monostable is usually satisfactory for this
purpose or this may also be achieved using software
applications.

2-4 Wire Conversion

The 2-4 Wire conversion circuit converts the
balanced full duplex signal at Tip and Ring of the
central office line into a transmit ground referenced
signal at VX (Transmit) of the MH88422. It also
converts the receive ground referenced signal at VR
(Receive) of the MH88422 into a balanced transmit
signal at Tip and Ring of the central office line.

In full duplex transmission, the Tip-Ring signal
consists of an audio signal from the central office as
well as an audio signal due to the VR input. There is
an internal cancellation circuit which prevents the V

signal appearing at the VX output. The degree to
which the 2-4 wire conversion circuit minimizes the
contribution of the VR signal at the VX output is
specified as transhybrid loss (THL).

The MH88422 is suitable to drive a COMBO II codec
or a VLSI modem device.

Line Impedance

The MH88422 provides a fixed Tip-ring impedance
which conforms to the following PTT requirements.

MH88422-1 Zin = 200 + 820 // 115nF

Germany

MH88422-2 Zin = 600

MH88422-3 Zin = 370 + 620 // 310nF

2-24

MH88422

Preliminary Information

Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied

Typical figures are at 25°C with nominal +5V supply and are for design aid only

Loop Electrical Characteristics*

* Loop 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.

Absolute Maximum Ratings* -

All voltages are with respect to AGND unless otherwise specified.

Parameter

Symbol

Min

Max

Units

DC Supply Voltage

-0.3

Storage Temperature

-55

+80

°C

DC Loop Voltage

Bat

-100

+100

Ringing Voltage

120

RMS

Loop Current

Loop

Recommended Operating Conditions

Parameters

Sym

Min

Typ

Max

Units

Test Conditions

DC Supply Voltages

4.5

5.0

5.5

Operating Temperature

°C

Ringing Voltage

RMS

Bat

=-48V

Characteristics

Sym

Min

Typ

Max

Units

Test Conditions

Ringing Voltage No Detect
-1 Variant only Detect

RMS

Refer to FT2 1TR2 for test
method

Ringing Voltage No Detect
All other Variants Detect

RMS

Externally
Adjustable

Ringing Frequency

On-Hook 2-wire Impedance

40k

1kHz

Operating Loop Current

Operating Loop Resistance

2800

Bat

=-48V, I

Loop

=15mA

Off-Hook DC -1 Variant
Voltage

-2 Variant

-3 Variant

6.0
6.0
4.0
4.0
6.0
6.0

9.6

6.0
7.8
9.0

12.5

V
V
V
V
V
V

Loop

=25mA

Loop

=42mA

Loop

=15-20mA

Loop

=26mA

Loop

=25mA

Loop

=42mA

Leakage Current (2-Wire to AGND)

100V

Leakage Current on Hook
(Tip to Ring)

Bat

=-50V

DC Resistance during dialling
-1 variant only

200

220

Loop

=20-40mA

Dial Pulse Distortion ON

-1 Variant OFF

All other Variant

+2
+8
+2

+10

ms
ms
ms

Preliminary Information

MH88422

2-25

DC Electrical Characteristics are over Recommended Operating Conditions unless otherwise stated.
Typical figures are at 25

C with nominal+ 5V supplies and are for design aid only.

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 600

unless otherwise stated

DC Electrical Characteristics

Characteristics

Sym

Min

Typ

Max

Units

Test Conditions

RVLC
LC

Supply Current

= -5.0V, Idle state

Power Consumption

= -5.0V, Idle state

Low Level Output Voltage
High Level Output Voltage

2.4

0.4

V
V

= 4mA

= 0.4mA

Low Level Input Voltage
High Level Input Voltage
Low Level Input Current
High Level Input Current

2.0

0.8

60
60

= 5.0V

= 0.0V

AC Electrical Characteristics

- MH88422 All Variants

Characteristics

Sym

Min

Typ

Max

Units

Test Conditions

Input Impedance VR

47k

Output Impedance at VX

Receive Gain (VR to 2-wire)

Frequency Response Gain
(relative to Gain @ 1kHz)

3.5

Input 0.5V at 1kHz

0
0

dB
dB

300 Hz
3400 Hz

Signal Output Overload Level

at 2-Wire

at VX

THD <5% @ 1kHz
I

Loop

=15 to 40mA

+2.0

+3.0

dBm

=4.5V to 5.5V

+2.0

+3.0

dBm

Total Harmonic Distortion

at 2-wire

at VX

THD

1.2
1.2

%
%

Input -10dB at 1kHz
DC loop = 25mA
V

=5.0V

Power Supply Reject Ratio

at 2-wire

at VX

PSRR

30
30

dB
dB

Ripple 0.1V,1kHz on
V

Transhybrid Loss

THL

Input -3.5dBm, 1kHz at
V

2-26

MH88422

Preliminary Information

Electrical Characteristics are over Recommended Operating Conditions unless otherwise stated.
Typical figures are at 25

C and are for design aid only.

AC Electrical Characteristics

- MH88422-2

Electrical Characteristics are over Recommended Operating Conditions unless otherwise stated.
Typical figures are at 25

C and are for design aid only.

AC Electrical Characteristics

- MH88422-1

Characteristics

Sym

Min

Typ

Max

Units

Test Conditions

2-Wire input Impedance
(200

+ 820

//115nF)

Zin

900

@ 1kHz

Return Loss at 2-Wire
(200

+ 820

//115nF)

20
20
20

22
24
26

dB
dB
dB

300-500Hz
500-2500Hz
2500-3400Hz

Longitudinal to Metallic Balance

40
55
53

65
60
60

dB
dB
dB

50-300Hz
300-1000Hz
1000-4000Hz

Idle Channel Noise

at 2-Wire

At VX

-72
-72

-58

dBmp
dBmp

Transmit Gain (2-Wire to VX)

-0.4

Input 0.5V @ 1kHz
Off -Hook

Frequency Response Gain
(relative to gain @ 1kHz)

-0.6
-0.6

dB
dB

300Hz
3400Hz

Characteristics

Sym

Min

Typ

Max

Units

Test Conditions

2-Wire Input Impedance
(600

)

ZIn

600

@ 1kHz

Return Loss at 2-Wire
(Reference 600

)

ERL

SFRL

20
14

20
18

500-2500Hz
200-3200Hz

Longitudinal to Metallic Balance
Metallic to Longitudinal Balance

58
53
60
40

60
55

dB
dB
dB
dB

200-1000Hz
1k-3kHz
200-1000Hz
1k-4kHz

Idle Channel Noise

at 2-Wire

at VX

13
13

dBrnC
dBrnC

Transmit Gain (2-Wire to Vx)

-0.4

Input 0.5V @ 1kHz
Off- Hook

Frequency Response Gain
(relative to gain @ 1kHz)

-0.6
-0.6

dB
dB

200Hz
3400Hz

2-27

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.

unless otherwise stated

Characteristics

Sym

Min

Typ

Max

Units

Test Conditions

2-Wire Input Impedance
(370

+ 620

// 310nF)

Zin

700

@ 1kHz

Return Loss at 2-Wire
(370

+ 620

// 310nF)

200-2400Hz

Longitudinal to Metallic Balance

300-3400Hz

Idle Channel Noise

at 2-Wire

at VX

-80
-80

-70
-68

dBmp
dBmp

Transmit Gain (2-Wire to Vx)

-0.4

Input 0.5V @ 1kHz
Off-Hook

Frequency 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 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 2k

Overall Transmit Gain =

-0.4 dB + 20log (R4 / (R4 + R3))

The output impedance of the VX output is about 10

therefore, with two external 1k

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 (47k

/(47k

+ Rext),

The input impedance of the VR input is 47k

therefore, an external 100k

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.

2-28

MH88422

Preliminary Information

Figure 3 - Typical Application Circuit

Figure 4 - Typical Application Circuit with Dummy Ringer and Monitor Phone

TIP

RLS

TXIN

RING

RVLC

AGND

VDD

TIP

RING

+5V

Audio Output

Audio Input

Ring Voltage & Loop

Current Detect Output

Loop Control Input

MH88422

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: 10

F, 6V Tantalum

2) C2: 0.1

F, 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.

TIP

RLS

TXIN

RING

RVLC

AGND

VDD

TIP

RING

+5V

Audio Output

Audio Input

Ring Voltage & Loop

Current Detect Output

Loop Control Input

MH88422

NOTES

1) C1: 10

F, 6V Tantalum

2) C2: 0.47

F, 100V

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.

R2 & C2 form a dummy ringer, where R2 is on the hybrid and C2 must be fitted by the customer.
To initate a manual call, lift the handset. The RVLC pin will indicate that the line has been seized by the monitor phone.
The required number is then dialled and the connections established. During this phase the Loop Control

input should be turned ON to ensure that when the relay K1 is operated and the monitor phone removed from the line loop
current is still maintained. On receipt of the correct progress tones the external controller should control the data transmission
and clear down the call when it is completed by turning the Loop Control OFF. For applications in some countries such as
Germany the MH88422 can not be used with an optional telephone but may still be used in a combined telephone/fax machine.

K1b

K1a

Line

Relay

Input

Internal
dummy
Ringer
Resistor

K1b

5) R6 adjust the monitor phone detect sensitivity typically set to 200 ohms for 5mA limit on the

-1 variant and 75 ohms on the -3 variant.

6) K1b is added to remove the dummy ringer when using a combined phone on the-1 variant

and is not required for all other variants.

Preliminary Information

MH88422

2-29

Figure 5 - Typical Application Circuit for Gain Adjustment

Figure 6 - Typical Application Circuit with Meter Pulse Filter and Coupling Capacitor

TIP

RLS

TXIN

RING

RVLC

AGND

VDD

TIP

RING

+5V

Audio Output

Audio Input

Ring Voltage & Loop

Current Detect Output

Loop Control Input

MH88422

1) C1: 10

F, 6V Tantalum

2) C2: 0.1

F, 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.

RECEIVE GAIN:
Audio input to Tip-Ring
=20log(47k/(R5+47K))+3.5dB

= -6.5dB

TRANSMIT GAIN:
Tip-Ring to Audio Output
=20log(R4(R3+R4))-0.4dB

= -6.4dB

Note:

The VX output gain has been reduced by 6dB
(to -6.4dB) and the VR input gain has been

reduced by 9.9dB (to -6.4dB) in this example.

5) (R3+R4) should be > 2k

to prevent overload.

100k

TIP

RLS

TXIN

RING

RVLC

AGND

VDD

TIP

RING

+5V

MH88422

1) C1: 10

F, 6V Tantalum

2) C2: 0.1

F, 250V (Typical Value)

3) D1:180VDC Foldback Diode,

Note:

Meter pulses at high frequency (16kHz or 12kHz) and high level (10V RMS) are used in some countries.

The VX amplifier has a low pass filter which attenuates by typically 15dB at 16kHz (For the -1 variant).

If this proves insufficient for specific applications, an LC reject filter is recommended.

e.g. TISP4180, TISP5180.

Meter

Pulse

Detector

Meter
Pulse
Filter

Loop Control Input

Current Detect Output

Ring Voltage & Loop

Audio Input

Audio Output

2-30

MH88422

Preliminary Information

Figure 7 -Typical Application Circuit showing UK Dummy Ringer Circuit.

Figure 8 - Mechanical Data for 14 Pin DIL Hybrid

TIP

RLS

TXIN

RING

RVLC

AGND

VDD

TIP

RING

+5V

MH88422

1) C1: 10

F, 6V Tantalum

2) C2:1.8

F, 250V (Normally in master socket)

3) D1:180VDC Foldback Diode, e.g. TISP4180, TISP5180

Notes:

Loop Control Input

Current Detect Output

Ring Voltage & Loop

Audio Input

Audio Output

1.42 Max

(36.1 Max)

0.19 Max (4.8 Max)

0.10 Typ

(2.54 Typ)

0.020 + 0.005

(0.5 + 0.12)

0.063 Max(1.6 Min)

0.26+0.015 (6.6+0.4)

AAAA

AAA

AAAA

AAA

(22.9 Typ)

0.95 Max

(24.2 Max)

0.90 Typ

0.27 Max

(6.9 Max)

0.08 Typ (2 Typ)

0.200+0.010

(5.08+0.25)

Notes:
1) Not to scale
2) Dimensions in inches).
3) (Dimensions in millimetres).

*Dimensions to centre of pin.

Preliminary Information

MH88422

2-31

Figure 9 - Mechanical Data for 14 Pin Surface Mount Hybrid

Figure 10 - Recommended Footprint for 14 Pin Surface Mount Hybrid

AAAA

AAA

1.08 Max

(27.4 Max)

0.20 + 0.005

(5.0 + 0.12)

1.42 Max

(36.1 Max)

0.19 Max

(4.8 Max)

0.063 Max

(1.6 Max)

0.33 Max
(8.4 Max)

AAAA

AAA

0.125+0.015±

(3.18+0.38)

0.060 Typ

(1.52)

0.10 Typ

(2.54 Typ)

0.20 + 0.010
(5.08 + 0.25)

0.08 Typ (2 Typ)

AAAA

Notes:
1) Not to scale
2) Dimensions in inches).
3) (Dimensions in millimetres).

*Dimensions to centre of pin.

0.1

(2.5)

0.89

(22.6)

0.04

(1.02)

0.16

(4.06)

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