9-129

Advance Information

MH89726/728

Functional Description

The MH89726/728 are 10 pin SIL thick film hybrid

circuits that may be used to extend the loop range

capability of the MT8972 (DNIC).

For a detailed

description of the DNIC, refer to the MT8972 data

sheet.

Figure 1 shows a block diagram of the Loop

Extender Circuit (LEC).

The LEC takes the line

equalizes, and amplifies it before sending it to the

termination network. The gain of the TX amplifier is

adjustable by using an external resistor between

gain condition exists. This gain results in a transmit

signal power level of +10 dBm.

On the receive side, the LEC receives the differential

line signal from the transformer, and performs first-

order pre-cancellation as well as gain and phase

adjustment of the receive signal. The signal is then

MT8972.

Tables 1 and 2 show typical results obtained by

using the LEC with the MT8972.

Actual distance

achieved may vary depending on the characteristics

of the transmission cable being used.

Most

important factors include cable attenuation (dB/km),

bridged taps, and crosstalk interference.

The LEC is powered typically from ±15 Volts if dual

power supplies are available. Alternately 0 to +30

Volts, or -30 to 0 Volts can be used if only a single

power supply is available.

Applications

Figure 3 shows a typical connection diagram of the

respectively. Pins 5, 6 and 7 are the power supply

pins, with the voltages being supplied as indicated in

a transmit power level of +10 dBm.

and are used to match the characteristic impedance

of the transmission line. This provides the 4 wire to 2

wire hybrid conversion necessary for twisted pair

transmission.

Figure 4 shows a typical application of the LEC. On

short loops, jumpers (or switches) could be used to

remove the LEC from the circuit. For longer loops,

the jumpers could be changed to include the LEC in

the signal path.

Figure 3 - Typical Connection Diagram for MT8972 and MH89726/728

MT8972

(DNIC)

MH89726

or

MH89728

+ 5V

22

1

21

11

D1

D2

10

987654

321

To Power Supplies

0.33

µF

1.0

µF

100k

Ω

D1, D2

IN5817

MH89726

160 kbit/s

MH89728

80 kbit/s

3.9k

Ω, 5%

430

Ω, 2%

15nF, 5%

3k

Ω, 5%

430

Ω, 2%

12nF, 5%

Power

Supply

DUAL

SINGLE

SINGLE

Pin 7

(V +)

+ 15V

GND

+ 30V

Pin 6

GND

GND*

GND*

Pin 5

(V-)

- 15V

- 30V

GND

*Use 0.33

µF capacitor to GND