AFE FOR CCD/CIS SIGNAL PROCESSOR
BL8531H
OVERRALL TRANSFER FUNCTION
The overall transfer function can be calculated as follows.
if gain mode = 0,
ADCout = [(Vin+Input_Offset)* PGA_Gain]/(2*REF)*4096,
if gain mode = 1,
ADCout = [(Vin+Input_Offset)* PGA_Gain * 1.5]/(2*REF)*4096,
where REF is equal to (REFT-REFB) and Input _Offset means the DAC value of the input offset register. The
analog offset range of the input offset register is varied between 150mV and -150 mV. The 8-bit data format for
the input offset register is straight binary coding. Thus, an all 'zeros' data word corresponds to -150 mV. An all
'ones' data word corresponds to 150 mV.
To maximize the dynamic range of the ADC input, it is necessary to program the input offset register code to
move the ADC code corresponding to the black level towards 'zero'.
In case of processing CIS signal, 4-bit of the gain & clamp control register are allocated to control CIS clamp
level. Like the input offset register, the 4-bit data format is straight binary coding. An all 'zeros' data word
corresponds to 0.1 V and an all 'ones' data word corresponds to 1.5 V.
INPUT COUPLING CAPACITOR
Because of the DC offset present at the output of CCD, some kind of DC restoration is required. In case of
CDS enable mode, to simplify input level shifting, a DC decoupling capacitor is used in conjuction with the
internal input circuitry.
The capacitor charging or discharging depends on the clamping time, the analog input resistance of the AFE
and the output resistance of the circuit driving the coupling capacitor.
The clamping time is typically (n*T), where n is the number of periods CDSCLK1 is asserted and T is the period
of assertion. CDSCLK2 should not be asserted during clamping time. And, STRTLN must be low in line clamp
mode for clamping operation. The analog input resistance of the AFE is equal to 1 kW. The recommended input
coupling capacitor is more than 0.01uF. Thus, to extend the clamping time, the time a transport motor moves the
scanner carriage can be available, for example.
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SAMSUNG [ SAMSUNG ]
