Driving Capacitive Loads

The MAX4137/MAX4138 provide maximum AC perfor-

mance with no output load capacitance. This is the

case when they are driving a correctly terminated trans-

mission line (i.e., a back-terminated 75

Ω cable).

However, the MAX4137/MAX4138 are capable of

driving capacitive loads up to 10pF without oscillations,

but with reduced AC performance.

Driving large capacitive loads increases the chance of

oscillations in most amplifier circuits. This is especially

true for circuits with high loop gain, such as voltage fol-

lowers. The amplifier’s output resistance and the load

capacitor combine to add a pole and excess phase to

the loop response. If the frequency of this pole is low

enough and phase margin is degraded sufficiently,

oscillations may occur.

A second problem when driving capacitive loads

results from the amplifier’s output impedance, which

looks inductive at high frequencies. This inductance

forms an L-C resonant circuit with the capacitive load,

which causes peaking in the frequency response and

degrades the amplifier’s gain margin.

The MAX4137/MAX4138 drive capacitive loads up to

10pF without oscillation. However, some peaking (in the

frequency domain) or ringing (in the time domain) may

occur (Figure 3).

1-Input/4-Output

Video Distribution Amplifiers

_______________________________________________________________________________________

9

Figure 2. MAX4138 Typical Operating Circuit

N.C.

1

VCCO

2

OUT1

3

OUT2

5

OUT3

8

AGND

9

OUT4

10

VCCO

11

N.C.

12

OUT1

OUT2

OUT3

OUT4

MAX4138

SW1

SW2

SW3

SW4

AGND

4

AGND

21

IN+

20

VEEI

19

RG

18

RF

17

VCCI

16

SEL3

15

SEL4

14

N.C.

13

0.1

µF

0.1

µF

0.1

µF

0.1

µF

75

Ω*

75

Ω*

75

Ω*

75

Ω*

75

Ω*

VEEO

6

VEEO

7

0.1

µF

N.C.

24

SEL1

23

SEL2

22

VIDEO

IN

IN

* TO TERMINATE TRANSMISSION LINE