17

www.national.com

Applications Information

1.0 Introduction

The LM9822 is a high performance scanner Analog Font End

(AFE) for image sensor processing systems. It is designed to

work with color CCD and CIS image sensors and provides a full 3

channel sampling, gain and offset correction system, coupled

with a 14 bit high speed analog to digital converter. A typical

application of the LM9822 is in a color flatbed document scanner.

The image sensing and processing portion of the system would

be configured similar to that shown in Figure 1.

Figure 1: LM9822 in Basic Color Scanner

2.0 CDS Correlated Double Sampler

The LM9822 uses a high-performance CDS (Correlated Double

Sampling) circuit to remove many sources of noise and error from

the image sensor output signal. It also supports CIS image sen-

sors with a single ended sampling mode.

Figure 2 shows the output stage of a typical CCD and the result-

ing output waveform:

Figure 2: CDS

Capacitor C1 converts the electrons coming from the CCD’s shift

register to an analog voltage. The source follower output stage

(Q2) buffers this voltage before it leaves the CCD. Q1 resets the

voltage across capacitor C1 between pixels at intervals 2 and 5.

When Q1 is on, the output signal (OS) is at its most positive volt-

age. After Q1 turns off (period 3), the OS level represents the

charge injection from Q1, thermal noise from the ON resistance

of Q1, and other sources of error. When the shift register clock

(Ø1) makes a low to high transition (period 4), the electrons from

the next pixel flow into C1. The charge across C1 now contains

that voltage is subtracted from the OS at the end of period 4, the

principal of Correlated Double Sampling.

3.0 CIS Mode (CDS Off, Selectable Signal Polarity)

The also LM9822 supports CIS (Contact Image Sensor) devices.

The output signal of a CIS sensor (Figure 3) differs from a CCD

signal in two primary ways: its output usually increases with

increasing signal strength, and it does not usually have a refer-

ence level as an integral part of the output waveform of every

pixel.

Figure 3: CIS

When the LM9822 is in CIS (CDS off) mode (Register 0, B5=1), it

each pixel (depending on the signal polarity setting (Register 0,

sampled as the reference level.

4.0 Programmable Gain

The output of the Sampler drives the input of the x3 Boost gain

stage. The gain of each x3 Boost gain is 3V/V if bit B5 of that

color’s gain register (register 4,5, or 6) is set, or 1V/V if bit B5 is

cleared. The output of each x3 gain stage is the input an offset

DAC and the output of each offset DAC is the input to a PGA

(Programmable Gain Amplifier). Each PGA provides 5 bits of gain

correction over a 0.93V/V to 3V/V (-0.6 to 9.5dB) range. The x3

Boost gain stage and the PGA can be combined for an overall

gain range of 0.93V/V to 9.0V/V (-.6 to 19dB). The gain setting for

each color (registers 4, 5 and 6) should be set during calibration

to bring the maximum amplitude of the strongest pixel to a level

just below the desired maximum output from the ADC. The PGA

gain is determined by the following equation:

If the x3 Boost gain is enabled then the overall signal gain will be

three times the PGA gain.

ASIC

RAM

8 Output Data

8

CCD Control

AFE Control

To Host

Other scanner elements

omitted for simplicity.

RS (RESET)

e-

(from shift register)

OS

Q1

Q2

C1

Ø1

RS

OS

12

3

4

5

OS (CCD)

12

3

4

5

OS (CIS)

PGA Gain

V

V

----





0.933 + .0667 (value in bits B4-B0)

=

Equation 1: PGA Gain