LM9823CCWM NSC [National Semiconductor], LM9823CCWM Datasheet - Page 20

• Manufacturer Part Number: LM9823CCWM
• Description: LM9823 3 Channel 48-Bit Color Scanner Analog Front End
• Manufacturer: NSC [National Semiconductor]

Applications Information
less than the initial error. If the LM9823 is operating in CDS
mode and multiple lines are used to charge up the clamping
capacitors after power-up, then a clamp capacitor value of
0.01µF should be significantly greater than the calculated
C
If the LM9823 is operating in CIS mode, then significantly larger
clamp capacitors must be used. Fortunately, the output imped-
ance of most CIS sensors is significantly smaller than the output
impedance of CCD sensors, and R
the 50
R
7.0 Power Supply Considerations
The LM9823 analog supplies (V
+5V source. The two analog supplies are brought out individually
to allow separate bypassing for each supply input. They should
not be powered by two or more different supplies.
Each supply input should be bypassed to its respective ground
with a 0.1µF capacitor located as close as possible to the supply
input pin. A single 10µF tantalum capacitor should be placed near
the V
The V
supplied by a clean, low noise linear power supply, with a 0.1µF
ceramic capacitor and a 10µF tantalum capacitor placed near the
V
plane should be provided to isolate the noisy digital output signals
from the sensitive analog supply pins. If the V
than V
are both at 5.0V, then they should be supplied by a common lin-
ear regulator, with separate analog and digital power and ground
planes.
To minimize noise, keep the LM9823 and all analog components
as far as possible from noise generators, such as switching power
supplies and high frequency digital busses. If possible, isolate all
the analog components and signals (OS, reference inputs and
outputs, V
the digital ground plane. The two ground planes should be tied
together at a single point, preferably the point where the power
supply enters the PCB.
8.0 Serial Interface and Configuration Registers
The serial interface is used to program the configuration registers
which control the operation of the LM9823. The SEN, SCLK, SDI
and SDO signals are used to set and verify configuration register
settings. In addition, MCLK must be active during all serial inter-
face activity. MCLK is used to register the level of the SEN input
and drives the logic that process information input on the SDI line.
9.0 Sample Mode Register Settings
A brief overview of the sample mode register and the bit locations
is give in Table 2: Configuration Register Parameters on page
14. The function of each bit is summarized in the following sec-
tions.
9.1 Output Driver Mode
The Output Driver Mode bit is normal set to 0. This bit can be set
CLAMP MIN
CLAMP
D
and DGND pins. If possible, a separate power and ground
A
D
A
supply pins to provide low frequency bypassing.
, a separate linear regulator should be used. If V
value, the clamp capacitors will charge faster.
input can be powered at 3.3V or 5.0V. Power should be
from the LM9823 internal clamp switch. With a smaller
A
, AGND) on an analog ground plane, separate from
value and can virtually always be used.
A
) should be powered by a single
CLAMP
(Continued)
will be dominated by
D
voltage is lower
D
and V
A
20
to 1 to reduce the slew rate of the output drivers.
9.2 DOE (Data Output Edge) Setting
The Data Output Edge bit selects which edge of MCLK is used to
clock output data onto the output pins. For lowest noise perfor-
mance, this bit should be set to 0. With this setting, new data is
placed on the D7-D0 pins on every falling edge of MCLK. See
Diagrams 1 through 6 and Diagram 13 for more information on
data output timing for the different Divide By modes, and detailed
timing of the output data signals.
The bit can be set to 1 to adjust the data output timing for some
applications, but the noise performance of the LM9823 may be
somewhat degraded.
9.3 CDS Enable
The CDS Enable bit determines whether the sampling section of
the LM9823 operates in Correlated Double Sampling mode or in
Single Ended Sampling mode. CDS mode is normally used with
CCD type sensors, while Single Ended mode is normally used
with CIS type sensors.
9.4 Signal Polarity
Whether the LM9823 is operating in Correlated Double Sampling
Mode, or Single Ended Sampling mode, the basic sampling oper-
ation is the same. First a reference level is sampled, then a signal
level is sampled. For CDS mode operation, if the signal level is
lower in voltage than the reference level, the Signal Polarity bit
should be set to 0. This is the normal setting for CCD type sen-
sors. If the signal level is more positive than the reference level,
the Signal Polarity bit would be set to 1 for Positive Polarity mode.
When Single Ended Mode is selected, the Signal Polarity bit
determines which internal reference voltage is used to compare
with the input signal. Most CIS type sensors have a positive polar-
ity type output, and in this case the Signal Polarity Bit should be
set to 1. In this case, the internal V
level during the Reference Sampling period.
In addition, the Signal Polarity bit determines which internal refer-
ence voltage is used during the Clamping interval. If Signal Polar-
ity = 0, V
used.
9.5 SMPCL
The SMPCL setting controls when the clamping action occurs
during the acquisition cycle. If SMPCL is set to 0, the Clamp will
be on for 1 MCLK before the reference sampling point. If SMPCL
is set to 1, clamping will occur in the interval after the reference
sampling point, and before the signal sampling point. In this case,
the clamping time is dependent on the present “divide by” mode,
and the settings of the CDSREF bits.
9.6 CDSREF
The CDSREF setting is provided to allow adjustable sampling
points for the reference sample at the higher “divide by” modes.
This may be useful to optimize the timing of the Reference Sam-
pling point for particular CCD sensors. Diagram 7 shows how the
various settings of CDSREF can be used to delay the Reference
Sampling point. Care must be taken to avoid setting CDSREF to
REF+
is used for clamping, if Signal Polarity = 1, V
REF-
is used as the reference
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REF-
is