AD9396SLASHPCB datasheet(13/48 Pages) AD

13 / 48 page

AD9396

Rev. 0 | Page 13 of 48

Note that the SOG signal is always negative polarity. For more

detail on setting the SOG threshold and other SOG-related

functions, see the Sync Processing section.

The PLL characteristics are determined by the loop filter design,

the PLL charge pump current, and the VCO range setting. The

loop filter design is shown in Figure 6. Recommended settings

of the VCO range and charge pump current for VESA standard

display modes are listed in Table 9.

GAIN

SOG

1nF

RAIN

47nF

BAIN

47nF

8nF

80nF

1.5k

FILT

PVD

Figure 4. Typical Clamp Configuration for RGB/YUV Applications

Figure 6. PLL Loop Filter Detail

Clock Generation

Four programmable registers are provided to optimize the

performance of the PLL. These registers are:

A PLL is employed to generate the pixel clock. In this PLL,

the HSYNC input provides a reference frequency. A voltage

controlled oscillator (VCO) generates a much higher pixel clock

frequency. This pixel clock is divided by the PLL divide value

(Register 0x01 and Register 0x02) and phase compared with the

HSYNC input. Any error is used to shift the VCO frequency

and to maintain the lock between the two signals.

•

The 12-bit divisor register (R0x01, R0x02). The input

HSYNC frequency range can be any frequency which,

combined with the PLL_Div, does not exceed the VCO

range. The PLL multiplies the frequency of the HSYNC

signal, producing pixel clock frequencies in the range of

10 MHz to 100 MHz. The divisor register controls the

exact multiplication factor.

The stability of this clock is a very important element in provi-

ding the clearest and most stable image. During each pixel time,

there is a period during which the signal slews from the old

pixel amplitude and settles at its new value. This is followed by a

time when the input voltage is stable before the signal must slew

to a new value. The ratio of the slewing time to the stable time is

a function of the bandwidth of the graphics DAC and the

bandwidth of the transmission system (cable and termination).

It is also a function of the overall pixel rate. Clearly, if the

dynamic characteristics of the system remain fixed, then the

slewing and settling time is likewise fixed. This time must be

subtracted from the total pixel period, leaving the stable period.

At higher pixel frequencies, the total cycle time is shorter and

the stable pixel time also becomes shorter.

•

The 2-bit VCO range register (R0x03). To improve the

noise performance of the AD9396, the VCO operating

frequency range is divided into four overlapping regions.

The VCO range register sets this operating range. The

frequency ranges for the lowest and highest regions are

shown in Table 7.

Table 7.

VCORNGE

Pixel Rate Range

12 to 30

30 to 60

60 to 120

120 to 150

PIXEL CLOCK

INVALID SAMPLE TIMES

•

The 5-bit phase adjust register (R0x05). The phase of the

generated sampling clock can be shifted to locate an

optimum sampling point within a clock cycle. The phase

adjust register provides 32 phase-shift steps of 11.25° each.

The HSYNC signal with an identical phase shift is available

through the HSOUT pin.

The coast pin or the internal coast is used to allow the PLL to

continue to run at the same frequency, in the absence of the

incoming HSYNC signal or during disturbances in HSYNC

(such as equalization pulses). This can be used during the

vertical sync period or any other time that the HSYNC signal is

unavailable. The polarity of the coast signal can be set through

the coast polarity register. Also, the polarity of the HSYNC

signal can be set through the HSYNC polarity register. For both

HSYNC and coast, a value of 1 is active high. The internal coast

function is driven from the VSYNC signal, which is typically a

time when HSYNC signals can be disrupted with extra

equalization pulses.

Figure 5. Pixel Sampling Times

Any jitter in the clock reduces the precision with which the

sampling time can be determined and must also be subtracted

from the stable pixel time. Considerable care has been taken in

the design of the AD9396 clock generation circuit to minimize

jitter. The clock jitter of the AD9396 is less than 13% of the total

pixel time in all operating modes, making the reduction in the

valid sampling time due to jitter negligible.

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