MAX4142 データシートの表示（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

IN+

MAX4142

A1

OUT+

RF

RG

A3

RG

RF

A2

IN-

SENSE+

GND

SENSE-

OUT-

5

4

3

2

CL = 15pF

1

0

-1

CL = 5pF

-2

CL = 10pF

RL

-3

-4

-5

100k

1M

10M 100M

1G

FREQUENCY (Hz)

Figure 5. MAX4142 Small-Signal Response with Capacitive

Load

Figure 4. Connection of SENSE+ and SENSE- to a Remote

Load

Using SENSE+ and SENSE-

The MAX4142 has two output voltage-sense pins,

SENSE+ and SENSE-. These pins are normally con-

nected to the MAX4142’S OUT+ and OUT- pins. In

some long-line applications, it may be desirable to con-

nect SENSE+ to OUT+ and SENSE- to OUT- at the

load, instead of the typical connection at the part

(Figure 4). This compensates for the long line’s resis-

tance, which otherwise leads to an IR voltage error.

When using this technique, keep the sense lines’

impedance low to minimize gain errors. Also, keep

capacitance low to maximize frequency response. The

gain of the MAX4142 is approximated by the following

equation:

( ) ( ) AV

=

1+





RF



+

∆RSENSE+ +

RG

∆RSENSE −







where ∆RSENSE+ and ∆RSENSE- are the SENSE+ and

SENSE- trace impedances, respectively. For the

MAX4142, RF is 700Ω and RG is 700Ω.

Additionally, mismatches in the SENSE+ and SENSE-

traces lead to common-mode gain errors. However,

these errors are effectively eliminated by the

MAX4142’s common-mode feedback (see the

Common-Mode Feedback section).

Driving Capacitive Loads

The MAX4142 provides maximum AC performance

when driving no output load capacitance. This is the

case when driving a correctly terminated transmission

line (i.e., a back-terminated cable).

In most amplifier circuits, driving large-load capaci-

tance increases the chance of oscillations. The amplifi-

er’s output impedance and the load capacitor combine

to add a pole and excess phase to the loop response.

If the pole’s frequency is low enough and phase margin

is degraded sufficiently, oscillations may occur. A sec-

ond concern when driving capacitive loads results from

the amplifier’s output impedance, which looks inductive

at high frequencies. The inductance forms an L-C reso-

nant circuit with the capacitive load. This causes peak-

ing in the frequency response and degrades the

amplifier’s phase margin.

The MAX4142 drives capacitive loads up to 25pF with-

out oscillation. However, some peaking may occur in

the frequency domain (Figure 5).

To drive larger-capacitance loads or to reduce ringing,

add isolation resistors between the amplifier’s outputs

and the load (Figure 6).

The value of RISO depends on the capacitive load

(Figure 7). With higher capacitive values, bandwidth is

dominated by the RC network formed by RISO and CL;

the bandwidth of the amplifier itself is much higher.

Also note that the isolation resistor forms a divider that

decreases the voltage delivered to the load.

8 _______________________________________________________________________________________

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