AD9981SLASHPCB datasheet(43/44 Pages) AD

43 / 44 page

AD9981

Rev. 0 | Page 43 of 44

Adding a series resistor of value 50 Ω to 200 Ω can suppress

reflections, reduce EMI, and reduce the current spikes inside of

the AD9981. If series resistors are used, place them as close to

the AD9981 pins as possible, (although try not to add vias or

extra length to the output trace to get the resistors closer).

If possible, limit the capacitance that each digital output drives

to less than 10 pF. This is easily accomplished by keeping traces

short and by connecting the outputs to only one device. Loading

the outputs with excessive capacitance increases the current

transients inside of the AD9981 and creates more digital noise

on its power supplies.

Digital Inputs

Digital inputs on the AD9981 (HSYNC0, HSYNC1, VSYNC0,

VSYNC1, SOGIN0, SOGIN1, SDA, SCL and CLAMP) were

designed to work with 3.3 V signals, but are tolerant of 5.0 V

signals. Therefore, no extra components need to be added if

using 5.0 V logic.

Any noise that gets onto the Hsync input trace adds jitter to the

system. Therefore, minimize the trace length and do not run

any digital or other high frequency traces near it.

REFERENCE Bypass

The AD9981 has three reference voltages that must be bypassed

for proper operation of the input PGA. REFLO and REFHI are

connected to each other through a 10 µF capacitor. REFCM is

connected to ground through a 10 µF capacitor. These refer-

ences are used by the input PGA circuitry to assure the greatest

stability. Place them as close to the AD9981 pin as possible.

Make the ground connection as short as possible.

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