Complete VGA Interface with

EMI Suppression

_______________________________________________________________________________________

9

resistor to the input. This results in an equivalent imped-

ance of 75

Ω at the DAC output, which indicates to the

video controller’s internal load-detection circuit that the

load is disconnected and the video controller shuts down

the video output. Figure 2 and Table 1 demonstrate how

the MAX9511 load-detection circuit operates in conjunc-

tion with the video controller load detection.

Output Video Signal Level Shift

The video signal at the MAX9511 output is shifted

upwards by approximately 240mV from the input to

keep the output stage of the video driver in a linear

region of operation. At the connector, the video signal

is attenuated by 6dB, canceling the 6dB gain of the

video driver. The 240mV level shift is also attenuated by

6dB, and hence the actual video signal seen by the

load is only 120mV higher than the video signal at the

input of the MAX9511. Monitors and other display

devices AC-couple the input signal so the 120mV level

shift should not affect the displayed video image.

75

Ω

REF

CURRENT DAC

MONITOR

VIDEO CONTROLLER

LOWPASS FILTER

FOR EMI SUPPRESSION

75

Ω

Figure 1. Conventional Load-Detection Scheme

37.5

Ω

REF

CURRENT DAC

MONITOR

VIDEO CONTROLLER

G = 2

75

Ω

MAX9511

-75

Ω

OUT

75

Ω

Figure 2. Load-Detection Scheme with MAX9511

Table 1. Function of Load-Detection

Scheme

AMPLIFIER OUTPUT

MAX9511

INTERNAL

SWITCH

RESISTANCE

AT DAC

OUTPUT

Connected to External Load

Open

37.5

Ω

Not Connected to External Load

Closed

75

Ω