AD9983A/PCBZ Analog Devices Inc, AD9983A/PCBZ Datasheet - Page 13

• Manufacturer Part Number: AD9983A/PCBZ
• Description: Pb-free EVALUATION Kit AD9983A
• Manufacturer: Analog Devices Inc
• Series: Advantiv®r

Specifications of AD9983A/PCBZ

• Main Purpose: Video, Video Processing
• Embedded: No
• Utilized Ic / Part: AD9983A
• Primary Attributes: 3 x 8-Bit 140 MSPS ADC's
• Secondary Attributes: Integrated PLL & VCO
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

CLOCK GENERATION
A PLL is used to generate the pixel clock. The Hsync input pro-
vides a reference frequency to the PLL. A voltage controlled
oscillator (VCO) generates a much higher pixel clock frequency.
The 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 maintain lock
between the two signals.
The stability of this clock is a very important element in
providing 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. Then there is a
time when the input voltage is stable, before the signal must
slew to a new value (see Figure 6). 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.
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 AD9983A clock generation circuit to
minimize jitter. The clock jitter of the AD9983A is low in all
operating modes, making the reduction in the valid sampling
time due to jitter negligible.
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 7. Recommended settings
of the VCO range and charge pump current for VESA standard
display modes are listed in Table 10.
PIXEL CLOCK
8.2nF
C
P
Figure 6. Pixel Sampling Times
Figure 7. PLL Loop Filter Detail
FILT
INVALID SAMPLE TIMES
1.5kΩ
R
Z
C
82nF
Z
PV
D
Rev. 0 | Page 13 of 44
Four programmable registers are provided to optimize the
performance of the PLL. These registers are the 12-Bit Divisor
Register, the 2-Bit VCO Range Register, the 3-Bit Charge Pump
Current Register, and the 5-Bit Phase Adjust Register.
The 12-Bit Divisor Register
The input Hsync frequencies can accommodate any Hsync as
long as the product of the Hsync and the PLL divisor falls
within the operating range of the VCO. The PLL multiplies the
frequency of the Hsync signal, producing pixel clock
frequencies in the range of 10 MHz to 140 MHz. The divisor
register controls the exact multiplication factor. This register
may be set to any value between 2 and 4095 as long as the
output frequency is within range.
The 2-Bit VCO Range Register
To improve the noise performance of the AD9983A, the VCO
operating frequency range is divided into four overlapping
regions. The VCO range register sets this operating range. The
frequency ranges for the four regions are shown in Table 8.
Table 8. VCO Frequency Ranges
PV1
0
0
1
1
The 3-Bit Charge Pump Current Register
This register varies the current that drives the low pass loop
filter. The possible current values are listed in Table 9.
Table 9. Charge Pump Current/Control Bits
Ip2
0
0
0
0
1
1
1
1
The 5-Bit Phase Adjust Register
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. Phase adjust is still available if an
external pixel clock is used. 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 from equalization
pulses). This can be used during the vertical sync period or at
any other time that the Hsync signal is unavailable.
PV0
0
1
0
1
Ip1
0
0
1
1
0
0
1
1
Pixel Clock
Range (MHz)
10 to 21
21 to 42
42 to 84
84 to 140
Ip0
0
1
0
1
0
1
0
1
Current (μA)
50
100
150
250
350
500
750
1500
KVCO
Gain (MHz/V)
150
150
150
150
AD9983A