AD9984A datasheet(39/44 Pages) AD

39 / 44 page

AD9984A

Rev. 0 | Page 39 of 44

0x2E—Bits[7:0] Test Register 6

Read/write bits for future use. Must be written to 0xE0 for

proper operation.

0x34—Bit[2] SOG Filter Enabler

When this bit is set to 1, the SOG does not pass pulses less than

250 ns in width. This reduces spurious signals that can improperly

drive the PLL circuit. Default for this bit is 0 or off.

0x36—Bit[0] VCO Gear Select

This bit allows the VCO to select a lower gear to run lower pixel

clocks while remaining in a more linear range.

Table 78. VCO Gear Select Bit

Value

Result

Normal VCO setting.

Enables lower VCO clock output.

0x3C—Bits[7:4] Test Bits

Must be set to 0x0 for proper operation.

0x3C—Bit[3] Auto Gain Matching Hold

This bit controls whether the auto gain matching function runs

continuously or runs once and holds the result. Continuous

updates are recommended because they allow the AD9984A to

compensate for drift over time, temperature, and so on.

If one-time updates are preferred, they should be performed

every time the part is powered up and when there is a mode

change. To perform a one-time update, auto gain matching

must first be enabled (Register 0x3C, Bits[2:0]). Next, this bit

(auto gain matching hold) must first be set to 1 to let the auto

gain matching function operate and settle to a final value.

The auto gain matching hold bit should then be set to 0 to hold

the gain values that the auto circuitry calculates. The AD9984A

auto gain matching circuit’s maximum settle time is 10 updates.

For example, if the update frequency is set to once every 64 Hsyncs,

the maximum settling time would be 640 Hsyncs (10 × 64 Hsyncs).

Table 79. Auto Gain Matching Hold Bit

Value

Result

Disables auto gain updates and holds the current

auto gain values.

Allows auto gain to update continuously.

1The power-up default setting is 0.

0x3C—Bits[2:0] Auto Gain Matching Enable

These bits enable or disable the auto gain matching function.

Table 80. Auto Gain Matching Enable Bits

Value

Result

000

Auto gain matching disabled.

110

Auto gain matching enabled.

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