AD9822
FUNCTIONAL DESCRIPTION
grounded, a zero volt input corresponds to the ADC’s zero-scale
output. The OFFSET pin may also be used as a coarse offset
adjust pin. A voltage applied to this pin will be subtracted from
the voltages applied to the red, green, and blue inputs in the
first amplifier stage of the AD9822. The input clamp is dis-
abled in this mode. For more information, see the Circuit
Operation section.
The AD9822 can be operated in four different modes: 3-Channel
CDS Mode, 3-Channel SHA Mode, 1-Channel CDS Mode,
and 1-Channel SHA Mode. Each mode is selected by program-
ming the Configuration Register through the serial interface.
For more detail on CDS or SHA mode operation, see the
Circuit Operation section.
3-Channel CDS Mode
Timing for this mode is shown in Figure 2. CDSCLK1 should
be grounded in this mode. Although not required, it is recom-
mended that the falling edge of CDSCLK2 occur coincident
with or before the rising edge of ADCCLK. The rising edge of
CDSCLK2 should not occur before the previous falling edge of
ADCCLK, as shown by tADC2. The output data latency is three
ADCCLK cycles.
In 3-Channel CDS Mode, the AD9822 simultaneously samples
the red, green and blue input voltages from the CCD outputs.
The sampling points for each Correlated Double Sampler (CDS)
are controlled by CDSCLK1 and CDSCLK2 (see Figures 8 and
9). CDSCLK1’s falling edge samples the reference level of the
CCD waveform. CDSCLK2’s falling edge samples the data
level of the CCD waveform. Each CDS amplifier outputs the
difference between the CCD’s reference and data levels. Next,
the output voltage of each CDS amplifier is level-shifted by an
Offset DAC. The voltages are then scaled by the three Program-
mable Gain Amplifiers before being multiplexed through the
14-bit ADC. The ADC sequentially samples the PGA outputs
on the falling edges of ADCCLK.
The offset and gain values for the red, green, and blue channels
are programmed using the serial interface. The order in which
the channels are switched through the multiplexer is selected by
programming the MUX register.
1-Channel CDS Mode
This mode operates in the same way as the 3-Channel CDS
mode. The difference is that the multiplexer remains fixed in
this mode, so only the channel specified in the MUX register is
processed.
The offset and gain values for the red, green and blue channels
are programmed using the serial interface. The order in which
the channels are switched through the multiplexer is selected by
programming the MUX register.
Timing for this mode is shown in Figure 3.
1-Channel SHA Mode
Timing for this mode is shown in Figure 1. It is recommended
that the falling edge of CDSCLK2 occur coincident with or before
the rising edge of ADCCLK, although this is not required
to satisfy the minimum timing constraints. The rising edge of
CDSCLK2 should not occur before the previous falling edge of
ADCCLK, as shown by tADC2. The output data latency is three
clock cycles.
This mode operates in the same way as the 3-Channel SHA
mode, except that the multiplexer remains stationary. Only the
channel specified in the MUX register is processed.
The input signal is sampled with respect to the voltage applied
to the OFFSET pin. With the OFFSET pin grounded, a zero
volt input corresponds to the ADC’s zero scale output. The
OFFSET pin may also be used as a coarse offset adjust pin. A
voltage applied to this pin will be subtracted from the voltages
applied to the red, green, and blue inputs in the first amplifier
stage of the AD9822. The input clamp is disabled in this mode.
For more information, see the Circuit Operation section.
3-Channel SHA Mode
In 3-Channel SHA Mode, the AD9822 simultaneously samples
the red, green and blue input voltages. The sampling point is
controlled by CDSCLK2. CDSCLK2’s falling edge samples the
input waveforms on each channel. The output voltages from the
three SHAs are modified by the offset DACs and then scaled by
the three PGAs. The outputs of the PGAs are then multiplexed
through the 14-bit ADC. The ADC sequentially samples the
PGA outputs on the falling edges of ADCCLK.
Timing for this mode is shown in Figure 4. CDSCLK1 should
be grounded in this mode of operation.
The input signal is sampled with respect to the voltage applied
to the OFFSET pin (see Figure 10). With the OFFSET pin
REV. A
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