Driving Capacitive Loads

The MAX4135/MAX4136 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 MAX4135/MAX4136 are capable of dri-

ving 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 MAX4135/MAX4136 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/6-Output

Video Distribution Amplifiers

_______________________________________________________________________________________

9

Figure 2. MAX4136 Typical Operating Circuit

OUT1

1

VCCO

2

OUT2

3

OUT3

5

OUT4

8

AGND

9

OUT5

10

VCCO

11

OUT6

12

OUT1

OUT2

OUT3

OUT4

OUT5

OUT6

MAX4136

SW1

SW2

SW3

SW4

SW5

SW6

AGND

4

AGND

21

IN+

20

VEEI

19

RG

18

RF

17

VCCI

16

SEL4

15

SEL5

14

SEL6

13

0.1

µF

0.1

µF

0.1

µF

75

Ω*

75

Ω*

75

Ω*

75

Ω*

75

Ω*

75

Ω*

75

Ω*

VEEO

6

VEEO

7

0.1

µF

SEL1

24

SEL2

23

SEL3

22

VIDEO

IN

IN

* TO TERMINATE TRANSMISSION LINE

0.1

µF