MAX463 Maxim, MAX463 Datasheet - Page 8

• Manufacturer Part Number: MAX463
• Description: Two-Channel / Triple/Quad RGB Video Switches and Buffers
• Manufacturer: Maxim

Two-Channel, Triple/Quad
RGB Video Switches and Buffers
The MAX463–MAX470 have a bipolar construction,
which results in a typical channel input capacitance of
only 5pF, whether the channel is on or off. This low
input capacitance allows the amplifiers to realize full
AC performance, even with source impedances as
great as 250Ω. It also minimizes switching transients
because the driving source sees the same load
whether the channel is on or off. Low input capaci-
tance is critical, because it forms a single-pole RC low-
pass filter with the output impedance of the signal
source, and this filter can limit the system’s signal
bandwidth if the RC product becomes too large.
The MAX465/MAX466/MAX469/MAX470’s amplifiers are
internally configured for a gain of two, resulting in an over-
all gain of one at the cable output when driving back-ter-
minated coaxial cable (see the section Driving Coaxial
Cable ). The MAX463/MAX464/MAX467/MAX468 are
internally configured for unity gain.
To realize the full AC performance of high-speed ampli-
fiers, pay careful attention to power-supply bypassing
and board layout, and use a large, low-impedance
ground plane. With multi-layer boards, the ground
plane should be located on the layer that is not dedi-
cated to a specific signal trace.
To prevent unwanted signal coupling, minimize the
trace area at the circuit's critical high-impedance
nodes, and surround the analog inputs with an AC
ground trace (analog ground, bypassed DC power
supply, etc).
MAX463–MAX470 have been separated with AC
ground pins (GND, V+, V-, or a hard-wired logic input)
to minimize parasitic coupling, which can degrade
crosstalk and/or stability of the amplifier. Keep signal
paths as short as possible to minimize inductance,
and ensure that all input channel traces are of equal
length to maintain the phase relationship between the
R, G, and B signals. Connect the coaxial-cable shield
to the ground side of the 75Ω terminating resistor at
the ground plane to further reduce crosstalk (see
Figure 1).
Bypass all power-supply pins directly to the ground
plane with 0.1µF ceramic capacitors, placed as close
to the supply pins as possible. For high-current loads,
it may be necessary to include 10µF tantalum or alu-
minum-electrolytic capacitors in parallel with the 0.1µF
ceramics. Keep capacitor lead lengths as short as
possible to minimize series inductance; surface-mount
(chip) capacitors are ideal.
8
_______________Detailed Description
Power-Supply Bypassing and Board Layout
_______________________________________________________________________________________
The analog input pins to the
Figure 1. Low-Crosstalk Layout. Return current from the
termination resistor does not flow through the ground plane.
Connect all V- pins to a large power plane. The V- pins
conduct heat away from the internal die, aiding thermal
dissipation.
Differential gain and phase errors are critical specifica-
tions for an amplifier/buffer in color video applications,
because these errors correspond directly to changes in
the color of the displayed picture in composite video
systems. The MAX467–MAX470 have low differential
gain and phase errors, making them ideal in broadcast-
quality composite color applications, as well as in RGB
video systems where these errors are less significant.
The MAX467–MAX470 differential gain and phase errors
are measured with the Tektronix VM700 Video
Measurement Set, with the input test signal provided by
the Tektronix 1910 Digital Generator as shown in Figure 2.
Measuring the differential gain and phase of the
MAX469/MAX470 (Figure 2a) is straightforward because
the output amplifiers are configured for a gain of two,
allowing connection to the VM700 through a back-termi-
nated coaxial cable. Since the MAX467/MAX468 are
unity-gain devices, driving a back-terminated coax
would result in a gain of 1/2 at the VM700.
Figure 2b shows a test method to measure the differen-
tial gain and phase for the MAX467/MAX468. First,
measure and store the video signal with the device
under test (DUT) removed and replaced with a short
circuit, and the 150Ω load resistor omitted. Then do
another measurement with the DUT and load resistor in
the circuit, and calculate the differential gain and phase
errors by subtracting the results.
COAX
COAX
Differential Gain and Phase Errors
CURRENT
CURRENT
RETURN
RETURN
R
R
T
T