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MAX498

Quad/Triple, SPDT, RGB Switches with 250MHz Video Buffer Amplifiers

Maxim Integrated 

Quad/Triple, SPDT, RGB Switches

with 250MHz Video Buffer Amplifiers

MAX186/MAX188

FULL POWER-DOWN

14

RISO = 0Ω

12

100pF

10

47pF

8

6

4

0pF

2

0

-2

-4

-6

1M

10M

100M

1G

FREQUENCY (Hz)

MAX186/MAX188

FULL POWER-DOWN

14

RISO = 6.8Ω

12

10

150pF

8

100pF

47pF

6

4

0pF

2

0

-2

-4

-6

1M

10M

100M

1G

FREQUENCY (Hz)

Figure 3a. Small-Signal Gain vs. Frequency and Load

Capacitor (RL = 100Ω, RISO = 0Ω)

Figure 3b. Small-Signal Gain vs. Frequency and Load

Capacitor (RL = 100Ω, RISO = 6.8Ω)

+1

VIN -1

+2

VOUT

-2

+100

VIN

-100

+200

VOUT

-200

TIME (10ns/div)

TIME (10ns/div)

Figure 4a. Large-Signal Pulse Response with CL = 100pF and

RISO = 5.1Ω

Coaxial Cable Drivers

High-speed performance, excellent output current

capability, and an internally fixed gain of +2 make the

MAX498/MAX499 ideal for driving back-terminated 50Ω

or 75Ω coaxial cables to ±2.5V.

In a typical application, the MAX498/MAX499 drive a

back-terminated cable (Figure 1). The back-termination

resistor, at the output, matches the impedance of the

cable’s driven end to the cable’s impedance, eliminating

signal reflections. This resistor, along with the load-

termination resistor, forms a voltage divider with the load

impedance, which attenuates the signal at the cable’s

output by one-half. The MAX498/MAX499 operate with

an internal +2V/V closed-loop gain to provide unity gain

at the cable’s output.

Figure 4b. Small-Signal Pulse Response with CL = 100pF and

RISO = 5.1Ω

Capacitive-Load Driving

In most amplifier circuits, driving large capacitive loads

increases the likelihood of oscillation. This is especially

true for circuits with high loop gains, such as voltage

followers. The amplifier’s output resistance and the

capacitive load form an RC filter that adds a pole to the

loop response. If the pole frequency is low enough (as

when driving a large capacitive load), the circuit-phase

margin is degraded and oscillation may occur.

The MAX498/MAX499 drive capacitive loads up to

100pF without sustained oscillation, although some

peaking may occur (Figures 3a and 3b). When driving

larger capacitive loads, or to reduce peaking, add an

isolation resistor (RISO) between the output and the

capacitive load (Figures 4a, 4b, and 5).

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