of the input signal. The default voltage threshold is 150 mV. When connected to an
ac-coupled graphics signal with embedded sync, it will produce a noninverting
digital output on SOGOUT. (This is usually a composite sync signal, containing
both vertical and horizontal sync infor mation that must be separated before
passing the horizontal sync signal to Hsync.) When not used, this input should be
left unconnected. For more details on this function and how it should be
configured, refer to the Sync-on-Green section.
CLAMP External Clamp Input
This logic input may be used to define the time during which the input signal is
clamped to ground. It should be exercised when the reference dc level is known to
be present on the analog input channels, typically during the back porch of the
graphics signal. The CLAMP pin is enabled by setting control bit Clamp Function to
1, (register 0FH, Bit 7, default is 0). When disabled, this pin is ignored and the
clamp timing is determined internally by counting a delay and duration from the
trailing edge of the Hsync input. The logic sense of this pin is controlled by Clamp
Polarity register 0FH, Bit 6. When not used, this pin must be grounded and Clamp
Function programmed to 0.
COAST Clock Generator Coast Input (Optional)
This input may be used to cause the pixel clock generator to stop synchronizing
with Hsync and continue producing a clock at its current frequency and phase.
This is useful when processing signals from sources that fail to produce horizontal
sync pulses during the vertical interval. The COAST signal is generally not
required for PC-generated signals. The logic sense of this pin is controlled by
Coast Polarity (register 0FH, Bit 3). When not used, this pin may be grounded and
Coast Polarity programmed to 1, or tied HIGH (to VD through a 10 k resistor) and
Coast Polarity programmed to 0. Coast Polarity defaults to 1 at power-up.
REF BYPASS Internal Reference BYPASS
Bypass for the internal 1.25 V band gap reference. It should be connected to
ground through a 0.1 ?F capacitor. The absolute accuracy of this reference is
?4%, and the temperature coefficient is ?50 ppm, which is adequate for most
AD9883A applications. If higher accuracy is required, an external reference may
be employed instead.
MIDSCV Midscale Voltage Reference BYPASS
Bypass for the internal midscale voltage reference. It should be connected to
ground through a 0.1 ?F capacitor. The exact voltage varies with the gain setting
of the Blue channel.
FILT External Filter Connection
For proper operation, the pixel clock generator PLL requires an external filter.
Connect the filter shown in Figure 6 to this pin. For optimal performance, minimize
noise and parasitics on this node.
POWER SUPPLY
VD Main Power Supply
These pins supply power to the main elements of the circuit. They should be
filtered and as quiet as possible.
VDD Digital Output Power Supply
A large number of output pins (up to 25) switching at high speed (up to 110 MHz)
generates a lot of power supply transients (noise). These supply pins are identified
separately from the VD pins so special care can be taken to minimize output noise
transferred into the sensitive analog circuitry. If the AD9883A is interfacing with
lower voltage logic, V DD may be connected to a lower supply voltage (as low as
2.5 V) for compatibility.
PVD Clock Generator Power Supply
The most sensitive portion of the AD9883A is the clock generation circuitry. These
pins provide power to the clock PLL and help the user design for optimal
performance. The designer should provide quiet, noise-free power to these pins.
GND Ground
The ground return for all circuitry on-chip. It is recommended that the AD9883A be
assembled on a single solid ground plane, with careful attention given to ground
current paths.
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