MAX4142 데이터 시트보기 (PDF) - Maxim Integrated

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MAX4142

250MHz, Low-Power, High-Output-Current, Differential Line Driver

Maxim Integrated 

250MHz, Low-Power, High-Output-Current,

Differential Line Driver

differential signal that is amplified by the receiver. The

unique topology of the MAX4142 (Figure 1) utilizes two

amplifiers (A1 and A2) to provide differential inputs and

outputs, and a third amplifier (A3) to provide common-

mode feedback. The common-mode feedback amplifi-

er senses common-mode voltage at the MAX4142

output and forces this voltage to zero, effectively

removing common-mode voltages from the transmis-

sion line. This technique improves CMR for systems

with imperfectly balanced transmission-line imped-

ances.

Grounding, Bypassing,

and PC Board Layout

Observe the following guidelines when designing your

PC board:

• High-frequency design techniques must be followed

when designing the PC board for the MAX4142.

• The printed circuit board should have at least two

layers: the signal layer and the ground plane.

• Do not use wire-wrap boards; they are too inductive.

• Do not use IC sockets; they increase parasitic

capacitance and inductance.

• Use surface-mount power-supply bypass capacitors

instead of through-hole capacitors. Their shorter lead

lengths reduce parasitic inductance, leading to

superior high-frequency performance.

• Keep signal lines as short and as straight as possi-

ble. Do not make 90° turns; round all corners.

• The ground plane should be as free from voids as

possible.

Input Stage Circuitry

The MAX4142 includes internal protection circuitry that

prevents damage to the precision input stage from

large differential input voltages. This protection circuitry

consists of five back-to-back Schottky protection

diodes between IN+ and RG, and IN- and RG (Fig-

ure 2). The diodes limit the differential voltage applied

to the amplifiers’ internal circuitry to no more than 10VF,

where VF is the diode’s forward voltage drop (about

0.4V at +25°C).

For a large differential input voltage (exceeding 4V), the

MAX4142 input bias current (at IN+ and IN-) increases

according to the following equation:

Input current = [(VIN+ - VIN-) - 10VF] / 1.4kΩ

A differential input voltage as high as 10V will cause

only 2.1mA to flow—much less than the 10mA absolute

maximum rating.

IN+

MAX4142

A1

OUT+

RF

2RG

RF

SENSE+

SENSE-

A2

OUT-

IN-

Figure 2. MAX4142 Input Protection Circuit

IN-

MAX4142

OUT-

700Ω

1.4k

700Ω

OUT+

IN+

Figure 3. MAX4142 Shutdown Equivalent Circuit

Shutdown Mode

The MAX4142 can be put into low-power shutdown

mode by driving SHDN high. The amplifier output is

high impedance in this mode; thus the impedance at

OUT is that of the feedback resistors (2.8kΩ) (Figure 3).

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